Evaluation ofthe Potential for Composting

Gypsum Wallboard Scraps

Evaluation of the Potential for

Composting Gypsum Wallboard Scraps

Final Report

Prepared for

The Recycling Technology Assistance Partnership (ReTAP)A program for the Clean Washington Center

A division of the Pacific NorthWest Economic Region (PNWER)2200 Alaskan Way, Suite 460

Seattle, WA 98121

June 25, 1997

Prepared by:

E&A Environmental Consultants, Inc.19110 Bothell Way NE, Suite 203

Bothell, Washington 98011

This recycled paper is recyclable

Copyright © 1997 CLEAN WASHINGTON CENTER. All rights reserved. Federal copyright lawsprohibit reproduction, in whole or in part, in any printed, mechanical, electronic, film or other distributionand storage media, without the written consent of the Clean Washington Center. To write or call forpermission: CWC Alaskan Way, Suite 460, Seattle, Washington 98121 (206) 443-7746.

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 ............................................................................................................................................. 82.1.1 Initial Mix Characteristics................................................................................................................................. 82.1.2 Bulking Material Discussion and Selection..................................................................................................... 82.1.3 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..........................................................................................................................................133.2.1 Process Monitoring .........................................................................................................................................143.2.2 Temperature and Oxygen Control ...................................................................................................................153.2.4 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.......................................................................................................445.8.1 Effect Of Gypsum On Soil .............................................................................................................................445.8.2 Compost Product Analyses ............................................................................................................................455.8.3 Product Use Recommendations.....................................................................................................................48

5.9 FIELD IMPLEMENTATION OBSERVATIONS.............................................................................................495.9.1 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 availableupon request

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

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

Appendix D: Test PlanAppendix E: Bin Schematic Drawing

List of Tables

Table l Initial Mix Development Characteristics and Their RelevanceTable 2 Renton WWTP Biosolids CharacteristicsTable 3 Bulking Material CharacteristicsTable 4 Recommended Mixes for the Bin Composting Project (Volumetric Ratios)Table 5 Composting Process Control Parameters and Their RelevanceTable 6 Summary of Equipment and Materials NeededTable 7 Process Monitoring Schedule SummaryTable 8 Process Monitoring MethodologyTable 9 Volumetric Mix RatiosTable 10 Multipliers for Gypsum Board to Compensate for Volume IncreaseTable 11 Mix Ratios With and Without Recycled Screen OversTable 12 Compost Product Quality AnalysesTable 13 Gypsum Compost Radish Germination After 7 Days

List of Figures

Figure l Temperature Profile for Mixes 1, 2, 3, and 4 - Trial 1Figure 2 Temperature Profile for Mix 3 & 4 - Trial 2Figure 3 Temperature Profiles for Successful Mixes - 1 & 2 from Trial 1 and 3 and 4

from Trial 2Figure 4 Mix Total Energy GenerationFigure 5 BTU's/CY/DayFigure 6 Airflow for Trial 1 MixesFigure 7 Trial 1 Oxygen ContentsFigure 8 Airflow for Trial 2 MixesFigure 9 Trial 2 - Oxygen ContentFigure 10 Trial 1 Exhaust Ammonia Generation RatesFigure 11 Trial 1 Exhaust Dimethyl Sulfide ContentFigure 12 Trial 2 Exhaust Dimethyl SulfideFigure 13 Gypsum Wallboard Paper ReductionFigure 14 Volume Increase Due to Gypsum Processing

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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.

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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

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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.

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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.

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• 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

6

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.

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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.

Table 1: Initial Mix Development Characteristics and Their RelevanceParameter Relevance Desired Condition/Adjustment

Porosity Needed for air distribution <1200 lb/cy initial mix bulk densityMoisture content Provides moisture for microbes < 60% moisture (&>50%)Available carbon Substrate for microbial growth Generate pathogen reduction tempsNutrient content Needed for microbial growth C:N ratio near 30:1 preferredpH Required for optimum microbial

growth6 to 7.5 preferred

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

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(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 CharacteristicsParameter Level

From belt presses:total solids (%) 21.8%

total volatile solids (%) 60.2%From digesters:

volatile acids (mg/l) 39alkalinity (mg/l) 8840

pH 7.8

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 - 167o 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.

