Evaluation of the Potential for Composting Gypsum Wallboard Scraps Final Report

Table of Contents

EXECUTIVE SUMMARY............................................................................................................ 1

1.0 INTRODUCTION................................................................................................................... 4

1.1 PROJECT OBJECTIVES................................................................................................................................................... 4

1.2 PROJECT OVERVIEW....................................................................................................................................................... 5

1.3 PROJECT RESPONSIBILITIES....................................................................................................................................... 6

2.0 EXPERIMENTAL DESIGN.................................................................................................. 8

2.1 MIXES EVALUATED........................................................................................................................................................... 8

2.1.1 Initial Mix Characteristics.............................................................................................................................................. 8

2.1.2 Bulking Material Discussion and Selection................................................................................................................ 8

2.1.3 Recommended Initial Mixes........................................................................................................................................ 10

2.2 EVALUATION CRITERIA................................................................................................................................................ 11

3.0 COMPOSTING BIN OPERATION.................................................................................... 12

3.1 MIXING AND BIN LOADING......................................................................................................................................... 12

3.2 PROCESS CONTROL...................................................................................................................................................... 13

3.2.1 Process Monitoring...................................................................................................................................................... 14

3.2.2 Temperature and Oxygen Control.............................................................................................................................. 15

3.2.4 Moisture Control.......................................................................................................................................................... 17

3.3 BIN BREAKDOWN AND SCREENING........................................................................................................................ 19

3.4 SUMMARY OF EQUIPMENT AND MATERIALS USED........................................................................................... 19

4.0 PROCESS MONITORING AND SAMPLE COLLECTION........................................... 20

METHODOLOGY....................................................................................................................... 20

4.1 PROCESS MONITORING SCHEDULE........................................................................................................................ 20

4.2 PROCESS MONITORING METHODOLOGY............................................................................................................. 21

4.3 DATA RECORDING.......................................................................................................................................................... 21

4.4 PRODUCT TESTING........................................................................................................................................................ 21

5.0 TEST RESULTS................................................................................................................. 23

5.1 TEMPERATURE COMPARISON................................................................................................................................... 24

5.2 ENERGY COMPARISON FOR ALL MIXES................................................................................................................ 27

5.3 AIR REQUIREMENTS AND OXYGEN MONITORING.............................................................................................. 31

5.4 ODOR MONITORING OF EXHAUST GASSES......................................................................................................... 35

5.5 PAPER DEGRADATION.................................................................................................................................................. 39

5.6  VOLUME CHANGE OF GYPSUM DUE TO CHIPPING............................................................................................ 40

5.7 SCREENED FRACTION................................................................................................................................................... 43

5.8 FINAL COMPOST PRODUCT QUALITY.................................................................................................................... 44

5.8.1  Effect Of Gypsum On Soil........................................................................................................................................... 44

5.8.2  Compost Product Analyses....................................................................................................................................... 45

5.8.3  Product Use Recommendations................................................................................................................................ 48

5.9 FIELD IMPLEMENTATION OBSERVATIONS........................................................................................................... 49

5.9.1 Germination Results..................................................................................................................................................... 49

6.0 SUMMARY............................................................................................................................ 51

7.0 ACKNOWLEDGMENT....................................................................................................... 54

8.0 REFERENCES.................................................................................................................... 55

Appendices:

Appendix A:                 Mix Ratios (Not included in this electronic report but available upon request

Appendix B:                 Energy Spread Sheets (Not included in this electronic report but available upon request

Appendix C:                 Lab Data (Not included in this electronic report but available upon request)

Appendix D:                 Test Plan

Appendix E:                 Bin Schematic Drawing

List of Tables

Table l              Initial Mix Development Characteristics and Their Relevance

Table 2 Renton WWTP Biosolids Characteristics

Table 3 Bulking Material Characteristics

Table 4 Recommended Mixes for the Bin Composting Project (Volumetric Ratios)

Table 5 Composting Process Control Parameters and Their Relevance

Table 6 Summary of Equipment and Materials Needed

Table 7 Process Monitoring Schedule Summary

Table 8 Process Monitoring Methodology

Table 9 Volumetric Mix Ratios

Table 10           Multipliers for Gypsum Board to Compensate for Volume Increase

Table 11           Mix Ratios With and Without Recycled Screen Overs

Table 12           Compost Product Quality Analyses

Table 13           Gypsum Compost Radish Germination After 7 Days

List of Figures

Figure l Temperature Profile for Mixes 1, 2, 3, and 4 - Trial 1

Figure 2            Temperature Profile for Mix 3 & 4 - Trial 2

Figure 3            Temperature Profiles for Successful Mixes - 1 & 2 from Trial 1 and 3 and 4                              from Trial 2

Figure 4            Mix Total Energy Generation

Figure 5            BTU's/CY/Day

Figure 6            Airflow for Trial 1 Mixes

Figure 7            Trial 1 Oxygen Contents

Figure 8            Airflow for Trial 2 Mixes

Figure 9            Trial 2 - Oxygen Content

Figure 10          Trial 1 Exhaust Ammonia Generation Rates

Figure 11          Trial 1 Exhaust Dimethyl Sulfide Content

Figure 12          Trial 2 Exhaust Dimethyl Sulfide

Figure 13          Gypsum Wallboard Paper Reduction

Figure 14          Volume Increase Due to Gypsum Processing

EXECUTIVE SUMMARY

Gypsum wallboard is used in the construction of all types of new buildings.  During construction, scrap wallboard is generated and added to the wastestream from building sites.  This creates a cost for the contractor and fills valuable landfill space.  The wallboard consists of mostly calcium sulfate and paper.  Currently, only one reuse option exists, which is the recycling of the board into new wallboard product.  This reuse method is quite successful, but does not use the wallboard waste paper fraction.  The waste paper still poses a disposal problem for the wallboard manufacturers.  While a market exists for the gypsum powder, not all generators of board will have economical access to this market.  Many generators will only have access to options which will take the whole board scrap.  Therefore, it is important to discuss and examine options for recycling the paper and gypsum powder together.  This report describes the results of a study undertaken to examine the feasibility of recycling scrap gypsum wallboard as a bulking agent in the composting process. 

Four mixes were examined with different mix ratios of gypsum, yard debris, and biosolids.  The mixes all reached temperatures suitable for pathogen destruction (as per EPA regulations), and there were not significant differences in odor production.  A comparison was made between the fine pieces (less than a quarter inch) and the larger pieces of paper (approximately 2 inch diameter) which appeared in the shredded wallboard.  The smaller pieces degraded nearly completely, and the larger paper pieces degraded an average of 40% by weight during the process.  The product quality was not hindered from the addition of the gypsum.  Some parameters were higher, as might be expected.  Calcium content rose in direct proportion to the gypsum fraction.  Organic content dropped as more gypsum was added.  Boron content was not affected.  A germination test was done to determine if the material had any toxic effects.  Germination was not affected by the addition of gypsum.  The screened end product had some noticeable differences, such as the presence of gypsum powder in greater quantities as the mix ratio increased.  There was no paper present in the screened product, and very little remained in the overs.   

Processing of the materials was examined as well.  Two different methods (crushing by hand and grinding with a chipper) showed that the material volume increased after processing.  These ratios were taken into account when recommendations for mixes were developed.  In addition, a field trial with processing of the board was observed.  The best way to control dust from the processing seemed to be to grind the board simultaneously with a prescribed volumetric ratio of yard debris, and to keep the hopper full to limit the escape of dust from the top.

The composting industry may wish to consider the use of gypsum wallboard to supplement the other bulking agents received at the site in times of low supply.    For facilities which receive biosolids, it is important to have an adequate supply of bulking material in order to provide the necessary porosity, balance the carbon to nitrogen ratio to within the appropriate range (25-35:1), and absorb the excess water present in the biosolids, which generally arrive on site between 15%-25% solids.  The addition of a dry bulking agent will help hit the target range for initial mix total solids (40%-50% solids).    The gypsum wallboard, with its paper content, can provide all of these things.   If a facility is regularly receiving high volumes of grass during one part of the season and does not have an adequate supply of  woody bulking material to provide porosity, a mix supplemented with chipped wallboard may be an appropriate measure to help prevent the generation of odors.

In areas with large yard debris composting facilities, it may be difficult to obtain enough of all the green bulking agents needed for a proper mix.  Gypsum wallboard should be considered as a supplement to wood and yard debris.  The conclusions of this report show no detrimental effects (aside from minor aesthetic issues) in the product or in the off gasses.  In addition, the tip fees from the wallboard will bring revenue to the site, helping to ensure profitability.  If yard debris and other woody material are not available, the shortfall can be filled with wallboard, to the extent that the mix recipe will allow. 

If an existing gypsum reuse option for new wallboard exists in the area close to a compost facility, it is likely that the scrap gypsum is not going to make it to the compost pile.  It is likely, though , that the gypsum recycling plant is creating a disposal problem for themselves, with all of the scrap paper stripped off of the old board scraps.  This material could also be incorporated into the compost pile, serving as a carbon source and a moisture absorber.  Again, there may be the benefit of tip fees generated from the receipt of this material.       

1.0       INTRODUCTION

The purpose of this project was to evaluate the potential for using composting as a means of recycling scrap gypsum wallboard generated in construction and demolition projects.  Currently there are few reuse options for this material, which contains both gypsum and paper.  There is an established market for the gypsum powder (in the production of new wallboard), but the paper is not reused in the process.  Paper has been shown to break down well in the composting process and serve as a source of carbon.  Gypsum and paper will absorb moisture, and calcium is a common soil sweetener.  In addition, the paper is fairly heavy and provides bulk and structural integrity (if not too wet) to the piles, which aids in the even dispersion of air throughout the pile.  This report describes the test mixes, testing plan, and project results.

The wastewater treatment process at Renton Wastewater Treatment Plant (a King County facility) generates wastewater solids (20% solids content).  These biosolids are currently utilized in a variety of offsite reuse options, including land application for agriculture and silviculture (forestry), and composting.  The composting of biosolids requires a bulking agent to balance the water content of the biosolids, add the required carbon content, and provide porosity for proper air distribution. 

