PILOT-SCALE TESTING OF THE AERATED STATIC PILE COMPOSTING PROCESS

Ellen B. Huffman Charlotte-Mecldenburg Utility Department

Charlotte, North Carolina

James E. Shelton, Ph.D. Department of Soil Science

North Carolina State University Mountain Horticulture Crops and Research Center

Fletcher, North Carolina

Julia D. Bellamy Black & Veatch

Charlotte, North Carolina

INTRODUCTION

The Charlotte-Mecklenburg Utility Department (CMUD) provides water and wastewater services to a population of approximately 450,000 in the city of Charlotte (North Carolina) and a major portion of Mecklenburg County. As a result of growth and urbanization in the Charlotte-Mecklenburg area and the promulgation of the federal sewage sludge regulations (40 CFR Part 503), CMUD desired to diversify its biosolids management program to meet long-term requirements. A diversified management program allows for increased reliability and flexibility of an overall residuals management program. Aerated static pile composting was selected as a long-term biosolids management alternative, along with alkaline stabilization and land application, as part of CMUD’s efforts to develop a multi-faceted biosolids management program. This decision led to the implementation of a pilot-scale compost demonstration project (Pilot Project).

Composting is a biological stabilization process that under controlled aerobic conditions decomposes the organic constituents of the biosolids. The heat produced by the biological activity during composting destroys any pathogens present in the bjosolids. The stabilization process also minimizes odors so that the product can be readily stored and marketed. The resulting final composting product is a stable, humus-like material that is suitable for use as a soil amendment and a low-nutrient fertilizer.

In addition to the Pilot Project, CMUD is participating in a research project conducted by The Water Resources Research Institute (WRRI) of the University of North Carolina. The research project involves evaluating the chemical and physical properties of the compost product to determine product consistency. Potential end uses of the composted biosolids are also behg evaluated for various agronomic and horticultural crops. Studies include the evaluation of the impacts of composted biosolids on plant growth, metds uptake by plants, nitrate and metals leaching potential, and nitrogen mineralization rates.

PURPOSE AND GOALS OF PILOT PROJECT

The purpose of the Pilot Project was to determine process operating parameters, alternative composting design mixes, and to generate product that could be used for research and demonstration projects. The objectives and goals of the Pilot Project are as follows:

To become familiar with aerated static pile compost operations and equipment. To collect operations data for the evaluation of the compost process and the impacts of various design mixes on process performance.

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To determine product characteristics and evaluate if the product meets "exceptional quality sludge" criteria established in the Part 503 sewage sludge regulations. To determine the effects of process variables on end product quality. To determine the suitability of the finished product for various compost end uses.

REGULATORY REQUIREMENTS

Composting of biosolids with subsequent use in land application, including distribution and marketing, is regulated by federal regulations contained in 40 CFR Part 503: Standards for the Use and Disposal of Sewage Sludge. These regulations address pathogen reduction criteria, vector attraction reduction (VAR) criteria, pollutant concentrations, and management practices.

Composted biosolids utilized for distribution and marketing purposes are required to meet the Class A pathogen reduction criteria established in the Part 503 regulations. Composted biosolids which meet the pollutant concentration limits, Class A pathogen rcduction, and VAR requirements are considered "exceptional quality (Ea)." These composted biosolids will have less recordkeeping and monitoring requirements and do not have to meet annual or cumulative loading limits.

For a composted biosolids product to meet the Class A pathogen reduction requirements in the Part 503 regulations, the product must meet the fecal coliform density limit of 1,000 MPN/g TS (or Salmonella sp. less than 3 MPN/4 g TS). In addition, the composting process must meet the criteria for a Process to Further Reduce Pathogens (PFRP). For an aerated static pile composting process to be classified as a PFRP, the temperature of the biosolids should be maintained at 55°C or higher for a minimum of three days. The VAR requirements for composting requires that biosolids be treated in an aerobic process for a minimum of 14 days during which time the temperature of the biosolids is above 40°C and the average temperature of the biosolids is higher than 45°C.

