Using Finished Compost to Treat and Control Emissions from Active Biosolids Composting

Background The Los Angeles County Sanitation Districts (LACSD) own and operate Tulare Lake Compost (TLC), a biosolids composting facility in the San Joaquin Valley of California (Figure 1). TLC was permitted to process up to 500,000 wet tons of biosolids per year, sufficient for all biosolids generated by LACSD wastewater treatment plants. During the composting process, volatile organic compounds (VOCs) and ammonia (NH3) are generated and may be emitted to the atmosphere. Emissions must be controlled to meet the requirements of TLC's air quality permit issued by the San Joaquin Valley Air Pollution Control District (SJVAPCD), which caps emissions of both VOCs and NH3 on a mass basis. Since the SJVAPCD is in non-attainment for ozone and PM 2.5, extremely stringent emissions limits are placed on their precursors, specifically VOCs and NH3, respectively. The original design of windrow composting piles with fabric cover emissions control technology for TLC did not perform adequately during the summer season. Through a series of test piles, LACSD has evaluated an alternative composting technology with the extended aerated static pile (eASP) configuration. Its emissions control device is a biofilter cover layer, composed of previously finished compost, which treats VOC and NH3 emissions from the active compost below it. To satisfy permit requirements at TLC's design capacity, lifecycle emissions from eASPs must be kept below emission factor targets of 0.20 lbs-VOC and 1.79 lbs-NH3 per ton of active compost mix. TLC produces Class A exceptional quality compost, with requirements to meet the Process to Further Reduce Pathogens (PFRP) and Vector Attraction Reduction (VAR). PFRP requires maintaining a core temperature of 131 °F for 3 days for pathogen removal, while VAR requires a core temperature of 104 °F for 14 days to reduce vector (e.g. rats, flies, mosquitos) attraction. Methodology and Implementation Baseline eASP Configuration eASP design was generally based on literature, experience from emissions testing consultants, and site visits to an existing facility utilizing the process. The active compost portion of the baseline eASP consisted of multiple adjoining rows with overall dimensions of 150' long, 140' wide, and 7' high (Figure 2). The active compost was topped with 2.5' of finished compost to form the biofilter cover layer. That 2.5' layer consisted of 1' of screened finished compost above of 1.5' of unscreened finished compost. The coarser unscreened material was intended to distribute emissions laterally across the pile, while the finer screened material above would be more effective at VOC and NH3 removal. Each row within the baseline eASP was aerated through an air header; block walls were placed on two sides of the eASP to limit uncontrolled air intrusion and retain heat (Figures 2 and 3). Sprinklers on top of the eASP irrigated the biofilter cover layer to maintain adequate moisture content in the biofilter cover for emissions control (Figure 4). Aeration Testing Adjustments to aeration timing and spacing parameters were tested. The intended purpose was to (a) improve the reliability of meeting PFRP/VAR temperature compliance, and (b) reduce capital expenditures (CapEx). More air was provided early in the composting process lifecycle in comparison to the baseline eASP (Table 1). The additional aeration was intended to increase biological activity to promote faster heating of the pile and meeting PFRP and VAR temperature requirements sooner in the eASP lifetime, thereby reducing the risk of non-compliance. The spacing between air headers was increased, only providing air beneath every other row as shown in Figure 5, while using the increased early aeration schedule from part a. This was to simplify construction and operation of the eASP by eliminating the need to reroute air headers. Irrigation Testing Decreased irrigation was tested to determine the relationship between irrigation applied to the eASP versus the amount of leachate produced and VOC emissions. Irrigation is critical to maintaining moisture to support the biology in the biofilter cover on top of the eASP, but too much irrigation leads to excessive leachate production. Leachate treatment or disposal is expensive so it's important to minimize the amount of leachate produced during the composting process while providing enough irrigation to not exceed the emissions limits for the facility. Temperature Monitoring eASP core temperatures were monitored continuously with online temperature probes (ReoTemp EcoProbes). Core temperatures were measured at 38' and 40' depths for PFRP and VAR compliance. Emissions Monitoring VOC emissions were monitored with SCAQMD Method 25.3 for total non-methane non-ethane organic carbon (TNMNEOC) and helium was used as a tracer gas for air flow measurement. Ammonia emissions were monitored by both manual GASTEC tube readings and SCAQMD Method 207.1. Results Baseline eASP Configuration VOC and NH3 emission factors were 0.05 and 0.04 lbs per ton of compost mix, respectively, well below the targets previously discussed. The eASP biofilter cover technology was able to control emissions during the composting process well below levels required for full capacity use of the facility. With respect to core temperature requirements, all zones in the baseline eASP passed the PFRP and VAR criteria, but one zone significantly lagged others. The average time for all zones to meet PFRP for the pile was 7.5 days with a minimum of 4 days and a maximum of 23 days. VAR was met at an average of 18.5 days. Such a cold zone increases the risk of non-compliance, which would require re-composting that zone. In response, aeration adjustments were implemented in the subsequent eASP to promote faster heating through increased biological activity. Aeration Optimization to Reduce PFRP/VAR Compliance Risk Four piles were tested with increased early aeration. The VOC emission factors ranged from 0.12 to 0.20 lbs per ton of compost mix, either below or just meeting the emission factor target. NH3 emissions were negligible for the two test piles that relied on solely Gastec tube measurements and 0.03 lbs per ton of compost mix for the test piles utilizing Method 207.1, well below the target threshold. With respect to core temperature requirements, all zones passed the PFRP and VAR criteria. The average time for all monitored zones to meet PFRP was 3.5 days with a minimum of 3 and a maximum of 4 days. VAR was met at an average of 14.5 days. There were no lagging zones and the faster heating is apparent in Figure 6, in which the four eASPs with increased early aeration shown as the gray lines have higher core temperatures early (days 1-10) in the composting process relative to the baseline aeration schedule shown as the blue line. Providing more air early in the eASP lifecycle can yield consistent temperature compliance throughout the eASP, but also yields higher VOC emissions. Aeration Optimization to Reduce CapEx for Implementation With wider air header spacing (aeration 'b'), the VOC emission factor was 0.08 lbs per ton of compost mix; NH3 emissions measured with Gastec tubes came back with minimal readings. All zones passed PFRP and VAR requirements. The average time for all monitored zones to meet PFRP was 3.2 days with a minimum of 3 and a maximum of 4 days. VAR was met at an average of 14.2 days. As shown in Figure 7, temperatures are similar between the four eASPs with air headers under every row (aeration 'a') shown as the gray lines and the eASP with wider air header spacing (aeration 'b') shown as the green line. Thus, the wider air header spacing did not appear to adversely affect VOC emissions control or compost temperatures, so an eASP can operate with fewer blowers and twice the distance between air headers as reported in literature. This is a major benefit for TLC as it allows for simplified eASP construction without the need to reroute air headers and avoids the expense of a future capital improvement project to install permanent air headers in-between the existing air headers. Irrigation Optimization to Minimize Leachate Production The relationship between irrigation rate versus leachate production and VOC emissions is shown in Figure 8. More leachate is produced with increased irrigation rate. Lower VOC emissions correlate with increased irrigation. Establishing these relationships is significant as it provides a valuable tool to balance leachate production and emissions for the facility. Conclusion eASP with biofilter cover is a viable technology for controlling composting emissions at TLC. Changes to the aeration schedule improved temperature to meet PFRP/VAR requirements and the operation with wider air header spacing was successful. Establishing the relationship between irrigation/emissions/leachate production helps with striking a balance for operati