1.1       PROJECT OBJECTIVES

The primary objective of this project was to assess the feasibility of composting as a process for recycling gypsum wallboard.  Specific project objectives are summarized as follows:

1.      Evaluate process for:
·        Ability to break down paper.
·        Reduction of the volume of material to be disposed/utilized.
·        Impact on final product calcium and sulfur content.
·        Impact on final product soil salinity and pH.
·        Production of ammonia and sulfur production in exhaust gas.
·        Ability to utilize gypsum.

2.      Develop recommendations for demonstration scale testing, including:
·        Most suitable supplemental bulking materials and initial mix ratios.
·        Appropriate detention time.
·        Aeration system sizing.
·        Process monitoring and testing requirements
·        Wallboard processing (grinding) to control dust.

3.      Establish bulking materials and composting process controls that provide the most effective breakdown of wallboard scraps with the best end product quality.

4.      Develop the following information for developing a full scale conceptual design and cost estimate:
·        Mass balance
·        Detention time
·        Processing equipment needed
·        Process control strategy

1.2       PROJECT OVERVIEW

This project entailed the composting of four different mixes using wastewater solids and several different biosolids/bulking material/gypsum  ratios in 21 cubic foot composting bins.  A cement mixer was used to mix the bulking materials and wastewater solids.  The mixes were manually loaded into the bin composters and composted/cured for an eight week period in which temperature, oxygen, and moisture were maintained within optimum ranges.  During the eight week process, monitoring information was collected. At the end of the process, the volume and weight of the product was determined.  In addition, the product was screened manually and the final product tested for several product quality parameters to determine the benefits or detrimental effects of the addition of the gypsum wallboard.

1.3       PROJECT RESPONSIBILITIES

Project responsibilities were defined as follows:

E&A Environmental Consultants, Inc.:

·        Oversee bin setup, material mixing, and bin loading.
·        Oversee procurement of bulking materials needed for the bin scale operation.
·        Provide the bin composters and process monitoring equipment.
·        Provide training to Renton WWTP personnel on bin composter operation, process monitoring, and data management.
·        Conduct a minimum of four site visits to oversee operation.
·        Oversee bin breakdown and screening.
·        Transport the bins to the composting site.
·        Collect and ship samples to appropriate laboratories as instructed in the testing plan.
·        Conduct all process monitoring and data entry as instructed by the testing plan.

East Division Reclamation Plant @ Renton agreed to provide the following:

·        Provide an area to protect the bins from the rain and sun, and a gravel or paved surface for supporting the bins and mixing the feedstocks.
·        Provide utilities, including single phase electrical power and water for moisture adjustment.
·        Provide access to office space in construction trailer and a desk for placement of controller unit.

2.0       EXPERIMENTAL DESIGN

2.1       MIXES EVALUATED

2.1.1    Initial Mix Characteristics

The composting process begins with the development of an initial mix that has suitable characteristics to promote thermophilic composting.  These initial mix characteristics are summarized in Table 1.

2.1.2    Bulking Material Discussion and Selection

In order to create an optimum initial mix, a bulking material is added to the wastewater solids.  Gypsum wallboard and yard debris served as the bulking agents for this project.  The bulking material is added to increase the solids content to a suitable range, increase the porosity of the initial mix, and add energy (readily degradable carbon source) to the mix if the wastewater solids provide an inadequate contribution of energy to the mix. 

The composting of wastewater biosolids has been studied closely, and it is fairly well known how much energy the wastewater solids will contribute to the mix.  The fresh solids typically have a high energy content.  The volatile solids content of an organic material is a good indication of the energy contained in the material.  The solids content of this material (approximately 20%) would result in an initial mix with no additional water requirements.  A materials balance analysis was performed to determine the need for additional water.

Renton (WWTP) biosolids characteristics are described in Table 2.  The data was provided by Renton personnel and is from digester number five, which is used to blend materials from the other four digesters before biosolids are sent to the dewatering building.  The solids content and volatile solids data are from samples collected after the belt press process.

Table 2 - Renton WWTP Biosolids Characteristics

There are many locally available materials that could potentially be used as a bulking agent.  The ideal bulking material has a solids content greater than 60 percent, provides enough energy to allow the maintenance of thermophilic conditions (113%F - 167^o F), provides structure and porosity to the mix, and is readily available at a reasonable cost.  The ideal particle size for a bulking material is dependent on several factors.  In general, the coarser the bulking material, the more porosity and less available carbon provided to the mix.  A coarse bulking material also typically needs to be screened to produce a product for sale.

A goal in developing the initial mixes was to test different bulking material ratios in order to evaluate the effects of adding different amounts of wallboard.  The characteristics of several bulking materials are summarized as follows.  Again, for this project, yard debris and gypsum wallboard were used as bulking agents.

2.1.3    Recommended Initial Mixes

Based on early discussions, test evaluation mixes are presented in Table 4.  The table displays the volumetric ratios as well as the cubic feet of each feedstock utilized in each mix (which is 21 cubic feet in total).  A mass balance initial mix ratio for each of these mixes is presented in Appendix A.

total ft³                                                                                  21                                                  47                                16

These mixes were designed based on the assumed percent solids of the biosolids and the yard debris.  An evaluation was made in the field based on the condition (moisture content) of these materials since conditions can change from day to day.  The bulking material ratios were not modified during mixing.

2.2       EVALUATION CRITERIA

Throughout the project, data was collected for evaluating the different bulking materials and the overall viability of composting.  Evaluation criteria are summarized as follows:

·        Gypsum content
·        Paper degradation
·        Volume and weight reduction
·        Heat generation
·        Energy generation
·        Ammonia volatilization and sulfur gas generation (odor impact)
·        Product quality
·        Volatile solids reduction

3.0       COMPOSTING BIN OPERATION

Detailed instructions for operating the bins are presented in this section.

3.1       MIXING AND BIN LOADING

Mixing the biosolids with the bulking agent (gypsum, yard debris, etc.) is the single most critical task in composting.  Attention to detail is important to control and achieve proper mixing.  The function of mixing is to thoroughly combine the biosolids and bulking agents to create a uniform, compostable mass.  The ratio, as well as the method of combining the biosolids and bulking agent, will affect the physical properties of the mixture.  The goal of mixing is to control the solids content of the mix and to create a mass that is sufficiently porous to allow air to flow uniformly through it.  The mix must possess structural integrity sufficient to maintain porosity when built into the compost pile.  In addition, mixing provides for the dispersal of the biosolids throughout the mass to expose maximum biosolids surface area to the microorganisms responsible for decomposition.

Mixing and bin loading entailed the following steps:

1.      Each bin was prepared by opening the top, checking that the aeration pipe was in place, and placing a two inch layer of coarse woody material over the top of the aeration pipe.

2.      Each feedstock material was loaded into a nine cubic foot cement mixer by way of five gallon buckets according to the specified mix ratio.

3.      Each bucket was weighed and recorded prior to loading.; each batch mix contained a maximum of 30 gallons.

4.      Materials were loaded into the mixer in the following order: half of the bulking material, all of the wastewater solids, remainder of the bulking material.

5.      Each batch was mixed until a homogenous mix was produced (approximately 5 to 8 minutes).

6.      The resulting mix was unloaded into a wheelbarrow and transported to the appropriate bin, where it was loaded manually into the bin through the top.

7.      Approximately one liter of each batch was put aside for the purpose of producing a compost sample for analysis.

8.      After the bin was full to within three inches of top (4 to 5 batches), the top on the inner box was replaced, the insulation was put in place, then the top on the outside box was replaced.

3.2       PROCESS CONTROL

Composting is a controlled biological process designed to rapidly convert waste organic material into a humus-rich material that is useful for a variety of purposes associated with landscaping and growing plants.   The controlled aspect allows the process to be completed efficiently.  Process control requires that appropriate monitoring be undertaken and process adjustments be completed based on performance.  The extent of monitoring and control for composting varies widely, depending on the complexity of the composting method used and the degree of process optimization desired.  Since compost is a product that is utilized for plant growth and landscaping, the character of the final product is critical to successful marketing.  In addition, the proper control of process parameters (temperature, oxygen levels, etc.) is an effective odor control method.

3.2.1    Process Monitoring

Process monitoring entails the regular collection of data pertinent to the composting process.  In addition, the data should be examined to determine if and what process adjustments need to be made.  Process control parameters and their relevance are summarized in Table 5.

In this project, process monitoring consisted of the daily determination of temperature, the weekly determination of oxygen content, and testing for moisture content at the beginning and end of the project.  Process monitoring methodology is presented in Section 4.  Process control adjustments are discussed in the next subsection.

3.2.2    Temperature and Oxygen Control

Both temperature and oxygen are controlled by adjusting the volume of air provided to the composting mass.  In the bin composter, these parameters are controlled by adjusting the aeration rate and frequency.  An increase in the amount of aeration air reduces bin temperature and increases the oxygen concentration.  Decreasing the volume of aeration air has the opposite effect on temperature and oxygen concentration.  Too little air can inhibit microbial activity and reduce temperature as well as produce conditions under which odors may be generated.  In the bin system, the provision of aeration air for temperature control typically results in the maintenance of aerobic conditions, and aeration changes for increasing the oxygen concentration are typically not required. 

Temperature Control Strategy

1.      In this project, the temperature of the bins was be maintained between 50^oC and 60^oC.

2.      Initially, the aeration rate was be adjusted to maintain temperatures of 55^oC for three consecutive days, to meet U.S. EPA pathogen reduction criteria. 

3.      After pathogen reduction was accomplished, aeration was adjusted to maintain temperatures between 40^oC and 50^oC, a level considered optimal for organic matter degradation.

Aeration Control Strategy

The volume of aeration air provided to the bin composter can be controlled in the following two ways:

1.      Increase the air flow rate by way of the rotameter (2 to 8.5 cfm).

2.      Increase the aeration off time with the "Compost Captain" controller

The bins used a programmable logic computer (PLC) designed to control four aeration blowers and record temperature in four piles.  The PLC can be operated in the following two modes:

1.      Manual setting of the blower off time.  In this mode, the on-time is fixed at two minutes and the off time can be increased from a minimum of two minutes off to a maximum of 21 minutes off (2 minutes on/2 minutes off to 2 minutes on/21 minutes off).

