Optimizing a Large Scale Biofilter Treating Compost Exhaust Air

Summary of ProjectSynagro operates the South Kern Industrial Center (SKIC) compost facility in Bakersfield, CA, taking approximately 600 tons per day of biosolids from Southern California area wastewater treatment plants and producing a final compost product that meets Class A Exceptional Quality (EQ) standards under EPA 40 CFR Part 503 requirements. s. Composting is achieved in aerated static piles with negative air flow from two primary and two secondary compost operations exhausting processed air to biofilters. The two primary biofilters are 20,000 ft2 each with 6–7 feet of organic media and the two secondary biofilters are 11,000 ft2 each with 6–7 feet of organic media and. The wood chip based biofilters have pre humidification chambers, surface irrigation and laterals embedded in gravel for maximum air flow distribution. The hot compost exhaust air is cooled with ambient air pulled by cooling air fans which is mixed with the compost air prior to the biofilter. The lack of an easy water discharge (no sewers available) creates a water optimization challenge within the bio-filter process and at the site. This challenge is both in quantity of water sent to the on-site retention pond and also the quality issues associated with Ammonia (NH3) compounds removed by the biofilters. The SKIC compost and biofilter operation began in 2007, and has air permits issued by the San Joaquin Valley Air Pollution Control District to remove 80% of the Volatile Organic Compounds (VOCs) and 80% of the ammonia (NH3) leaving the biofilters. Each time the media is replaced, an extensive air emissions testing program is required to demonstrate compliance with the air permit.While the SKIC facility has not experienced any permit exceedances during the bi-annual testing events, it was believed that improvements in the air flow distribution network within each biofilter media would assist Synagro in optimizing water usage and maximize regulated constituent removal efficiencies. To assist in these optimization projects, Webster Environmental Associates, Inc. (WEA) was retained in the summer of 2011, to evaluate the current conditions, conduct testing and devise a plan to restore the biofilter to be able to meet performance on a long term basis. A second objective was to extend the life of the media because Synagro had been replacing the media every 2 years at a cost of about $400,000 for each occurrence. To accomplish both of these goals a detailed Process Engineering Sampling and Monitoring protocol was developed for use by the SKIC operational staff.The staff also desired to better understand the fate and removal of nitrogen from the biofilters. While the constituent of concern is Ammonia (NH3) which is removed readily – greater than 97% in the last source test – gaining additional knowledge for the speciation of N compounds and the potential process optimization were of great interest to the Synagro operational team.Status of the ProjectA diagnostic evaluation was conducted by WEA and several options were presented to Synagro to maximize the biofilter performance and minimize water flow to the retention basin. Air flow through the media provided for permit compliance it was not however always optimal in achieving superior Empty Bed Contact Time (EBCT). Synagro elected to proceed with a plan to install a new lateral system with a new configuration and additional gravel in the media underdrain. The added gravel improved the filters Empty Bed Contact Time (EBCT) and provides sufficient structural stability to allow the operational staff the ease of utilizing front end loaders to mix and fluff the biofilter media without harming the updated air flow lateral network. Other modifications such as better access to the “clean outs” were installed to improve the drainage within the biofilter and also to reduce the operational time required for the periodic flushing required. New media was placed onto all four biofilter beds and the system was started up in August, 2011 and a San Joaquin Valley Air Pollution Control District (SJVAPCD) source test was completed in September, 2011. The new biofilters met the APCD permit requirements, with a 97–99% removal efficiency of NH3 and an 88–94% removal efficiency of VOCs vs. the permitted levels of 80% for NH3 and 80% for VOC's.The Process Engineering and Monitoring Protocol developed by WEA were implemented by the Synagro operational staff. The scope of this plan includes: monitoring of the inlet temperature, pressure, flow and ammonia and the biofilter bed moisture, pH and ammonia. To assist in creating greater data consistency provided during the field sampling, a special sampling device was built for Synagro by Chuck E. Schmidt (CES Testing) to perform surface sampling and to check for even air distribution. An evaluation of the fate of ammonia in the biofilter, in terms of conversion to nitrates will be presented. By monitoring the biofilter and meeting process control objectives, Synagro's goal is to achieve a media life of 3–4 years for significant cost savings, while meeting air emission performance standards.