April 27, 2009 | General

Composting Trials Evaluate VOC Emissions Control

BioCycle April 2009, Vol. 50, No. 4, p. 33
Technology evaluated for its ability to comply with California air quality districts’ limits on total volatile organic compound emissions from composting operations.
Charles E. Schmidt, Thomas R. Card and Bernhard Kiehl

SEVERAL air quality districts in California have adopted, or are set to propose, regulations to control volatile organic compound (VOC) emissions from various composting operations. South Coast Air Quality Management District (SCAQMD) adopted its rules in 2003. These include Rule 1133, Composting and Related Operations-General Administrative Requirements; Rule 1133.1, Chipping and Grinding Activities; and Rule 1133.2, Emission Reductions from Co-Composting Operations. San Joaquin Valley Air Pollution Control District (SJVAPCD) adopted Rule 4565, which covers composting operations whose throughput consists entirely or in part of biosolids, animal manure or poultry (including amendments, typically derived from green waste). Rule 4566, which is in draft form (see “Controlling Composting Emissions,” BioCycle January 2009), will apply to green waste composting.
A common minimum requirement in these regulations is implementation of a mitigation measure demonstrating a VOC reduction of at least 80 percent. To be able to assess success in respect to this mitigation goal, SCAQMD has established “baseline life cycle emission factors” for emissions from uncontrolled windrow composting. Composting facilities in these air districts may need to change current operations to comply with these rules.
SCAQMD has collected and published data on green waste composting and biosolids/green waste composting emissions. The green waste values are provided in both a unit emission factor form (pounds of pollutant per hour and per 1,000 square feet (lbs/hr-1,000 ft2) and for a total compost cycle (pounds of pollutant emitted per ton of material in the windrow (lbs/ton). The biosolids/green waste numbers are for full cycle only; no unit emission factor values were provided. Table 1 summarizes those emissions factors.
The sampling technology utilized by SCAQMD was direct measurement, a modified USEPA flux chamber technology. The hydrocarbon sample collection and analysis, for the most part, was per SCAQMD Method 25.3 for total nonmethane non-ethane organic (TNMNEO) compounds by cold water trap and canister analysis.
Table 2 summarizes both the SCAQMD and SJVAPCD requirements. The accepted life cycle emission factors set by SCAQMD for windrow composting of green waste are 4.75 lbs TNMNEO/ton. The accepted life cycle emission factors for green waste/biosolids windrow composting are 1.78 lbs TNMNEO/ton. The compost emissions from SCAQMD data appear unrealistically low, and are significantly outside other published data sets.
One technology being evaluated for its effectiveness in reducing VOC emissions in relation to baseline emission factors and emission reduction targets is the GORE™ Cover Composting System (GORE System). The goal of the assessments, conducted at pilot-scale, was to measure the fugitive air emissions of total VOCs, as determined by the SCAQMD modified flux chamber technology and the SCAQMD Method 25.3 for TNMNEO compounds over the lifecycle of the composting operation, and to assess the control efficiency of the cover system.
The GORE System uses a specially designed expanded PTFE membrane (similar to the one used in GORE-TEX® garments) laminated between two strong polyester layers. The results from these investigations indicate that the GORE System is able to meet strict VOC emission control goals for green waste (SJVAPCD Draft Rule 4566) as well as for biosolids cocomposting (SCAQMD Rule 1133.2, SJVAPCD Rule 4565). This control value was determined by comparing the under the cover flux measurements with the on the cover flux measurements.
Emissions testing was done with biosolids/wood chip mixtures, biosolids/green waste and green waste only at two facilities. Fugitive air emissions were determined by measuring the emissions of VOCs or TNMNEO compounds at multiple locations on the test piles on top of the GORE cover on key and representative days of the composting cycle for both composting and finishing phases, plotting the life cycle emissions for the process over the full cycle, and computing the emissions on a “per ton” basis of feedstock. Testing included measuring emissions on the cover throughout the composting cycle, with the cover removed at pile breakdown and mixing, and after pile mixing.
The control efficiency of TNMNEO compounds was determined by burying flux chambers under the cover, measuring emissions of the compost under the cover and directly on the pile surface, and then expressing the ratio of the emissions measured on top of the cover to the compost emissions below the cover, all expressed as percent control of the potential to emit. SCAQMD-modified EPA surface emission isolation flux chamber technology was used to evaluate composting emissions during the field trials.
The first test was conducted in the summer of 2005 in southern California at a location called Site 1 in this report. This was just a screening test, so only mid-compost cycle data was taken on a limited basis; full life cycle emissions could not be generated from this study. However, the test objective was achieved since the testing design was a “proof of concept” approach where the control efficiency of the GORE technology was evaluated. A more complete test was conducted in Washington State during the winter of 2007 at a location called Site 2, where life cycle emissions were determined as well as control efficiency.
Two aerated static GORE System piles were constructed at the Site 1 facility. One pile was composting green waste and one was composting biosolids/green waste. The control was a nonaerated windrow that was composting green waste per Site 1 standard operating procedures. These data were collected only to generate unit emission rates. The test was not designed to accurately measure full compost cycle emissions.
Green waste composting was evaluated at Site 1 by constructing two test piles with different start dates, and testing fugitive air emissions through the cover on key days: at 28 days in the composting cycle (end of GORE Phase I composting); with the cover removed; and on the green waste compost after compost mixing. Emissions data were used to compose an emissions profile and emissions estimate for study compounds for the green waste composting cycle.