North Carolina General Statute 143-251.1 and 15A NCAC 2H.200 require that biosolids removed in the course of treatment from a permitted wastewater treatment facility must be disposed of in a manner authorized by the Division of Environmental Management (DEM). Approval from DEM was obtained for the operations of the Pilot Project, with approved material usage for research purposes only. Subsequently, CMUD obtained a distribution and marketing permit from DEM for alkaline stabilized biosolids and composted biosolids in May 1994. Any composted biosolids generated during the Pilot Project in accordance with this permit may be distributed for use.

PROCESS DESCRIPTION

Compost Operations. The Pilot Project was located at a covered storage area at CMUD's Sugar Creek Wastewater Treatmcnt Plant (WWTP). The covered storage area has a concrete pad which has approximately 4,000 square feet of work space. No new construction was required to accommodate the Pilot Project. Advantages of locating the Pilot Project at the Sugar Creek W P ' s covered storage area included a firm work surface for the operation of mobile equipment and protection of materials and operations from the elements. The Pilot Project primarily utilized dewatered biosolids cake from the CMUD's McAlpinc Creek Wastewater Management Facility (WWMF), because this will be the primary biosolids source for CMUD's full-scale aerated static pile compost facility. The centrifuge dewatered cake (approximately 18 to 20% TS) was hauled by truck to the covered storage area at the Sugar Creek WWTP.

The Pilot Project consisted of ten composting cycles. Variations between cycles included the investigation of different bulking agents, amendments, and biosolids sources. Each cycle of the Pilot Project involved the construction of two discrete pilot-scale compost piles (approximately 40 cubic yards (cy) each). Pile construction for each cycle involved the delivery of raw materials (biosolids, bulking agent, and amendment), mixing of raw materials with a cattle feed mixer, and construction of the two discrete piles. The required volume of biosolids, bulking agent, and amendment were transported by front-end loaders to the cattle feed mixer for mixing of the

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feed constituents. The mix of feed constituents was then placed on a one-foot layer of wood chips, which covered perforated piping for aeration purposes. Cover material, consisting of recycled compost, was then placed over each pile for insulation.

Aeration is required to maintain aerobic conditions for the biological processes, to remove heat produced by biological activity, and to remove water. The aeration rate is highly dependent on the type of feed stock constituents being composted and the temperatures encountered. Aeration requirements are highly site-specific and are dependent on the ratio of feedstock constituents. Aeration during each cycle of the Pilot Project was monitored closely to ensure that the piles received a sufficient amount of oxygen necessary for biological activity and to ensure that the piles are not overly aerated.

After the material had composted for 28 days, the piles were broken down and relocated for nonaerated curing. Due to space restrictions at the covered storage area, the compost piles were cured for 30 days in an unsheltered area. When storage of the product was required, the compost product was stored in an unsheltered area adjacent to the curing piles.

Process Monitoring. A comprehensive monitoring program was conducted to collect data on various physical, chemical, and biological parameters. The monitoring program consisted of three parts: (1) raw material testing, (2) routine process control testing, and (3) fmal product testing. Process control parameters were monitored in order to optimize performance and to produce consistent product that meets federal and state regulations.

The total solids (TS) and volatile solids (VS) content of the feedstocks are important factors that affect the composting process. Prior to pile construction for each cycle, a mass/energy balance was computed to determine the quantity of amendment necessary to provide a sufficient amount of biodegradable VS to facilitate composting. Input parameters to the madenergy balance included biosolids quantity, bulking agent quantity, measured TS and VS content of feedstock components, assumed biodegradable VS content of feedstock components, and the assumed energy content of feedstock components. An initial mix between 38 and 40% TS was targeted for each cycle of the Pilot Project. Composite samples of all compost feedstock constituents were collected for analysis of TS and VS content prior to pile construction.

Throughout the composting cycle, the piles were monitored for TS and VS content (weekly), temperature (daily), and odors (qualitatively). Temperature measurements were monitored at seven locations for each pile. General ambient conditions were also monitored during the course of the cycle.

The final compost product was analyzed for TS and VS content. In addition, laboratory analyses on the final product (after curing), for metals, nutrients, soluble salts, pH, fecal coliform, and Salmonella sp. were conducted. These data were used in evaluating potential end-use opportunities, as well as demonstrating specific federal and state regulatory requirements.