2.      Time and temperature setting.  This mode combines the manual setting of the blower-off time with a temperature feedback setting.  The temperature feedback dial on the Compost Captain is set for the maximum temperature desired.  When the temperature rises above this set point, as determined by a temperature probe placed in the bin, the controller automatically starts the aeration blower.  When the temperature falls below the set point, the blower is automatically turned off.

Specific operating instructions for the PLC used in this study are presented as follows:

1.      The controller was set on the time and temperature setting.

2.      The temperature feedback control was set at 60^oC until 55^oC had been maintained for three consecutive days.

3.      The temperature feedback was set at 50^oC after 55^oC has been maintained for three consecutive days.

4.      The rotameter was adjusted to deliver 2 cfm.

5.      The blower-off time was set at 20 minutes.

6.      If the bin temperatures were continually above the target level, the blower-off time was decreased.  If the temperatures were still above the target level, airflow was increased by way of the rotameter.

7.      If the bin temperatures were below the target level,  the airflow was decreased by way of the rotameter.  When the airflow was reduced to 2 cfm, the blower-off time was increased.

8.      If bin temperatures were below the target level at the lowest aeration setting (2 cfm, 2 minutes on/20 minutes off), the aeration blower was shut off.

9.      The goal to achieve was to adjust the rotameter and blower-off time, so that aeration was provided as near continuously as possible.

10.  All aeration adjustments were recorded on the daily operational log.

3.2.4    Moisture Control

Moisture levels, which were determined before and after the composting stage, were controlled through the following three methods:

·        An appropriate amount of bulking material was added to develop an initial mix with the desired moisture content.
·        Water was added manually through the top of the bin if necessary.
·        The airflow rate was increased to enhance evaporation.

Moisture Control Strategy

1.      The initial mix was adjusted to have a moisture content between 58% and 62%.
2.      During composting the moisture content was not allowed to drop below 45% until the last week of composting.
3.      At the time of bin breakdown and screening the moisture content should have been between 38% and 42%.

Moisture Control Instructions

1.      The moisture content and bulk density of the feedstocks was determined prior to developing the initial mix.  The mass balance spreadsheet was used to determine how much bulking material was needed to develop a mix that had a moisture content within the target range (58% to 62%).

2.      If the moisture content during composting declined below the lower process control limit of 45% (and composting was to continue at least seven additional days prior to screening), the mass balance spreadsheet was used to determine how many gallons of water needed to be added.  The water was added slowly through the top of the bin.  The compost agitator tool was used to facilitate the distribution of water throughout the composting mass.

3.      If, one week prior to screening, the moisture content was greater than 42%, the volume of aeration air provided was increased to enhance evaporation.  Removing the top off the bin also increased the rate of moisture loss.

3.3       BIN BREAKDOWN AND SCREENING

The time of bin breakdown was based on several factors, including moisture content and overall length of the project.  The procedure for breaking down the bins and screening the compost follows:

1.      Plastic sheeting was placed on the ground in front of the bin.
2.      The top and side of the bin was opened.
3.      The bin contents were shoveled into a wheelbarrow.
4.      A composite sample of the mix was collected for field bulk density measurement and laboratory analyses.
5.      The contents of the bin were manually passed through a 3/8” screen.
6.      The volume of screen overs and unders was recorded.
7.      The bulk density of screen overs and unders was determined.
8.      A composite sample of the screen overs and unders was collected for laboratory analysis.

3.4       SUMMARY OF EQUIPMENT AND MATERIALS USED

Equipment and supplies needed for the project are summarized in Table 6.

4.0       PROCESS MONITORING AND SAMPLE COLLECTION

METHODOLOGY

This section presents the process monitoring parameters for the project with the appropriate frequency and methodology for each.  It also discusses data recording and end product testing parameters.

4.1       PROCESS MONITORING SCHEDULE

The process monitoring schedule is summarized in Table 7.

4.2       PROCESS MONITORING METHODOLOGY

Process monitoring methodology is described in Table 8.

4.3       DATA RECORDING

All temperature data was recorded by a printer attached to the PLC.  Any operational activities that were conducted, i.e. water addition, were recorded on an additional form.  A separate form was kept for each mix.

4.4       PRODUCT TESTING

In order to determine the difference between the end products derived from each mix, the compost was tested for several parameters.  The addition of the gypsum was expected to have an effect on the pH and the levels of calcium and sulfur, since the wallboard is typically 92% calcium sulfate ore.  The product was tested for:

·        Calcium, sulfur, pH
·        Other nutrients:
        -total kjeldahl nitrogen
        -nitrate nitrogen
        -ammonium nitrogen
        -phosphorus
        -potassium
        -magnesium
        - zinc
·        Cation exchange capacity
·        Soluble salts
·        Volatile solids
·        Compost stability (CO₂ respiration rate)
·        Bulk density
·        Total solids, volatile solids, and bulk density of input feedstocks.

5.0       TEST RESULTS

This section discusses the results of the pilot test and defines the parameters that were observed in the most successful trials (mix ratios, temperatures, aeration needs, etc.).  Graphs are shown for temperature, aeration rates, oxygen levels, energy generation (BTU’s), and exhaust odors. 

Two trials were conducted, as a result of some temperature problems encountered in the first trial.  During the first trial, which began with mixing of the four batches on December 26, 1996, the third and fourth mixes did not come up to temperature.  This was largely due to the fact that on the day of mixing, the Seattle area experienced an intense rain/snow/sleet storm.  The materials were all kept under tarps, but when biosolids were transported from the dewatering facility, it was done in an open wheelbarrow, and water was taken on.  In addition, as the gypsum was chipped, it fell onto a tarp, and was immediately covered loosely with another tarp.  Despite this method, the gypsum still drew moisture out of the humid air.  As a result, the materials which were mixed later (Mix 1 was first, Mix 4 was last) were considerably wetter than those mixed earlier in the day (mixing spanned 8 hours).  The wetter mixes did not have the proper porosity to distribute air evenly, due to moisture levels that were too high.  When it became evident that Mixes 3 and 4 were too wet, the materials were removed and remixed with fresh gypsum, yard debris, and biosolids.  The remixing occurred on January 23, 1997.  The mix ratios are described in volumetric terms.  Table 9 shows the volumetric mix ratios for the 4 mixes which were examined in the two trials.  Mix 1 had no gypsum and Mixes 2 through 4 had gradually decreasing fractions of gypsum content.

Table 9:  Volumetric Mix Ratios

5.1       TEMPERATURE COMPARISON

Temperature profile comparisons for each of the mixes for each trial are shown in this section.

Figure 1 shows the temperatures achieved throughout the 28 day composting period for Trial l.  As the graph shows, Mix 1 (no gypsum) and Mix 2 (37.5% gypsum) achieved the temperatures which correspond to a Process to Further Reduce Pathogens (PFRP) (55⁰C), and Mix 3 and Mix 4 did not.  Again, this was due to the mixes becoming too wet due to the weather.  The ambient temperature is also plotted, and shows temperatures hovering around freezing for the first few days and gradually climbing.

Figure l

The temperature profile for Trial 2 is shown in Figure 2.  Trial 2 replicated the mix ratios for Mixes 3 and 4 in Trial 1, since they were too wet in the first trial. The remixing occurred on a day which was dry, so the mixes did not get overly moist and the temperatures reacted as expected Both Mix 3(25% gypsum) and Mix 4 (12.5% gypsum) achieved the PFRP temperatures.  Ambient temperatures again were low, but mostly above freezing.  These conditions are similar to those seen in Trial 1, with the exception of the ambient air being slightly drier (see section 5.2 for details of the energy balance).

Figure 2

In order to compare the temperature response of the four successful mixes, which spanned two trials,  the temperature graphs are combined in Figure 3.  This figure shows the temperature response of each mix as it relates to the process day of the respective trial.  The temperature curves are quite similar, with the exception of the when the peak occurred.  Trial 2 had peaks which occurred several days sooner in the process than did Trial 1.  This does not appear to be a function of the gypsum content of the mixes, but of the conditions of the trial.  The ambient temperatures were quite a bit lower in the early stages of the process in Trial 1, and therefore slowed the heating process. 

There does appear to be a trend of slowed temperature response with greater gypsum content.  The two mixes in Trial 1 were the control (no gypsum) and the mix with the greater fraction of gypsum.  The control heated up approximately four days faster than the high gypsum mix.  The two mixes from Trial 2 were the mixes with the least amount of gypsum and second greater fraction.  Again, the mix with the least amount of gypsum heated up faster.  Mix 4 heated up two days faster than Mix 3 , again showing that greater amounts of gypsum slowed the heating process.  This makes intuitive sense, since the inorganic gypsum replaces organic yard debris, thereby reducing the potential energy release.  None of the mixes had gypsum levels high enough to hinder temperature.  

Figure 3

5.2       ENERGY COMPARISON FOR ALL MIXES

One method of comparing the reaction of the mixes is to examine the energy generated by each mix (energy is expressed in British Thermal Units, or BTU’s).  While comparing temperatures shows the heat retained by each mix, energy calculations take into account the surrounding atmosphere (temperature and relative humidity) and therefore allow for a comparison of conductive and convective heat losses.  By adding the conductive and convective losses to the BTU’s required to heat the materials in the bin, a total energy generation rate can be calculated.  In addition, if ambient temperature and relative humidity are recorded for mixes that occur at different times and under different atmospheric conditions, accurate energy generation can be calculated for each mix and a true comparison of energy can be made.  A comparison of temperatures alone would not adequately describe the mix dynamics.  For instance, a mix in cold conditions (winter) may achieve the same temperature levels as a mix during warm conditions (summer), but the winter mix will show a much higher energy generation rate than the summer mix, because of compensation for the cold weather.