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Table 3: Bulking Material CharacteristicsBulking Material Solids

Content(%)

ParticleSize(% <3/8”)

BulkDensity(lb/cy)

EnergyContent

Availability/Cost

Medium bark 55 - 65 20 - 30 400 medium very avail., $12 - $14/cySawdust 45 - 55 100 500 Low med avail., $7 - $10/cyWood shavings 70 - 80 70 -80 300 Low limited avail.,$7 - $10/cyYard debris 50 - 60 70 -80 500 High med. avail., $3 - $5/cyWood waste 80 - 90 20 - 30 400 Very low med. avail., $3 - $5/cyGypsum Wallboard 80 - 85 20 - 30 400 Very low avail., likely no charge

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.

Table 4: 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 0.0Mix 2 1.0 5.25 1.5 10.55 1.5 5.20Mix 3 1.0 4.70 2.0 11.60 1.0 4.70Mix 4 1.0 5.00 2.5 14.80 0.5 1.20total 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.

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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

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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.

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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.

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Table 5: Composting Process Control Parameters and Their RelevanceParameter Relevance Desired Condition/Adjustment

Porosity • Maintain aerobic conditions • Adjust by turning or remixingMoisture Content • Microbial moisture requirement

• Reduce for efficient screening• Excess results in anaerobic

conditions

• Add moisture to keep > 40%• Reduce to 40 to 50% for screening• Adjust mix > dry bulk material

Oxygen Content • Aerobic conditions • Adjust aeration to maintain oxygen at16%

Temperature • Pathogen reduction• Weed seed destruction• Control biological process • Drying• Vector Attraction Reduction

• Satisfy time/temp requirements (3 days,55oC)

• Adjust aeration rate and frequency to maintain thermophilic temperatures• Increase aeration to dry (if needed)• Satisfy time/temp. requirements (15 days

avg. 400C)Odor • Anaerobic conditions

• Improper mix• Biological process problems

• Increase aeration or turning frequency• Adjust mix• Change composting temperatures

DecompositionRate

• Determines processing time • Adjust process conditions• Adjust initial mix

Visual / Qualitative • Experienced operators knowdesired characteristics

• Supplements laboratory testing• Use to adjust process

pH • Can inhibit biological process • Adjust mix or process• Add buffer or acid / base

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

14

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 50oC and 60oC.

2. Initially, the aeration rate was be adjusted to maintain temperatures of 55oC 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 40oC and 50oC, 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

15

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 60oC until 55oC had been maintained for

three consecutive days.

3. The temperature feedback was set at 50oC after 55oC 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:

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• 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.

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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.

Table 6: Summary of Equipment and Materials NeededItem Quantity

Bin composters 4Aeration controller and temperature probes 1Screen (3/8”) 1Thermocouples 4Hand-held digital thermometer 1Wheelbarrow (6 cubic feet) 1Cement mixer (9 cubic feet) 15 gallon buckets 8Fresh wastewater solids 25 cfYard debris 57 cfGypsum Wallboard (crushed) 18 cfFeedstock quantities assume a 15% contingency

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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.

Table 7: Process Monitoring Schedule SummaryMonitoring Parameter Frequency

Bin temperature DailyIntensive temperature monitoring Daily for first two weeks of processAeration rate and blower off time After every adjustmentOxygen E&A site visitsHeadloss Beginning and end of compostingPile volume (height of mix) Beginning and end of compostingSample collection Beginning and end of compostingMoisture content Beginning and end of compostingOther compost analyses Beginning and end of composting

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4.2 PROCESS MONITORING METHODOLOGY

Process monitoring methodology is described in Table 8.

Table 8: Process Monitoring MethodologyMonitoringParameter

Methodology

Temperature Read directly from the aeration controller/printed record.Aeration rate Read cfm directly off the rotameter.Aerationfrequency

Read blower off time directly from the aeration controller.

Oxygenconcentration

Push probe into the middle of the composting mass through the hole in the top ofthe bin. Connect air pump and oxygen sensor to probe. Start pump and readlevel from oxygen meter.

Bulk density Fill a 5 gallon bucket to the top with the desired material. Drop the bucket from aheight of 4 inches 3 times. Refill the bucket to the top. Weigh the bucket. Besure to tare the scale or subtract the weight of the bucket.