PILOT PROJECT RESULTS

To date, nine of the ten composting cycles have been completed for the Pilot Project. Table 1 summarizes several of the test mixes evaluated. Some of the test mixes were duplicated during the Pilot Project to confirm process performance and to produce product for the WRRI research project.

Variables evaluated during the Pilot Project included bulking agent source, amendment source, and biosolids source. Following is a discussion of the impacts of each variable observed during the Pilot Project.

Amendment Source. The first test mix evaluated during the Pilot Project was a 41 volumetric mix of wood chips to biosolids. An average temperature profile during the 28-day composting cycle for Test Mix 1 is presented in the attached figure. As illustrated in the figure, the compost pile constructed using Test Mix 1

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I I I I I I I I I I I I I I

4: 0 (u

0 s m 0 m 0 W

0 F

(3) arn)aradmal.

69

39-41 -29 -7

18 65

McAlpine

Sawdust 67 1:l

20 65

McAlpine

Sawdust 64

0.5: 1

Wood chips

65 3:l

None

Wood chips

55 2 1

None

failed to meet the Class A temperature performance criteria and the VAR requirement. This test mix was repeated during a duplicate cycle, and a similar temperature profile was observed.

Table 1 Summary of Compost Pilot Project

~

Evaluation Item

~

Test Mix I

Test Mix Test Mix 2 I 3

Test Mix 4

Test Mix 5

Test Mix 6

Biosolids TS, % v s , % Source

20 65

M cAl pine

22 63

McAlpine

17 58

Irwin

20 67

McAlpine

None Sawdust 77

0 5 1

~~ ~

Sawdust 77

0.51

Amendment Type TS, % Volumetric Ratio*

Sawdust 64

0.51

Pallet mulch

55 3: 1

Wood chips

63 1.5:l

~

Wood

63 1.51

chips Bulking Agent

Type TS, % Volumetric Ratio*

Wood

57 4 1

chips

Recyclc Type TS,% Volumetric Ratio*

Wood chips

41 1.51

Wood chips

41 1.51

None None

Mix TS, % 45 39-41 38-43 38-43

Product TS. % NA 60 60 60

10 Aeration

Cycle Time, min/hr

Temperature Criteria PFRP VAR

10 10 10

Failed Failed

Failed Failed

Passed Passed

Passed Passed

I Passed Passed Passed Passed

* Ratio of materials tc bios o 1 ids

The energy/mass balance used to determine the volumetric mix ratios indicated that sufficient biodegradable volatile solids were present to promote active composting using Test Mix 1. Similar problems achieving temperatures have been encountered with biosolids cakes of 20% TS or higher when wood chips were the only feedstock component due to low biodegradability of wood chips (Haug, 1Y93). It was concluded that an amendment (i.e,, sawdust) to provide more biodegradable volatile solids would be necessary in future test mixes. The assumed biodegradability factor for wood chips from 10 to 5 percent of the total volatile solids content was also reduced.

Test Mix 2 consisted of 3:l:l volumetric ratio of wood chips to sawdust to biosolids. The attached figure also presents the average temperature profde observed during the composting cycle for Test Mix 2. Two discreet compost piles were constructed using this test mix, and each had a temperature profde similar to the one presented in the figure. The highest observed temperature during the compost cycle was V C , while pile

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temperature averaged 58°C during the compost cycle. Both piles met the Class A performance criteria and the VAR requirement

Tesl Mix 3 consisted of 3:1:0.5 volumetric ratio of wood chips to biosolids to sawdust. As shown in the figure, reducing the quantity of sawdust did not have an adverse affect on the temperature profile to meet Class A and VAR temperature requirements.

Bulking Agent Source. As illustrated in the figure, composting was achieved using a volumetric mix ratio of 3:l:l of wood chips to biosolids and sawdust. Given the significant operational and maintenance cost associated with supplying wood chips for a full-scale composting facility, CMUD wanted to evaluate the use of alternative bulking agent sources.

One potential bulking agent identified by CMUD was a shredded wood product from crushed wood pallets produced by Mecklenburg County. The county was also evaluating alternative uses for this product; therefore, CMUD selected to further evaluate the use of this crushed wood waste as an alternative bulking agent source.