The energy for a mix is derived by calculating the conductive heat loss, convective/evaporative heat loss, and the energy required to heat the volume of materials from ambient to final temperature.  Conductive heat loss is the loss through the sides of the boxes (or in a full scale operation through the insulative cover material) due to the temperature difference between the compost and the ambient air.  Convective/evaporative losses are due to the energy required to vaporize water in the mix and be carried out in the exhaust.  The energy required to heat the materials from one temperature to another is a function of the specific heat of a material.  Specific heat is reported in BTU’s/lb/^oF, and if the weight of material and the change in temperature are known, BTU’s can easily be calculated.  Again, total energy for a mix is the convective/evaporative losses plus the conductive losses plus the energy required to heat the materials to final temperature.    

The specific heat of water is 1 BTU/lb/^oF.  The specific heat of other materials is generally lower than that of water, and inorganic materials are usually higher than organic materials.  As a result, the BTU’s required to heat the mixes with higher quantities of gypsum are slightly higher than those with less gypsum.  As can be seen in Figure 4, the portion of the total BTU generation related to the heating of the mass is a small fraction as compared to the convective and conductive heat losses from the mixes. 

Figure 4

Mixes 3 and 4 from Trial 1 showed the lowest BTU generation rate, as was predicted from the low temperatures.  This makes intuitive sense because of several factors.  First, the mixes are wetter and require more energy to heat up.  Second, the mixes never heated up sufficiently to produce significant convective or conductive heat losses.  Because the convective losses did not occur, little water was driven off, leaving the mix wet, and not allowing it to heat up properly.  Again, these mixes were too wet from the start as a result of the weather conditions and the gypsum drawing water from the moist air.  The result of this finding is a recommendation to grind wallboard on an as needed basis. If the board is chipped and stored, much more surface area is exposed, allowing for the gypsum to absorb more water from the air, making it difficult to use in a biosolid compost system, since the mix water is added with the biosolids.  Please note again that the volumetric content of the mixes are as follows:

                        Mix l                0% Wallboard

                        Mix 2               37.5% Wallboard

                        Mix 3               25% Wallboard

                        Mix 4               12.5% Wallboard

The mixes which were remixed during Trial 2 responded quite well.  The conditions were more favorable, and fresh dry gypsum was used.  Figure 5 shows a comparison of BTU’s/cubic yard/day for each of the four mixes in Trial 1 and the two remixes for Trial 2.  As can be seen, the energy generation rates for the remixed batches are similar to those from the first two mixes of Trial 1 (those which achieved proper temperature levels).  The weather during Trial 2 was slightly warmer, the air was generally a bit drier, and similar energy generation was observed.    

5.3       AIR REQUIREMENTS AND OXYGEN MONITORING

This section describes the airflow needs of the compost mixes as observed during the trials.  The air flow is reported in cubic feet per hour per dry ton of material (cf/hr/dt).  This is a standard design unit for aerated composting.  The typical range of aeration is from 500 - 5000 cf/hr/dt, depending on the energy level in the feedstocks.  Food waste, for instance, has a high energy potential and therefore needs more oxygen to stimulate the population of microbes and to strip excess heat out of the compost.  The beneficial organisms have an optimum range of temperatures in which they thrive.  If the upper end of this range is exceeded for an extended period of time, the beneficial microbe population will decrease and the composting process will be compromised.  If too little air is provided, the microbes are not supplied with sufficient levels of oxygen, and again the population will decrease, compromising the process.  

Figure 6 shows the aeration rates observed in Trial 1.  The aeration is adjusted by controlling the actual flow through the rotometer (0 - 10 cfm) and by controlling the blower on/off time.  The on time is always two minutes, and the off time can range from 0 - 21 minutes.  The flow rates in the graph were calculated as follows: 

       cf              min       on time

                      ____  *  60 ____  *  ______--                                     cubic feet     

                      min               hr        off time                     =        ___________________

______________________________________________        hour * dry ton organics

lbs. organics in mix * 2000 lb/ton * % solids of organics

Flow rates were adjusted during the trial to try to optimize the heat generated in the mix.  The airflow in Trial 1 was high to begin the composting, because heat is normally generated rather quickly with biosolids.  This mix was wetter than usual (due to the rain) and the aeration rate most likely stripped heat out faster than was optimum.  This may account for the delayed temperature peak as compared to Trial 2.   

Figure 6

Figure 7 shows the oxygen levels in the compost mixes during Trial 1.  Ambient air has 20.9% oxygen, and a well managed compost system should maintain oxygen levels in the range of 14% - 18%.  A very active compost pile with high energy feedstocks will require more air to meet this, while a mix with less energy would require less air.  It is necessary to monitor temperature and oxygen simultaneously, since they are so closely related.  Low temperatures could mean oxygen levels below the optimum range (0%-10%, too little air) or above the optimum range (>18%, too much air-heat is being stripped off).  Monitoring oxygen levels with temperature allows for aeration adjustments which will optimize the process and make efficient use of power.  Oxygen is monitored in a compost pile by using a hollow probe to draw a sample of air from the center of the pile across an oxygen sensor (many are commercially available).  Digital or analog meters read the level to a tenth of a percent.    Mix 3 and Mix 4 dropped below the optimum level of oxygen during Trial 1.  The aeration rate for these mixes was left a bit higher than the rates for Mix 1 and Mix 2, and the oxygen levels came back up.  Unfortunately, the moisture content of Mix 3 and Mix 4 did not promote microbial activity and the temperatures never came up to pathogen reduction levels.  Mix 2 and Mix 3 had identical oxygen contents.

Figure 7

Figure 8 shows the airflow for the mixes in Trial 2.  In an effort to bring the temperature up faster in the second trial than in the first, aeration was not started until several days into the composting process, and was kept at a lower rate.

As can be seen in Figure 9, oxygen levels for the two mixes came into the optimum range within five days and stayed there throughout the process.

Figure 9

5.4       
ODOR MONITORING OF EXHAUST GASSES

The exhaust gasses from the bins are all channeled through a port at the back of the bin.  Air is forces through the pile from the bottom, and the air escapes through a pipe a the top of the bin.  Odorous compounds can be detected with a Draeger Tube system, which uses a hand pump to draw a sample of air through a glass tube, specific to the compound of interest.  A color change occurs in the tube, indicating the level of the compound in the airstream on a scale on the tube.  The compounds of interest for these trials were ammonia, hydrogen sulfide, mercaptans, formic acids, and dimethyl sulfide.

persist. Windrow composting, for instance, can produce H₂S fairly often since the piles are usually turned at the most every three days. If a high energy feedstock is used, the oxygen introduced during turning will be used up within a couple of hours. Depending upon the size of the bulking agent, which determines the level of passive aeration, this can lead to anaerobic conditions and therefore the production of 1125. None of the mixes tested showed any detectable level of H₂S. This is likely due to the mixes remaining aerobic (as was seen in the oxygen graphs in Section 5.3).

Mercaptans are considered organic sulfides. They are distinguished by their intense vile odor. The odor produced by skunks is largely due to butyl mercaptan (Haug, 1993). These compounds occur in nature and provide the odor and taste of such plants as garlic and onions. Mercaptans are formed from sulfiw containing amino acids, with the greatest production under anaerobic conditions. If a compost pile has microsites which are under anaerobic conditions, mercaptans may be formed. If air and oxygen are introduced to these anaerobic microsites, which may occur as the material dries and air dispersion becomes beffer, the mercaptans can be oxidized to dimethyl sulfide. No mercaptans were detected in either trial. Dimethyl sulfide was present in each trial, and the levels are compared in Figure 11 (Trial 1) and Figure 12 (Trial 2). Dimethyl sulfide is detectable to the human nose on average at one part per billion in the air. The exhaust contained levels ranging from zero parts per million at the end of the trial to 2.25 parts per million midway through the process of Trial 1. Mix 1 and Mix 2 had similar levels. Mix 3 and Mix 4 were approximately half the levels of Mixes 1 and 2. No significant trend was observed, and no detrimental effect or increased odor was seen from the addition of the gypsum wallboard.

Figure 11 Trial 1 Exhaust Dimethyl Sulfide Content

Formic (fatty) acids occur in nature as constituents of fats, oils, and waxes. Long chained acids can be hydrolized to lower volatile acids such as acedic, propionic, and butyric acids. Volatile and fatty acids are readily degradable. One example, acetic acid (vinegar), has a recognizable and generally agreed upon objectionable odor. No formic acids was detected in either trial.

5.5       PAPER DEGRADATION

One of the interests in this study was to examine the degradation of the paper from the gypsum wallboard. Other recycling options for gypsum wallboard make use of the powder (such as reuse in new wallboard) but do not handle the paper, and it becomes a disposal liability. Composting could take the paper alone or with the crushed wallboard, and it would serve as a carbon source and a bulking agent to provide porosity to the pile. A degradation rate of the paper in the pile would allow for design of a mix ratio with recycled bulking agents.

For this study, torn sheets of the paper were weighed, put into net lifter bags, and buried in the piles. Several samples of the material were dried and weighed again to determine the average moisture content of the paper as it entered the compost. From this, a dry weight of paper at the start of the composting was calculated. After the composting was completed, the liner bags were removed, the paper was dried, and the samples weighed again. This gives the dry weight of the paper after composting. The difference between the dry weight at the beginning and the dry weight at the end was the mass degraded during composting.

Four samples were placed in the compost. Figure 13 shows the percent dry weight reduction for each of the four samples and the average. Reduction ranged from 25 - 60%, with an average of nearly 40%. As stated above, this data is for lifter bags which contained torn sheets of the paper (average of two inches by three inches). The majority of the paper in the mix was considerably smaller than this, after going through a four inch tree chipper. The product had very little paper left in it after composting, with the exception of these larger sheets.

process, and are a waste product. Some goes back into the manufacturing of new board, but the paper is still considered a disposal liability. The material is stacked in a criss-cross pattern on pallets, with no space between rows. In essence, the pallets hold a four by four by four foot solid block of gypsum wallboard. when this material is processed, it falls into a pile, and the chips and paper create space, increasing the volume of the material.