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

Pile volume Measure distance from top of bin to top of composting mass from each side ofthe bin. Volume is calculated in a spreadsheet based on this measurement.

Samplecollection

Remove the top of the inner and outer bins. Using a pitchfork or shovel dig ahole 8 to 12 inches into the composting mass. Collect a sample from within thishole.

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

20

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.

21

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 RatiosMix ID Biosolids Yard Debris Gypsum Board Gypsum

Contentparts parts parts % volume

Mix 1 - control 1.0 3.0 0.0 0%Mix 2 1.0 1.5 1.5 37.5%Mix 3 1.0 2.0 1.0 25%Mix 4 1.0 2.5 0.5 12.5%

22

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

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

-10

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

process day

tem

per

atu

re (

o C) mix 1 (0%)

mix 2 (37.5%)

mix 3 (25%)

mix 4 (12.5%)

PFRPambient

23

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).

24

Figure 2

Temperature Profile for Mixes 3 & 4, Trial 2

-10

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

process day

tem

per

atu

re (o

C)

mix 3 (25%)

mix 4 (12.5%)

PFRP

ambient

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

25

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

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

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

process day

tem

per

atu

re (

oC

)

mix 1 (0%)

mix 2 (37.5%)

mix 3 (25%)mix 4 (12.5%)

PFRP 55

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

26

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.

27

Figure 4

Mix Total Energy Generation

0

50,000

100,000

150,000

200,000

250,000

mix 1, trial 1 mix 2, trial 1 mix 3, trial 1 mix 4, trial 1 mix 3, trial 2 mix 4, trial 2

mix

ener

gy

(BT

U's

)

BTU's to heat mass

BTU generation

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:

28

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.

Figure 5BTU's/cy/day

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

mix 1, trial 1 mix 2, trial 1 mix 3, trial 1 mix 4, trial 1 mix 3, trial 2 mix 4, trial 2

mix

mix

en

erg

y (B

TU

's)

BTU's/cy/d

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

29

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.

30

Figure 6

Airflow for Trial 1 Mixes

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

process day

airf

low

(cf

/hr/

dt)

mix 1 (0%)

mix 2 (37.5%)

mix 3 (25%)

mix 4 (12.5%)

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.

31

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

Trial 1 Oxygen Contents

0

2

4

6

8

10

12

14

16

18

20

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

process day

per

cen

t mix 1 (0%)mix 2 (37.5%)

mix 3 (25%)

mix 4 (12.5%)

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.

Figure 8

32

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.

33

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.

34

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.

35

Figure 11 Trial 1 Exhaust Dimethyl Sulfide Content

36

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.

37

38

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.

39

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.

40

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.

41

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

42

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.

43

44

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 (Ksp = 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

45

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

46

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

47

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 screening2gyp/yd mix is a 50/50 mix of yard debris and gypsum composted for 30 days3stone solids are from Stoneway Cement Company

48

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,

49

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

50

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

51

.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.

52

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 Center2001 6th Avenue, Suite 2700

Seattle, WA 98121

Presented by:E&A Environmental Consultants, Inc.

19110 Bothell Way NE, Suite 203Bothell, 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 reusedin 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 usingcomposting as a means of recycling scrap gypsum wallboard generated in construction anddemolition projects. Currently there are few reuse options for this material, which contains bothgypsum and paper. There is an established market for the gypsum powder (in the production ofnew wall board), but the paper is not reused in the process. The paper would break down well inthe composing process and serve as a source of carbon. This testing plan describes the test mixesand 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 forrecycling 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 effectivebreakdown of wallboard scraps with the best end product quality

4. Develop the following information for developing a full scale conceptual design and costestimate• 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 severaldifferent biosolids/bulking material/gypsum ratios in 21 cubic foot compostin2 bins. A cement mixerwill be used to mix the bulking materials and wastewater solids. The mixes will be manually loadedinto the bin composters.. The mixes will be composted for an eight week period in whichtemperature, oxygen and moisture will be maintained within optimum levels. During the eight weekcomposting period process monitoring information will be collected. The composting period may beexpanded or shortened, depending on the product stabilization. At the end of the compostingprocess, the volume and weight of product will be determined. Jn addition, the product will bescreened 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 suitablecharacteristics to promote thermophilic composting. These initial mix characteristics are summarizedin Table 1.