Average temperature profiles observed for Test Mixes 3 and 4 are presented in the attached figure. Test Mix 3 represents the standard compost formulation using biosolids, sawdust, and wood chips as feedstock components. Test Mix 4 represents a compost mix prepared using a volumetric mix ratio of 3:1:0.5 of pallet mulch to biosolids to sawdust. Test Mix 3 achieved temperature levels expected for a biosolids composting operation. The highest observed temperature was 71"C, while the pile temperature for Test Mix 3 averaged 49°C during the compost cycle. As illustrated in the figure, Test Mix 4 did not achieve the thermophilic temperatures necessary for active composting. The highest observed temperature was 59"C, while pile temperature for Test Mix 4 averaged 37°C during the composting cycle. Using pallet mulch as a bulking agent did not produce temperatures necessary to comply with the Class A performance criteria or the vector attraction reduction requirements.

Two possible reasons could be responsible for the failure of compost piles constructed with pallet mulch as a bulking agent to achieve the required time and temperature requirements. First, the particle size of the pallet mulch is quite different from that of the wood chips. In general, the pallet mulch pieces are 2 to 4 inches in length but are only 0.5 inch or less wide. The wood chips, however, are 2 to 3 inches in length and up to an inch (or more) wide. This increase in particle size may provide more porosity in the compost mix, therefore allowing more complete aeration for microbiological activity in the compost mix. Data from sieve analyses performed on the wood chips and the pallet mulch are summarized in Table 2. Approximately 53% of the wood chips were retained on a 3/8-inch sieve while only 42.6% of the pallet mulch was retained on the same sieve. The second reason for the failure.of the pallet mulch pile could be attributed to slower biodegradability resulting from the manufacturing process of the pallets.

Biosolids Source. Biosolids generated at the McAlpine Creek W W M F are anaerobically digested and mechanically dewatered using centrifuges to 20% TS. Volatile solids content typically ranges between 63 and 67% VS. CMUD is constructing a full-scale aerated static pile composting facility at the McAlpine Creek WWMF. The regional composting facility is designed to receive biosolids from the McAlpine Creek WWMF and two other wastewater treatment plants. In addition, provisions are included for blending alum residuals with biosolids prior to composting.

The Alum Sludge Facilities: Altemalive Evaluation and Preliminary Design Studies (1993) prepared for CMUD recommended that dewatered alum residuals from the Franklin Water Treatment Plant (WTP) be transported to the full scale composting facility for blending with biosolids prior to composting. Based on projected residuals quantities, alum residuals from the Franklin WTP will be less that 6% by dry weight of the total residuals quantity. Currently, alum residuals from both the Franklin and Vest WTPs are received via the sanitary sewer at the Irwin Creek W P for treatment. Biosolids from the Irwin Creek WWTP have been land applied successfully for many years while containing up to 16% alum residuals on a dry weight basis. To

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evaluate the impact of alum residuals on the composting process and product quality, biosolids from the Irwin Creek WWTP were used to construct a compost pile for Test Mix 6.

Sieve Size

Table 2 Sieve Analyses of Wood Chips and Pallet Mulch

Wood Chips Pallet Mulch Percent Passing (%) Percent Passing (%)

1-112 inch 314 inch 112 inch 3/8 inch No. 4 No. 8 No. 16 No. 30 No. 50 No. 100

100 92.5 63.0 47.0 13.8 7.1 4.8 2.7 1.4 0.5

100 95.9 67.3 57.4 26.9 10.0 4.3 1.3 0.5 0.1

Average temperature profiles for Test Mix 5 and 6 are illustrated in the attached figure. Test Mix 5 represents a compost mix prepared using biosolids from the McAlpine Creek WWMF (source for all other tests during Pilot Project), recycled wood chips, wood chips, and sawdust. Test Mix 6 represents a compost mix prepared using biosolids from Irwin Creek WWTP, recycled wood chips, wood chips, and sawdust. From the temperature data, both piles were capable of complying with the Class A performance criteria and the VAR requirements. The presence of alum residuals did not appear to negatively impact the composting process.