To determine this volumetric increase, a test was conducted to examine two different processing methods. Six piles (thirty strips each, four inches wide, one-half inch thick, four feet long) of gypsum wallboard were set aside. The starting volume of each stacked pile was measured. Each stack was 1.67 cubic feet, due to the regularity of the stacking. The stacks were processed in two different manners. Three piles were sent through the chipper, and three piles were scored with a shop knife and crushed by hand with a 20 pound sledge hammer. This method was designed to simulate the use of a loader to crush the board. A loader could run over short stacks of the material. The material was then placed in 5 gallon buckets and the volume measured by counting the buckets.

In both cases, an increase in volume was observed. Figure 14 shows the increase for each of the three samples and the average for the two processing methods. The hand crushed material showed a much greater volume increase (average 130%), as the result of having more large chunks of gypsum in the processed material. The material fed through the chipper had smaller particles, but contained some large pieces of paper, which created space in the volume. An average volume increase of about 40% was seen from the chipper material.

Considering the volume increase and the desire to chip materials with yard debris in order to minimize dust (moisture in the yard debris will help knock down a portion of the dust), a multiplier should be used to determine the proper volumetric mix ratio for the chosen processing method.  These multipliers are based on the dry weight mix ratios established in the pilot test.  Targeting a dry weight mix ratio with products having different bulk densities will require different volumetric ratios.  Table 10 shows these ratios and multipliers for the three mixes tested in the pilot study.  The multiplier is used to determine the volume of raw board which is needed to hit the proper mix of chipped material, yard materials, and biosolids.
Table 10 - Multipliers for Gypsum Board to Compensate for Volume Increase

                                                                                          Chipper Processing     Hand Processing

  Mix      Gypsum         Dry weight     Volume          Gyp. vol.       Volume          Gyp. Vol.

                Content          ratio                ratio             multiplier        ratio              multiplier

               %Volume      bs:yd:gyp       bs:yd:gyp                             bs:yd:gyp

   1            0%               1.0:0.0:2.1        1.0:3.0:0.0          0.72              1.0:3.0:0.0          0.43

   2          37.5%            1.0:1:1:2.9        1.0:1.5:1.5          0.72              1.0:1.5:2.5          0.43

   3          25%              1.0:1.4:2.2        1.0:2.0:1.0          0.72              1.0:2.0:1.7          0.43

    4            12.5%             1.0:1.8:1.2        1.0:2.5:0.5          0.72           1.0:2.5:0.8           0.43

The volume multiplier is used to determine the volume of unchipped board which should be used to get the desired dry weight ratio of gypsum in the mix. For gypsum processed with a chipper, a volumetric ratio of 1:2:1 is used. For every 1 cubic yard of biosolids (or other nitrogenous source) and 2 cubic yards of yard debris, 1 cubic yard of chipped gypsum is needed. Using the multiplier, this mix ratio would require 0.7 cubic yards of unchipped board. If materials are hand processed, due to the lower density, a volume of 1.7 cubic yards of processed gypsum will be needed for every cubic yard of biosolids and two cubic yards of yard debris. Using the multiplier associated with hand processing, the same volume of gypsum is required (0.7 cubic yards of unchipped board). This is as expected, since the dry weight mix ratios remain unchanged despite changing processing methods.

5.7   SCREENED FRACTION

Materials which passed through and over a 3/8" screen were weighed and examined to determine the fraction of gypsum for the finished product generated from each of the mixes. If screen overs are recycled back into the mix, this information will help determine the volume of fresh gypsum to add to achieve the desired dry weight ratio and final product pH. Assuming that the wallboard and the yard debris obtain similar moisture contents and bulk densities from the composting process, the volumetric fraction will be the same as the mass fraction. Table 11 shows the mass fraction of gypsum contained in the screen overs.

Table 11 - Mix Ratios With and Without Recycled Screen Overs

       Mix without recycle               Gypsum                           Mix with recycle

         biosolid:yard debris;              fraction                             biosolid:yard debris

        gypsum                                  of overs (%)                      gypsum; recyle         

            1:3:0                                      0                                      1:3:0:0

         1: 2.5 : 0.5                                     15                                        1:1.7 : 0.3 :1

                1: 2:1                                      38                                        1:1.4 : 0.6:1

                1:1.5 1.5                                 45                                        1: 0.9:1.1:1

The mix ratios which show recycle content are calculated considering the content of gypsum and yard debris fractions in the screen overs. The fresh gypsum and yard debris totals are reduced in proportion to these contents. If gypsum is used for an extended period of time, there may be a steady supply of gypsum-laden recycle, and the operator should use these mix ratios.

5.8      FINAL COMPOST PRODUCT QUALITY

5.8.1   Effect Of Gypsum On Soil

The effect of wallboard debris on compost quality is an important consideration if this material is to be used as a composting feedstock. In understanding what effect this material has on the final compost product quality, it is useful to review the use of gypsum as a soil amendment, as well as its use as a compost amendment in mushroom cultivation. Gypsum has been used in compost for growing mushrooms for some time, to add porosity for proper aeration.

In agriculture, gypsum is primarily amended to sodic and heavy clay soils to improve the physical properties. A sodic soil develops when excess levels of sodium displace other ions on colloidal and other ion exchange surfaces. This causes dispersion of the soil clay particles, which in turn can result in poor soil structure and reduced infiltration rates. When wet, sodic soils are slippery; after drying, a crust which resists water infiltration and seedling emergence is formed. The addition of gypsum to a sodic or heavy clay soil causes the aggregation of colloidal particles. Calcium present in gypsum also displaces the sodium ions on the cation exchange sites. Gypsum is added to soils and poffing mixes as a source of the plant nutrients calcium and sulfur. Although gypsum is 38 % calcium by weight, it is a poor liming agent and despite its alkaline pH, tends to act as a pH buffer when amended to soil.

Mushroom cultivation entails the relatively brief composting (<7 days) of straw and manure, followed by the inoculation of the compost with mushroom spawn. Gypsum is typically added to the initial compost for the following reasons:

·     To enhance the physical structure of the mix through the aggregation of colloidal particles, which improves aeration and water holding capacity.

·     To supply calcium necessary for mushroom growth.

·     To act as a buffer to keep the pH from becoming excessively basic, which in turn, limits ammonia volatilization.

Based on this discussion, the use of wallboard as a composting feedstock should enhance the quality of the final compost product. Analyses of the final compost products are presented and discussed in the following sections.

5.8.2   Compost Product Analyses

Compost product quality analyses are presented in Table 12. The control compost mix (no gypsum) characteristics are typical for a biosolids/yard debris compost. This product has a moderate to high amount of plant nutrients QJ:P:K = 2.2:1.1:0.2), a slightly acidic pH of 6.3, a and slightly elevated soluble salt content of 5.7 mrnhoslcm. The low volatile solids content (50 percent), low C:N ratio (13), high cation exchange capacity (50.1 milliequivalentsll00 g) and presence of nitrate, are all indicators that the control compost is stable and well decomposed. The compost was tested for C0₂ respiration rate as well, which indicates stability. All products were in the range which represents very stable compost.

In the soil or compost environment, calcium sulfate acts as a pH buffer. Calcium ions react as a base, neutralizing acids; sulfate ions react as an acid, neutralizing hydroxyl ions (OH-) and other bases. All three wallboard mixes have a slightly lower pH than the control mix, indicating the acid generating reaction of gypsum was greater than the base generating reaction. The pH of the wallboard mixes is still within a favorable range for plant growth. However, an important question that needs to be addressed is: "what is the effect of the wallboard on the compost pH over a longer curing and storage period?" It should be noted that wallboard will not have the same effect on pH when other feedstocks are used. All facilities seeking to use this material should conduct a pilot project to determine the effect of wallboard on the quality of their compost products.

An interesting trend is noted where the conductivity of the compost is reduced as the amount of wallboard in the mix is increased. This is due to the low solubility of calcium sulfate in water, as evidenced by its low solubility constant (K_sp = 1.9 x l0^-4). Only a small amount of the calcium sulfate added immediately dissociates into calcium and sulfate ions. Consequently, the addition of gypsum wallboard to the mix actually reduces the overall conductivity of the compost product. This effect that the wallboard has on the compost product would be very beneficial when the compost is amended to soils or growing media with an elevated salt content.

The visual appearance of a compost product is a very important characteristic with respect to consumer acceptance and marketability. The compost mixes containing 57% and 48% wallboard contain numerous white specks that are quite visible. Upon closer examination, it is obvious that the white specks are gypsum and not paper. The gypsum flecks are not very noticeable in the mix containing 30% wallboard. In many end-use applications, the presence of the white gypsum specks may not be acceptable.

The more physically aggressive processing typical of a full scale facility would tend to reduce the appearance of the white gypsum specks. In the pilot project, the wallboard was coarsely ground, and the compost was not handled, mixed, or moved very much. In a full scale composting operation, the wallboard would typically be ground finer using a tub grinder, hammermill, or similar piece of equipment. In addition, the compost would undergo more physical mixing through initial mixing, pile construction, windrow turning, screening and material handling, and transfer. The net effect in a full scale operation would be a better blending of the wallboard into the compost, reducing its visibility in the final product.

Based on an examination of the product quality characteristics, the use of wallboard in a biosolids/yard debris compost at a high addition rate of 58% (dry weight basis) is technically feasible. The product quality analyses suggest that the compost would not hinder or retard plant growth. However, plant growth trials should be conducted to confirm this. Final product appearance may limit the amount of wallboard that can be added to the initial compost mix. Depending on the actual compost process and the intended compost market, addition of wallboard at a rate greater than 30% may affect the acceptance and marketability of the product.

5.8.3    Product Use Recommendations

Based on the final product quality characteristics, the wallboard compost could be readily used in most compost end-use applications. The acidic pH of these specific compost products could limit its use in some applications. Again, plant growth trials need to be performed to establish the effect of this product on plant growth. As discussed previously, the physical appearance of compost containing wallboard could also limit end use applications.

The high gypsum content of the compost product would have an added value in some end-use applications. In particular, the compost would have an added value in agricultural and horticultural applications where gypsum is used or needed. For example, compost containing wallboard would be very desirable for producing manufactured topsoils where the primary constituent is a heavy textured, clay soil. The gypsum would aggregate the clay particles producing a topsoil with an improved physical structure. Another added value application is the use of the compost for reclaiming highly saline or sodic soils. Again, the gypsum would improve the soil structure. A compost containing wallboard would also be desirable in applications where sulfur and calcium are limiting. This type of compost product would also provide a substantial amount of calcium without raising the pH of the soil or growth medium it is amended to.

