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 r