Product Quality. Product quality testing was conducted only for product that met the Class A performance criteria and the VAR requirements. The final compost product was analyzed for TS and VS content. In addition, laboratory analyses on the final product (after curing) for metals, nutrients, soluble salts, pH, fecal coliform, and Salmonella sp. were conducted.

Composite samples of screened compost product were collected for analysis of the ten metals regulated under the Part 503 regulations. The results from the analyses are summarked in Table 3. Results from these analyses indicate that the compost product generated is capable of meeting the Pollutant Concentration (PC) limits established in the Part 503 regulations. In fact, the concentrations of metals in the compost product were well below the PC limits.

Table 4 illustrates typical biological monitoring results obtained during the Pilot Project. Samples of the compost product were collected following the curing cycle. Fecal coliform levels typically exceeded the Class A limit of 1,OOO MPN/g TS. However, Salmonella sp. levels were below the limit established in the Part 503 regulations (3MPN/4g TS) and, therefore, were in compliance with the Class A requirement. Samples were also collected 60 days after the curing cycle to document the impacts of storage on product quality. Results were similar to those presented in Table 4.

Biosolids can be rich in calcium, magnesium, potassium, and sodium. Composted biosolids, therefore, may contain higher concentrations of soluble salts than is present in common nursery media, such as peat, bark, soil, or sand. While such minerals are essential for plant growth, excess can damage plant roots and inhibit germination, especially in container grown plants. Symptoms of excess salts can include yellowing of leaves, wilting, leaf burn, and slow growth of seedlings. Table 5 summarizes soluble salt concentrations of several test mixes from the Pilot Project. Total soluble salt concentrations should be below 10 mmhoskm to avoid injury

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to plants. As seen from the results, some of the test mixes have exceeded this level. In general, soluble salts should not be a major concern, but CMUD should monitor levels of cured and finished compost products to avoid management problems.

Sample Number

1 2 3 4 5 6 7

Table 3 Pollutant Concentrations of Biosolids Compost

Fecal Coliform snlmonelln sp. (MPN/g TS) (MPN/4 g TS)

770 < 1 10,Ooo < 1 820 <1

6,700 2,600 4,100 891

Parameter

Arsenic Cadmium chromium Copper Mercury Molybdenum Nickel Lead Selenium Zinc

Compost Mix

Test Mix 2a "3)

0.77 1.1 64 84

0.42 3 10 24

0.81 186

Totai Soluble Salts (mmhos/cm)

Test Mix 3 (mg/kg)

1.1 2.07 59.3 85.8 0.366 2.9 10.8 27.7 0.86 274

Composite Sample 10.4 Grab Sample 16.5

Test Mix 3* 5.5 Test Mix 5* 7.8

* Composite samples. I

Part 503 PC Limits (mglkg)

41 39

1,200 1,500

17 18

420 300 36

2,800

Table 4 Biological Monitoring Data for Biosolids Compost*

Table 5 Soluble Salt Concentrations

(1 Test Mix 2

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Table 6 presents typical values for pH, TS, and VS content, nutrient value, and micronutrient data of the composted biosolids. These values are typical of composted biosolids products.

Table 6 Composted Biosolids* Quality Data

~

PH TS, % vs, % Nitrogen (TKN), % Organic Nitrogen, % Iron, m a g Manganese, m?,/kn

6.1 57.65 79.68 1.43 1.33

5,955 393

I(coRepresentspostfrom Test Mix 2. II WRRI RESEARCH AND PRELIMINARY RESULTS

CMUD is participating in a research project conducted by WRRI and North Carolina State University to evaluate product consistency and potential end uses of the composted biosolids for various agronomic and horticultural crops. In addition, demonstrations with potential users for training public officials, extension personnel, and other potential users of the product are also being conducted. The research project will provide necessary data to evaluate the suitability of composted biosolids products for various end uses. The following objectives are to be accomplished as part of this research project: . Develop a standard formulation of feedstock components for compost production.

Evaluate physical and chemical properties and consistency of compost produced. Evaluate the potential end uses of the compost on various soil and plant types as well as a media

. . amendment for container and greenhouse production. Develop a comprehensive demonstration training program to field evaluate compost and familiarize users with product potential.