5.9       FIELD IMPLEMENTATION OBSERVATIONS

Cedar Grove Composting in Maple Valley, WA, recently brought a load of gypsum wallboard onsite for a brief trial to determine if the material would be suitable for use in their mix. The material was chipped with a Diamond Z grinder and mixed with yard debris at approximately a 1 to 1 volumetric ratio. In an effort to limit dust from the grinding operation, the wallboard material was loaded with the yard debris simultaneously. The moisture in the yard debris served to help knock down the dust generated from the grinding. Mixing the material on the ground with a front end loader before loading it into the grinder gave the best results. In addition, if the loader was kept full, the dust which escaped the process was limited to that which could escape from the discharge chute. Storage of materials on site should be done under cover (roof or tarp) to prevent the gypsum from becoming saturated with water. It is very difficult to handle and move when soaked with water.

5.9.1    Germination Results

A germination test was conducted in the Cedar Grove greenhouse at the composting facility. The greenhouse is built around an exhaust duct between the active compost piles and the odor control biofilter. The exhaust air from the compost process must be cooled slightly in order for the biofilter to perform properly, and the duct is designed to dissipate heat. The greenhouse uses this residual heat to grow plants for demonstration use of Cedar Grove Compost.

Six flats were set up with ten three-inch pots each. Each of the six flats contained a different soil mixture. The soil mixtures tested included mixes of finished compost, yard debris/gypsum compost, and stoneway cement solids. Two radish seeds were placed in each pot, and the germination rate for each soil mix was recorded. Table 13 shows the mixes and the seven day germination data.. The stoneway cement material consists of the solids which settle in a pond near the area that the trucks are cleaned at the Stoneway Cement Company's plant. The plant health was observed for several weeks after germination and no qualitative differences were seen.

Table 13: - Gypsum Compost Radish Germination After Seven Days

Soil mix                        Description                        Seeds germinated            % germination

2                 100% finished compost'                                       20                                100%

3                 l00%gyp/ydmix²                                                                   20                                100%

4                 50% fin comp/50% gyp/yd mix                             19                                 95%

5                 75% fin comp/25% stone solids³                                   11                                 55%

6                 90% fin comp/l0% stone solids                             19                                 95%

¹finished compost is defined as compost fresh off of the conveyor belt after screening

²gyp/yd mix is a 50/50 mix of yard debris and gypsum composted for 30 days

³stone solids are from Stoneway Cement Company

6.0  SUMMARY

The composting industry may wish to consider the use of gypsum wallboard to supplement the other bulking agents received at the site in times of low supply. For facilities which receive biosolids, it is important to have an adequate supply of bulking material in order to provide the necessary porosity, balance the carbon to nitrogen ratio to within the appropriate range (25:1 -35:1), and absorb the excess water present in the biosolids, which generally arrive on site between 15%-25% solids. The addition of a dry bulking agent will help hit the target range for the initial mix total solids (40%-50% solids) content. The gypsum wallboard, with its paper content, can provide all of these things. If a facility is regularly receiving high volumes of grass during one part of the season and does not have an adequate supply of woody bulking material to provide porosity, a mix supplemented with chipped wallboard may be an appropriate measure to help prevent the generation of odors. These benefits can be summarized as follows:

            Mix Parameter                                      Wallboard Benefit

1.   Porosity (bulk density)                       Low density; dilutes density of heavy feedstocks.

2.   C:N ratio                                          Adds carbon from paper.

3.   Moisture content                               Very dry; absorbs excess moisture in wet feedstocks.

In areas with large yard debris composting facilities, it may be difficult to obtain all the green bulking agents needed for a proper mix. Gypsum wallboard should be considered as a supplement to wood and yard debris. The conclusions of this report show no detrimental effects (aside from minor aesthetic issues) in the product or in the off gasses. In addition, the tip fees from the wallboard will bring revenue to the site, helping to ensure profitability. If yard debris and other woody material are not available, the site shortfall can be filled with wallboard, to the extent that the mix recipe will allow.

If an existing gypsum reuse option for new wallboard exists in the area close to a compost facility, it is likely that the scrap gypsum is not going to make it to the compost pile. It is likely,

though that the gypsum recycling plant is creating a disposal problem for themselves with all of the scrap paper stripped off of the old board scraps. This material could be incorporated into the compost pile, serving as a carbon source and a moisture absorber. As with the wallboard scrap, there may also be the benefit of tip fees generated from the paper alone.

The addition of either the crushed wallboard or the residual paper from other reuse options can be beneficial to the compost process. The addition will not adversely affect the process or the product quality, if used in the proper proportions. As with any feedstock, if used in proportions greater than those recommended, gypsum wallboard could diminish the quality of the product or the process balance to the point that the product either does not compost or is not suitable for reuse. With proper attention to the mix ratio, however, the skillful composter can successfully implement the addition of gypsum wallboard to a compost process.

The results of this study indicate that the addition of gypsum wallboard to a compost mix can be accomplished without hindering the end product quality. The pH of the finished compost was well within the acceptable range for end use in most situations. All of the mixes produced finished compost with a pH of approximately 6 (including the control). Each of the mixes met EPA pathogen reduction requirements. The only noticeable trend concerning temperatures was that the peak seemed to be delayed as the gypsum content increased. This was due to the displacement of some of the yard debris which had provided energy to the mix.

The odors generated from the mixes did not show discernible differences, with the exception of a higher production of ammonia from the control, which contained no gypsum wallboard. This showed that a properly operated compost facility which maintains aerobic conditions in its piles can successfully incorporate gypsum wallboard to a mix and not produce excessive odors, as might have been expected. Under anaerobic conditions, more sulfurous exhaust might be seen. This is an important point to recognize, since the issue of odor generation drives many decisions at compost facilities, including the incorporation of new feedstocks. A successful mix can be ruined by introducing a feedstock which upsets the balance of porosity, moisture content, and/or carbon to nitrogen ratio.

The aesthetics of the product is somewhat subjective, but the end product contained more visible gypsum (whitish chalky powder) as the ratio of wallboard in the initial mix was increased. Using the organic content of the end product to determine the initial mix may be a good way to determine what mix ratio is best for the needs of a particular facility. If a minimum organic matter percentage of 25% is desired, then a mix ratio of up to 1 part biosolids:1.5 yard debris:  1.5 part gypsum should be used. If an organic content of approximately 40% is desired, a mix ratio of 1 part biosolids : 2.5 parts yard debris : 0.5 parts gypsum should be used. Gypsum is recommended for use as an additive to compost for growing mushrooms. Gypsum (calcium sulfate) supplies the calcium necessary for mushroom metabolism (Stamets, Chilton).

This study shows that gypsum wallboard can be successfully incorporated into the composting process. Proper attention must be paid to ensure aerobic conditions are maintained (through aeration or mix porosity) in order to limit odors. In addition, dust must be controlled during the processing of the wallboard. If these two items are kept in check, the incorporation of this material can be successflilly achieved. The end use of the material is dependent upon the aesthetics and the desired organic content. Again, product end use, process control, and grinding optimization efforts will dictate the success of any gypsum wallboard compost project

.

7.0 ACKNOWLEDGMENT

ReTAP is a program of the Clean Washington Center, Washington State's lead agency for the market development of recycled materials. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. The MEP is a growing nationwide network of extension services to help smaller U.S. Manufacturers improve their performance and become more competitive. ReTAP is also sponsored by the U.S. Environmental Protection Agency and the American Plastics Council.

8.0  REFERENCES

California Fertilizer Association, Soil Improvement Committee. 1995. Western Fertilizer Handbook. Interstate Publishers, Inc. Danville, IL.

Devlin, R. M. 1966. Plant Physiology. Reinhold Publishing Corporation.

Epstein, E. 1997. The Science of Composting. Technomic Publishing Company, Inc. Lancaster, PA.

Gerrits, J. P. G. 1977. Gypsum, Mushroom Compost, and Ammonia Content. Neth. J.Agic. Sci. 25 (1977): 288-302.

Haug, R. T. 1993. The Practical Handbook of Composting. Lewis Publishers, CRC Press, Inc. Boca Raton, FL

Korcak, R. 1996. Scrap Construction Gypsum Utilization. USDA Agricultural Research Service. Beltsville, MD

Metcalf & Eddy, 1979. Wastewater Engineering - Treatment, Disposal, Reuse. McGraw-Hill

National Research Council, Board on Agriculture. 1989. Alternative Agriculture. National Academy Press. Washington, D. C.

Weust, P. Composting Techniques, Compost Ingredients and How to Supplement Compost. Penn State University

APPENDIX A

Mix Ratios

(Not included in this electronic report but available upon request)

Appendix B

Energy Spread Sheets

(Not included in this electronic report but available upon request)

APPENDIX C

Lab Data

(Not included in this electronic report but available upon request)
Appendix D

Testing Plan for Evaluating the Potential of Composting Gypsum Wallboard Scraps
Testing Plan for Evaluating the Potential

of Composting Gypsum Wallboard Scraps

DRAFT

December 10, 1996

Presented to:

The Clean Washington Center

2001 6th Avenue, Suite 2700

Seattle, WA 98121

Presented by:

E&A Environmental Consultants, Inc.

19110 Bothell Way NE, Suite 203

Bothell, WA 98011

1.0         Introduction

The wastewater treatment process at Renton Wastewater Treatment Plant (a King County facility) generates wastewater solids (20 % solids content) per month. These biosolids are currently reused in a variety of offsite reuse options, including land application for agriculture and silviculture (forest), and composting. The purpose of this project is to evaluate the potential for using composting as a means of recycling scrap gypsum wallboard generated in construction and demolition projects. Currently there are few reuse options for this material, which contains both gypsum and paper. There is an established market for the gypsum powder (in the production of new wall board), but the paper is not reused in the process. The paper would break down well in the composing process and serve as a source of carbon. This testing plan describes the test mixes and how the bin scale pilot project is to be Conducted.