Tests at Site 2 were intended to generate data that would accurately represent full cycle emissions. The test took place during the winter of 2007. The compost mix consisted of biosolids mixed with woodchips and overs. Table 2 provides a summary of the pile volume and mix metrics. The typical GORE System operations mode consists of three phases: Phase I – 28 days; Phase II – 14 days; and Phase III – 14 days.
Biosolids delivered to Site 2 were observed to be partially frozen when dumped from the trucks. The biosolids, wood chips and overs were mechanically mixed in small batches using a tractor-powered mixer device designed for mixing and delivering livestock feed. Total amount of material was 92.4 lbs of biosolids and 156.6 lbs of amendment. The pile was built with a front-end loader from the small mix batch piles and covered with the GORE cover three days after arriving on site. The initial pile volume was 365.4 cubic yards; density was 1,363 lbs/cy (50.5 lbs/sq. ft.). Due to the cold ambient temperatures and cold (frozen) biosolids, the compost mix was slow to achieve operating temperature. The pile did not achieve a 55°C core temperature until five days later. Based on the temperature profile, Day 1 of the compost cycle was chosen to be three days after mixing. The normal pile operating temperature was not achieved until 10 days after the initial covering.
The pile was operated in three phases. It was aerated by a positive pressure blower, which was operated per GORE staff direction. Phase 1 was nominally 28 days, Phase 2 and Phase 3 were 14 days each. The pile was covered for each phase, although most GORE System composting installations do not cover the Phase 3 operation. The pile was broken down and rebuilt using a front-end loader between each phase. It was decided not to provide any additional mixing besides that provided by the loader handling the solids.
Emissions sampling was initiated on compost Day 2. The base sample program was to take six samples from the cover surface (4 top and 2 sides). These samples were taken sequentially from three locations. The same locations were used for each sample day. In addition, two samples were taken from flux chambers located below the cover. During the transition periods between phases, samples were taken from the uncovered but undisturbed surface and the disturbed surface after handling by the loader, as well as the newly placed material prior to covering. The additional emissions caused by removing the cover were inconsequential because the uncovered transition period was normally only a few hours.
All sampling was completed under fairly cold ambient conditions with temperatures ranging from 0°C to 10°C. This had no effect on data quality as far as sample capture, storage, shipment and analysis.
When the cover was removed after Phase I, there was noticeable poor mixing. Large chunks (25 cm in diameter) of unmixed biosolids were observed. In addition, a sheen of unmixed biosolids coated the surface of the pile under two of the sampling locations. Poor mixing typically results in elevated VOCs generation from the composting process. Thus these test conditions represent a worst case scenario.
Rigorous field and laboratory quality control (QC) procedures were used to insure the proper collection and analysis of field samples from the studies conducted at both Site 1 and Site 2. Total organic compound QC in the laboratory included duplicate analysis of field samples, method blank determinations and method response to four-point calibration curves. All method QC testing was with method specifications, and these data indicate acceptable laboratory method performance.
The published life cycle emission factors (SCAQMD) are 4.75 lbs TNMNEO per ton of green waste and 1.78 lbs TNMNEO per ton of biosolids/green waste compost. Only unit surface emission values (lbs/hr-1000 ft2) were produced from the pilot at Site 1 (thus baseline emission factors could not be reported). A comparison of surface emissions, however, is possible with this data set. The green waste surface flux or emissions value for Day 28 compost was 0.00149 lbs/hr-1000 ft2, which is comparable to the SCAQMD criteria of 0.079 lbs/hr-1000 ft2. This is a demonstrated control efficiency of about 98 percent. Likewise the biosolids/green waste surface flux value for Day 28 compost was 0.00802 lbs/hr-1000 ft2. Unfortunately, there is not a comparison value for biosolids emission control in emission units. The results from this first field testing effort demonstrated that the fugitive air emissions from green waste and biosolids composting using the GORE System were less than 10 percent of the SCAQMD baseline emission rate values (lbs/hr-1000 ft2).
At Site 2, the biosolids/woodchips and overs “on top of the cover” surface flux values ranged from 0.0024 lbs/hr-1000 ft2 to 0.0236 lbs/hr-1000 ft2. The “under of the cover” surface flux values ranged from 0.0475 lbs/hr-1000 ft2 to 0.921 lbs/hr-1000 ft2. Since life cycle emissions were developed from these emission rate data and compost metric information, comparable life cycle emissions data (lbs/ton) can be compared to SCAQMD criteria. The biosolids/woodchips emissions were 0.20 lbs/ton as measured on the cover (multiple tests per pile per multiple days in the life cycle assessment) as compared to a 3.69 lbs/ton life cycle emissions as determined under the cover, or a 95 percent control of fugitive emissions from the life cycle process. This is also compared to the SCAQMD life cycle emission criteria of 1.79 lbs/ton for a demonstration of 89 percent control. These studies show better than a 90 percent control of fugitive air emissions from compost operations for biosolids/wood chip, biosolids/green waste and green waste mixtures.
The results from these investigations indicate that the GORE System is able to meet strict VOC emission control goals for green waste (SJVAPCD Draft Rule 4566) as well as for biosolids cocomposting (SCAQMD Rule 1133.2, SJVAPCD Rule 4565).
Charles E. Schmidt, PhD is with CE Schmidt Consulting located in Red Bluff, California. Thomas R. Card, PE is with Environmental Management Consulting in Enumclaw, Washington. Bernhard Kiehl is with W.L. Gore & Associates, Inc. in Elkton, Maryland.

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