Table 1: Initial Mix Development Characteristics and Their Relevance

Parameter Relevance Desired Condition / AdjustmentPorosity Needed for air distribution < 900 lb/cy initial mix bulk densityMoisture content Provides moisture for microbes <60% moisture (&>50%)Available Carbon Substrate for microbial growth Generate pathogen reductiontemps

pH requlred for optimum 6 to 7.5 preferredmicrobial 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 wastewatersolids. Gypsum wallboard and yard debris will act as the bulking agent for this project. The bulkingmaterial is added to 1.) increase the solids content to a suitable range, 2.) increase the porosity ofthe initial mix and 3.) add energy (readily degradable carbon source) to the mix, if the wastewatersolids provide an inadequate contribution of energy to the mix.

The composting of wastewater biosolids lids has been studied closely, and it is fairly will knownhow much energy the wastewater solids will contribute to the rnix. The fresh solids probably have ahigh energy content. The solids content of this material (approximately 20%) would result in aninitial mix no additional water requirements. A full mix ratio analysis will be performed to determinethe 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 energyto allow the maintenance of thermophilic conditions, provide structure and porosity to the mix andbe readily available at a reasonable cost. The ideal particle size for a bulking material is dependenton several factors. In general, the coarser the bulking material, the more porosity and less availablecarbon provided to the mix. A coarse bulking material also typically needs to be screened toproduce a product for sale. This can be advantageous as incorporation of the bulking material intothe final product typically increases the processing period, the screen residuals can be reused as abulking material, and there is smaller volume of final product.

A goal in developing the recommended initial mixes was to test differentbulking material ratios in order to evaluate the effects of adding differentamounts of wallboard. The characteristics of several bulking materials aresummarized as follows. Again, for this project, yard debris and gypsumwallboard 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/cyWood shavings 70 - 80 70-80 300 low limited avail., 7 -

$10/cyYard debris 50 - 60 70-80 500 high med. avail., $3- $5/cyWood waste 80 - 90 20 - 30 400 very low med. avail., $3 - $5/cyGypsum 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 tabledisplays the volumetric ratios as well as the cubic feet of eachfeedstock necessary for each mix (which is 21 cubic feet total). Amass balance initial mix ratio for each of these mixes is presentedin Appendix A.

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

Mix ID Biosolids Yard Debris Gypsum Boardparts ft3 parts ft3 parts ft3

Mix 1 - control 1.0 5.25 3.0 15.75 0.0 0.0Mix 2 1.0 5.25 1.5 10.55 1.5 5.20Mix3 1.0 4.70 2.0 11.60 1.0 4.70Mix 4 1.0 5.00 2.5 14.80 0.5 1.20total ft3 21 47 16total gallons 157 353 118

These mixes are designed based on the assumed percent solids of the bioso lids and the yard debris. Anevaluation will be made in the field based on the condition(moisture content) of these materials, and the bulking ratios may bemodified during mixing. Any changes will be noted and recordedfor inclusion in the final report and calculation of compostcharacteristics.

2.2 Evaluation Criteria

Throughout the project, data will be collected for evaluating the different bulking materials andthe 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 taskin composting. Attention to detail is important to control and achieve proper mixing. The function ofmixing is to intimately combine the biosolids and bulking agents to create a uniform, compostablemass. The ratio, as well as the method of combining the biosolids and bulking agent, will affect thephysical properties of the mixture. The goal of mixing is to control the solids content of the mix andto create a mass that is sufficiently porous to allow air to flow through it. The mix must possessstructural 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 biosolidssurface area to the microorganisms responsible for decomposition.

Mixing and bin loading will entail the following steps. This procedure may also be followed if a mixis 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 atwo 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 fivegallon buckets according to the specified mix ratio.