1.1             Project Objectives

The primary objective of this project is to assess the feasibility of composting as a process for recycling gypsum wall board. Specific project objectives are summarized as follows.

1.    Evaluate process for:

      ·     breaking down gypsum and paper

      ·     reducing the volume of material to be disposed/utilized

      ·     final product calcium and sulfur content

      ·     final product soil salinity and pH

      ·     ammonia production in exhaust gas

2.    Develop recommendations for demonstration scale testing, including

      ·     most suitable bulking materials and initial mix ratios

      ·     appropriate detention time

      ·     aeration system sizing

      ·     process monitoring and testing requirements

      ·     wall board processing (grinding) to control dust

3.    Establish bulking materials and composting process control that provide the most effective breakdown of wallboard scraps with the best end product quality

4.    Develop the following information for developing a full scale conceptual design and cost

      estimate

      ·     mass balance

      ·     detention time

      ·     processing equipment needed

      ·     process control strategy

1.2 Project Overview

This project will entail the composting of four different mixes using wastewater solids and several different biosolids/bulking material/gypsum ratios in 21 cubic foot compostin2 bins. A cement mixer will be used to mix the bulking materials and wastewater solids. The mixes will be manually loaded into the bin composters.. The mixes will be composted for an eight week period in which temperature, oxygen and moisture will be maintained within optimum levels. During the eight week composting period process monitoring information will be collected. The composting period may be expanded or shortened, depending on the product stabilization. At the end of the composting process, the volume and weight of product will be determined. Jn addition, the product will be screened manually and the final product will be tested for several product quality parameters.

1.3         Project Responsibilities

Project responsibilities are defined as follows.

E&A Environmental Consultants. Inc.

·      Oversee bin setup, material mixing and bin loading

·      Oversee procurement of bulking materials needed for the bin scale operation

·      Provide the bin composters and process monitoring equipment

·      Provide training to Renton WWTP personnel on bin composter operation, process monitoring and data management

·      Conduct a minimum of four site visits to oversee operation

·      Oversee bin breakdown and screening

·      Transport the bins to the composting site

·      Collect and ship samples to appropriate laboratories as instructed in the testing plan.

·      Conduct all process monitoring and data entry as instructed by the testing plan

Renton WWTP'P has agreed to provide the following:

·      An area to protect the bins from the rain and sun and a gravel or paved surface for supporting the bins and mixing the feedstocks.

·      Utilities including single phase electrical power and water for moisture adjustment

·      Access to office space in construction trailer and a desk for placement of controller unit

2.0      Experimental Design

2.1         Mixes to Be Evaluated

2.1.1          Initial Mix Characteristics

The composting process begins with the development of an initial mix that has suitable characteristics to promote thermophilic composting. These initial mix characteristics are summarized in Table 1.

Table 1: Initial Mix Development Characteristics and Their Relevance

            Parameter                                           Relevance                                    Desired Condition / Adjustment

Porosity                           Needed for air distribution                 < 900 lb/cy initial mix bulk density

Moisture content              Provides moisture for microbes         <60% moisture (&>50%)

Available Carbon              Substrate for microbial growth          Generate pathogen reduction temps

      pH                               requlred for optimum                     6 to 7.5 preferred

                                          microbial growth

2.1.2          Bulking Material Discussion and Selection

In order to create an optimum initial mix, a bulking material needs to be added to the wastewater solids. Gypsum wallboard and yard debris will act as the bulking agent for this project. The bulking material is added to 1.) increase the solids content to a suitable range, 2.) increase the porosity of the initial mix and 3.) add energy (readily degradable carbon source) to the mix, if the wastewater solids provide an inadequate contribution of energy to the mix.

The composting of wastewater biosolids lids has been studied closely, and it is fairly will known how much energy the wastewater solids will contribute to the rnix. The fresh solids probably have a high energy content. The solids content of this material (approximately 20%) would result in an initial mix no additional water requirements. A full mix ratio analysis will be performed to determine the need for additional water.

There are numerous locally available materials that could potentially be used as a bullring agent. The ideal bulking material will have a solids content greater than 6G percent, provide enough energy to allow the maintenance of thermophilic conditions, provide structure and porosity to the mix and be readily available at a reasonable cost. The ideal particle size for a bulking material is dependent on several factors. In general, the coarser the bulking material, the more porosity and less available carbon provided to the mix. A coarse bulking material also typically needs to be screened to produce a product for sale. This can be advantageous as incorporation of the bulking material into the final product typically increases the processing period, the screen residuals can be reused as a bulking material, and there is smaller volume of final product.

                                 A goal in developing the recommended initial mixes was to test different bulking material ratios in order to evaluate the effects of adding different amounts of wallboard. The characteristics of several bulking materials are summarized as follows. Again, for this project, yard debris and gypsum wallboard will be used as bulking agents.

Table 2: Bulking Material Characteristics

Bulking Material        Solids         Particle       Bulk         Energy            Availability/

                                    Content         Size          Density  Content                   Cost

                                     (%)                (% <3/8")          lb/cy)

Medium bark                55 - 65        20 - 30            400        medium     very avail., $12 -

                                                                                                           $14/cy

Sawdust                       45 - 55          100                500        low            med avail., $7 - $10/cy

Wood shavings              70 - 80        70-80              300        low            limited avail., 7 -

                                                                                                           $10/cy

Yard debris                   50 - 60        70-80              500        high           med. avail., $3- $5/cy

Wood waste                 80 - 90        20 - 30            400        very low    med. avail., $3 - $5/cy

Gypsum Wallboard        80 - 85        20 - 30            400        very low    avail., likely no charge

Yard debris is a material that is readily available at a reasonable cost.

2.1.3    Recommended Initial Mixes

Based on the above discussion, recommended test evaluation mixes are presented in Tabl& 3. The table displays the volumetric ratios as well as the cubic feet of each feedstock necessary for each mix (which is 21 cubic feet total). A mass balance initial mix ratio for each of these mixes is presented in Appendix A.

Table 3: Recommended Mixes for the Bin Composting Project (volumetric ratios)

        Mix ID                   Biosolids                     Yard Debris                  Gypsum Board

                                    parts           ft³                                     parts                ft³                                     parts              ft³

Mix 1 - control                    1.0                   5.25                  3.0                     15.75                   0.0                    0.0

Mix 2                                    1.0                  5.25                    1.5                    10.55              1.5                    5.20

Mix3                                     1.0                   4.70                  2.0                     11.60                   1.0                    4.70

Mix 4                                    1.0                   5.00                  2.5                     14.80                   0.5                    1.20

total ft³                                                                                         21                                             47                                                   16

total gallons                                                  157                                          353                                                118

These mixes are designed based on the assumed percent solids of the bioso lids and the yard debris. An evaluation will be made in the field based on the condition (moisture content) of these materials, and the bulking ratios may be modified during mixing. Any changes will be noted and recorded for inclusion in the final report and calculation of compost characteristics.

2.2        Evaluation Criteria

Throughout the project, data will be collected for evaluating the different bulking materials and

the overall viability of composting. Evaluation criteria are summarized as follows.

·      gypsum content

·      paper degradation

·      volume and weight reduction

·      heat generation

·      energy generation

·      ammonia volatilization and sulfur gas generation

·         product quality

·      volatile solids reduction

   3.0    Composting Bin Operation

Detailed instructions for operating the bins are presented in this section.

3.1        Mixing and Bin Loading

Mixing the biosolids with the bulking agent (gypsum, yard debris, etc.) is the single most critical task in composting. Attention to detail is important to control and achieve proper mixing. The function of mixing is to intimately combine the biosolids and bulking agents to create a uniform, compostable mass. The ratio, as well as the method of combining the biosolids and bulking agent, will affect the physical properties of the mixture. The goal of mixing is to control the solids content of the mix and to create a mass that is sufficiently porous to allow air to flow through it. The mix must possess structural integrity sufficient to maintain porosity when built into the compost pile. In addition, mixing provides for the dispersal of the biosolids throughout the mass to expose maximum biosolids surface area to the microorganisms responsible for decomposition.

Mixing and bin loading will entail the following steps. This procedure may also be followed if a mix is to be remixed with additional feedstock during the project as a result of poor performance.

1.    Prepare the bin by opening the top, checking that the aeration pipe is in place, and placing a two inch layer of coarse woody material over the top of the aeration pipe

2.    Each feedstock material will be loaded into a nine cubic foot cement mixer by way of five gallon buckets according to the specified mix ratio.

3.    Each bucket will be weighed and recorded prior to loading

4.    Each batch mix will contain a maximum of 30 gallons

5.    Materials are loaded into the mixer in the following order: half of the bulking material, all of the wastewater solids, remainder of the bulking material

6.    Each batch is mixed until a homogenous mix is produced (approximately 5 to 8 minutes)

7.    The resulting mix is unloaded into a wheelbarrow and transported to the appropriate bin, where it is loaded manually into the bin through the top

8.    Put aside approximately one liter of each batch for the purpose of producing a compost sample for analysis.

9.    After the bin is full to within three inches of top (4 to 5 batches), place the top on the inner box, put the insulation in place, then put the top on the outside box

3.2       Process Control

Composting is a controlled biological process designed to rapidly convert waste organic material into a humus rich material that is useful for a variety of purposes associated with landscaping and growing plants. The controlled aspect allows the process to be completed efficiently. Process control requires that appropriate monitoring be undertaken and process adjustments be completed based on performance. The extent of monitoring and control for composting varies widely depending on the complexity of the composting method used and the degree of process optimization desired. Since the compost is a product that is utilized for plant grown and landscaping, the character of the final product is critical to successful marketing.

3.3.1    Process Monitoring

Process monitoring entails the regular collection of data pertinent to the composting process. In addition, the data should be examined to determine if and what process adjustments need to be made. Process control parameters and their relevance are summarized in Table 4.