To date, the following studies have been conducted as part of the research project: (a) the evaluation of various compost sources for sod production, @) evaluation of seeding rates of bluegrass-fescue mixture for sod production in medium of composted biosolids, and (c) use of composted biosolids as a medium for growing dwarf nandina. A discussion of the preliminary research results follows.

Sod Production. A greenhouse study was conducted using a bluegrass-fescue mixture seeded on 1.5 inches of three types of compost (biosolids, fish waste, municipal solid waste). Fertilizer was not applied until after the first cutting, at which time 0, SO, and 100 ppm of a 20-20-20 soluble fertilizer solution was applied weekly. A slight salt injury was observed for grass grown in composted biosolids which received the highest fertilizer application. This did not result in inhibited growth and did not recur after the first cutting. Grass grown on biosolids compost had a significantly greater dry weight and surface density after the third cutting (90 days after seeding) than grass grown on other compost media.

Sod Seeding Rate Study. A greenhouse study was conducted to evaluate appropriate seeding rates for a bluegrass-fescue mixture seeded on composted biosolids. Seeding rates of 4, 8, 12, 16, 20, and 24 lbs/1,000 square foot were evaluated. Each plot received 100 ppm of 20-20-20 soluble fertilizer solution. Plant and root densities were evaluated on a scale of 1 to 10, with 1 considered the lowest rating and 5 or above considered acceptable. Seeding rates of 12 lb/1,000 square foot or greater produced an acceptable sod based on plant and root density, as indicated in Table 7. Preliminary results indicate that sod production with

7 4

composted biosolids in a greenhouse may be possible within five to six weeks and outdoors within five to six months. Typical sod production requires 15 to 18 months.

Seed Rate (lb/l,OOO f f)

Table 7 Effect of Seeding Rate on Plant Density, Root Density,

and Bio Mass Dry Weight for Bluegrass-Fescue Sod

Bio Mass Dry Weight (6) Plant Density Root Density

Compost Growth Rating

4 8 12 16 20 24

6 8 10

13.5 17 21

3.3 4.8 5.8 6.8 7.9 8.0

~ ~~~

3.0 4.2 5.4 6.3 7.1 7.8

Dwarf Nandina. A study was conducted to evaluate the effects of composted biosolids with various pine bark-compost mix ratios and fertilizer applications on growth performance of dwarf nandina. Volumetric mix ratios (with pine bark) of 0, 25, 50, 75, and 100% compost were evaluated along with fertilizer applications (of 5, 10, and 15 lbs/cy) of 18-6-12 slow release fertilizer. The control (0% compost) used in the study was a standard nursery media. Plant density ratings were evaluated on a scale of 1 to 10, with 1 the lowest rating and 5 or above an acceptable rating. Significant differences for growth ratings among various mixing ratios of composted biosolids and pine bark, as well as fertilizer application rate, were observed. Plant density rating was observed to be highest for 100% compost and lowest for the control as illustrated in Table 8. In addition, growth ratings were higher at fertilizer rates between 10 and 15 lb/cy as compared to the 5 lbky fertilizer rate.

Table 8 Growth Rating by Compost Pine Bark Mix Ratio

SUMMARY

To date, the Pilot Project has successfully produced a consistent humus-like material. Product quality testing indicates the test mixes meeting the Class A and VAR temperature performance criteria also meet the PC limits and pathogen limits required for classification as an EQ biosolids product. In addition, preliminary results of the WRRI research project indicate composted biosolids can be successfully used as a growing media for sod production and growth of container plants (dwarf nandina).

Future plans are to use the composted biosolids in demonstration projects with extension personnel, public officials, parks and recreation department personnel, and other potential users. This portion of the project will help to promote beneficial use, educate users, and assist in preliminary market development.

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REFERENCE

Alum Sludge Facilities: Alternative Evaluation and Preliminary Design Studies, Black & Veatch, Environmental Engineering & Technology, August 1993.

Haug, Tim, City of Los Angeles, in personal communication with Scott Carr, Black & Veatch, February 1, 1993.

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