3. Each bucket will be weighed and recorded prior to loading4. Each batch mix will contain a maximum of 30 gallons5. Materials are loaded into the mixer in the following order: half of the bulking material, all of the

wastewater solids, remainder of the bulking material6. 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 top8. 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 intoa humus rich material that is useful for a variety of purposes associated with landscaping andgrowing plants. The controlled aspect allows the process to be completed efficiently. Processcontrol requires that appropriate monitoring be undertaken and process adjustments be completedbased on performance. The extent of monitoring and control for composting varies widelydepending on the complexity of the composting method used and the degree of process optimizationdesired. Since the compost is a product that is utilized for plant grown and landscaping, thecharacter 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. Inaddition, the data should be examined to determine if and what process adjustments need to bemade. 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 remixingMoisture 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 oxygenat 16 %Temperature •• Pathogen reduction •• Satisfy time/temp requirements (3days, 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 tuningfrequency

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 testingcharacteristics •• Use to adjust process

pH Can inhibit biological process •• Adjust mix or process

Add buffer or acid / baseIn this project, process monitoring will entail the daily determination of temperature, theweekly determination of moisture content and the occasional determination of oxygencontent. If these parameters fall outside of the levels presented in Table 4, processadjustments 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 thecomposting mass, which in the bin composter are in turn controlled by adjusting the aeration rateand frequency. An increase in the amount aeration air reduces bin temperature and increases theoxygen concentration. Decreasing the volume of aeration air has the opposite effect on temperatureand 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 theoxygen concentration are typically not required.

Temperature Control Strategy1. 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 StrategyThe volume of aeration air provided to the bin composter can be controlled in the following twoways.

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 blowersand record temperature in four piles. The Compost Captain can be operated in the following twomodes.

1. Manual setting of the blower off time. In this mode, the on time is fixed at two minutes andthe off time can be increased from a minimum of two minutes off to a maximum of 21 minutesoff (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 timewith a temperature feedback setting. The temperature feedback dial on the Compost Captain isset for the maximum temperature desired. When the temperature rises above this set point, asdetermined by a temperature probe placed in the bin, the controller automatically starts theaeration blower. When the temperature falls below the set point, the blower is automaticallyturned 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 therotameter.

7. If the bin temperatures are below the target level, start decreasing airflow by way of therotameter. 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 mmonI2O mm off), turn off the aeration blower.

9. A goal to achieve in adjusting the rotameter and blower off time, is to have aeration providedas 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 controlledthrough 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 Strategy1. The initial mix should have a moisture content between 58 and 62 percent2. During composting the moisture content should not drop below 45 percent until the last week

of composting3. At the time of bin breakdown and screening the moisture content should be between 38 and

42 percent

Moisture Control Instructions1. 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 isneeded 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 of45 percent (and composting is to continue at least seven additional days prior to screening), usethe 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 thedistribution of water throughout the composting mass.

3. If, one week prior to screening the moisture content is greater than 42 percent, increase thevolume of aeration air provided to enhance evaporation. Removing the top off the bin willincrease 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 overalllength of the project. Specific instructions for breaking down the bins and screening the compost areprovided 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 NeededItem Quantity

Bin composters 4Aeration controller and temperature probes 1Screen (3/8") 1Thermocouples 4Hand held digital thermometer 1Wheelbarrow (6 cubic feet) 1Cement mixer (9 cubic feet) 15 gallon buckets SFresh wastewater solids 25 cfYard debris 57 cfGypsum Wallboard (crushed) 18 cfFeenstock 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 FrequencyBin temperature DailyIntensive temperature monitoring Daily for first two weeks of processAeration rate and blower off time After every adjustmentOxygen E&A site visitsHeadloss Beginning and end of compostingPile volume (height of mix) WeeklySample collection WeeklyMoisture content WeeklyOther 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 MethodologyParameterTemperature Read directly from the aeration controller/printed recordIntensive One of the bins will be fitted with 12 thermocouples for intensive temperaturetemperature monitoring. Temperature is taken by plugging the thermocouple lead into the

hand held thermometerAeration rate Read cfm directly off the rotameterAeration Read blower off time directly from the aeration controllerfrequencyOxygen Push probe into the middle of the composting mass through the hole in the topconcentration 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 thebucket. Be sure to tare the scale or subtract the weight of the bucket.

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

Pile volume Measure distance from top of bin to top of composting mass from each side ofthe 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 acollection 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 recordedon a spreadsheet form that is identical to the day-to-day monitoring schedule anyoperational activities that are conducted, i.e. water addition, are to be recorded onthis 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, thecompost will be tested for several parameters. The addition of the gypsum is exected tohave an effect on the pH and the levels of calcium and sulfur, since the wallboard istypically 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