Table 4: Composting Process Control Parameters and Their Relevance

  Parameter                    Relevance                                   Desired Condition/Adjustment

Porosity                          Maintain aerobic conditions                         Adjust by turning or remixing

Moisture Content            Microbial moisture requirement               · Add moisture to keep > 40%

                                            Reduce for efficient screening                     Reduce to 40 to 50% for screening

                                            Excess results in anaerobic                        · Adjust mix> dry bulk material

                                            conditions

Oxygen Content            · Aerobic conditions                                      · Adjust aeration to maintain oxygen at 16 %

Temperature                  · Pathogen reduction                                     · Satisfy time/temp requirements (3 days, 55~C)

                                            Weed seed destruction                                   Adjust aeration rate and frequency to

                                          · Control biological process                           maintain temperature between 40 and 50~C

                                          · Drying                                                           · Increase aeration to dry (if needed)

Odor                                Anaerobic conditions                                      Increase aeration or tuning frequency

                                            Improper mix                                                 · Adjust mix.

                                          · Biological process problems                     · Change composting temperatures

Decomposition Rt.     Determines processing time                            Adjust process conditions

                                                                                                                      Adjust initial mix

Visual / Qualitative      Experienced operators knows desired        · Supplements laboratory testing

                                          characteristics                                                · Use to adjust process

pH                                     Can inhibit biological process                      · Adjust mix or process

                                                                                                                      Add buffer or acid / base

In this project, process monitoring will entail the daily determination of temperature, the weekly determination of moisture content and the occasional determination of oxygen content. If these parameters fall outside of the levels presented in Table 4, process adjustments need to be made. Process monitoring methodology is presented in section 4. Process control adjustments are discussed as follows.

3.3.2    Temperature and Oxygen Control

Both temperature and oxygen are controlled by adjusting the volume of air provided to the composting mass, which in the bin composter are in turn controlled by adjusting the aeration rate and frequency. An increase in the amount aeration air reduces bin temperature and increases the oxygen concentration. Decreasing the volume of aeration air has the opposite effect on temperature and oxygen concentration. In the bin system, the provision of aeration air for temperature control, typically results in the maintenance of aerobic conditions, and aeration changes for increasing the oxygen concentration are typically not required.

Temperature Control Strategy

1.    In this project, the temperature of the bins will be maintained between 50 and 60⁰C.

2.    Initially, the aeration rate will be adjusted to maintain temperatures of 55⁰C for three consecutive days, to meet U.S. EPA pathogen reduction criteria.

3.    After this has been accomplished, aeration will be adjusted to maintain temperatures between 40 and 50⁰C, a level considered optimal for organic matter degradation.

4.    If necessary, aeration will be increased prior to bin breakdown and screening in order to reduce the moisture content to a suitable level for screening (38 to 45 %).

Aeration Control Strategy

The volume of aeration air provided to the bin composter can be controlled in the following two ways.

1.    Increasing the air flow rate by way of the rotameter (2 to 8.5 cfm)

2.    Increasing the aeration off time with the Compost Captain controller

The Compost Captain is a programmable logic computer desi2ned to control four aeration blowers and record temperature in four piles. The Compost Captain can be operated in the following two modes.

1.      Manual setting of the blower off time. In this mode, the on time is fixed at two minutes and the off time can be increased from a minimum of two minutes off to a maximum of 21 minutes off (2 mm on/2 mm off to 2 mm on/21 mm off)

2.      Time and temperature setting. This mode combines the manual setting of the blower off time with a temperature feedback setting. The temperature feedback dial on the Compost Captain is set for the maximum temperature desired. When the temperature rises above this set point, as determined by a temperature probe placed in the bin, the controller automatically starts the aeration blower. When the temperature falls below the set point, the blower is automatically turned off.

Specific operating instructions for the Compost Captain are presented as follows.

1.    Set the controller on the time and temperature setting.

2.    Until 55⁰C has been maintained for three consecutive days set the temperature feedback control at 60⁰C.

3.    After 55⁰C has been maintained for three consecutive days, set the temperature feedback at

50⁰C.

4.    Adjust the rotameter to deliver two cfm.

5.    Set the blower off time at 20 minutes.

6.    If the bin temperatures are continually above the target level, start decreasing the blower off time. If the temperatures are still above the target level, increase airflow by way of the rotameter.

7.    If the bin temperatures are below the target level, start decreasing airflow by way of the rotameter. When the airflow is reduced to two cfm, begin increasing the blower off-time.

8.    If bin temperatures are below the target level at the lowest aeration setting (2 cfm, 2 mm onI2O mm off), turn off the aeration blower.

9.    A goal to achieve in adjusting the rotameter and blower off time, is to have aeration provided as near continuously as possible.

10.   Record all aeration adjustments on the daily operational log.

3.3.4 Moisture Control

Moisture levels, which will be determined before and after the composting stage, are controlled

through the following three methods.

·      Adding an appropriate amount of bulking material to develop an initial mix with the desired moisture content

·      Adding water manually through the top of the bin

·      Increasing the airflow rate to enhance evaporation

Moisture Control Strategy

1.    The initial mix should have a moisture content between 58 and 62 percent

2.    During composting the moisture content should not drop below 45 percent until the last week of composting

3.    At the time of bin breakdown and screening the moisture content should be between 38 and

42 percent

Moisture Control Instructions

1.    Determine the moisture content and bulk density of the feedstocks prior to developing the initial mix. Use the mass balance spreadsheet to determine how much bulking material is needed to develop a mix that has a moisture content within the target range (58 to 62 percent).

2.    If the moisture content during composting declines below the lower process control limit of

45 percent (and composting is to continue at least seven additional days prior to screening), use the mass balance spreadsheet to determine how many gallons of water need to be added.

Add the water slowly through the top of the bin. Use the compost agitator tool to facilitate the distribution of water throughout the composting mass.

3.    If, one week prior to screening the moisture content is greater than 42 percent, increase the volume of aeration air provided to enhance evaporation. Removing the top off the bin will increase the rate of moisture loss.

3.4       Bin Breakdown and Screening

The time of bin breakdown will be based on several factors including moisture content and overall length of the project. Specific instructions for breaking down the bins and screening the compost are provided as follows.

1.    Place plastic sheeting on the ground in front of the bin.

2.    Open the top and side of the bin.

3.    Shovel the bin contents into a wheelbarrow.

4.    Collect a composite sample of the mix for field bulk density measurement and laboratory analyses.

5.    Manually pass the contents of the bin through a 3/8" screen.

6.    Record the volume of screen overs and unders.

7.    Determine the bulk density of screen overs and unders.

8.    Collect a composite sample of the screen overs and unders for laboratory analysis.

3.5       Summary of Equipment and Materials Needed

Equipment and supplies needed for the project are summarized in Table 5.

Table 5: Summary of Equipment and Materials Needed

                               Item                                           Quantity

Bin composters                                                              4

Aeration controller and temperature probes                      1

Screen (3/8")                                                                 1

Thermocouples                                                              4

Hand held digital thermometer                                         1

Wheelbarrow (6 cubic feet)                                            1

Cement mixer (9 cubic feet)                                           1

5 gallon buckets                                                             S

Fresh wastewater solids                                                 25 cf

Yard debris                                                                    57 cf

Gypsum Wallboard (crushed)                                          18 cf

Feenstock quantities assume a 15%  percent contingency

4.0         Process Monitoring and Sample Collection Methodology

4.1       Process Monitoring Schedule

The process monitoring schedule is summarized in Table 6.

Table 6: Process Monitoring Schedule Summary

                 Monitoring Parameter                                              Frequency

Bin temperature                                                  Daily

Intensive temperature monitoring                          Daily for first two weeks of process

Aeration rate and blower off time                         After every adjustment

Oxygen                                                              E&A site visits

Headloss                                                            Beginning and end of composting

Pile volume (height of mix)                                  Weekly

Sample collection                                                Weekly

Moisture content                                                 Weekly

Other compost analyses                                       beginning and end of process

4.2       Process Monitoring Methodology

Process monitoring methodology is described in Table 7.

Table 7: Process Monitoring Methodology

Monitoring                                                        Methodology

Parameter

Temperature      Read directly from the aeration controller/printed record

Intensive            One of the bins will be fitted with 12 thermocouples for intensive temperature

temperature        monitoring. Temperature is taken by plugging the thermocouple lead into the

                         hand held thermometer

Aeration rate      Read cfm directly off the rotameter

Aeration             Read blower off time directly from the aeration controller

frequency

Oxygen              Push probe into the middle of the composting mass through the hole in the top

concentration      of the bin. Connect air pump and oxygen sensor to probe. Start pump and read

                         level from oxygen meter.

Bulk density       Fill a 5 gallon bucket to the top with the desired material. Drop the bucket

                         from a height of 4 inches 3 times. Refill the bucket to ihe top. Weight the

                         bucket. Be sure to tare the scale or subtract the weight of the bucket.

Headloss            Connect magnehelic gauge to barb on ingoing aeration pipe. Read headloss

                         from magnehelic gauge

Pile volume        Measure distance from top of bin to top of composting mass from each side of

                         the bin. Volume is calculated in a spreadsheet based on this measurement

Sample               Remove the top of the inner and outer bins. Using a pitchfork or shovel dig a

collection            hole 8 to 12 inches into the composting mass. Collect a sample from within

                         this hole.

4.3       Data Recording

With the exception of the intensive temperature monitoring data, all data will be recorded on a spreadsheet form that is identical to the day-to-day monitoring schedule any operational activities that are conducted, i.e. water addition, are to be recorded on this form. A separate form is to be kept for each mix.

4.4       Product Testing

In order to determine the difference between the end products derived from each mix, the compost will be tested for several parameters. The addition of the gypsum is exected to have an effect on the pH and the levels of calcium and sulfur, since the wallboard is typically 92% calcium sulfate ore. The product will be tested for:

·    calcium, sulfur, pH

·    other nutrients

-total kjehldal nitrogen

-nitrate nitrogen

-ammonium nitrogen

-phosphorus

-potassium

-magnesium

- zinc

·    cation exchange capacity

·    soluble salts

·    volatile solids

·    compost stability (C0₂ respiration rate)

·    bulk density

·    sieve analysis

·    total solids, volatile solids and bulk density of input feedstocks

·        Lead & mercury

APPENDIX E

Bin Schematic Drawing