The City of Los Angeles Hyperion Treatment Plant (HTP) has conducted a multiphase study of thermophilic anaerobic digestion to meet federal (Part 503 Biosolids Rule) and local (Kern County ordinance) requirements for Exceptional Quality Class A biosolids. The study consisted of five Phases of full-scale testing.

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To treat odors associated with two of the City's largest interceptor sewer systems the City of Los Angeles has been testing various odor control strategies for several years in order to develop the most cost-effective approach. The Air Treatment Facilities (ATF's) for the interceptors are located in the public right-of-way and therefore safety concerns played a major role in process selection. After careful evaluation of alternatives, Bio-trickling filters (BTF) were selected as they do not require the use of chemicals.Initial testing at the test facility located at the Hyperion Treatment Plant was conducted on a fullscale, single stage bio-trickling filter followed in series by a bench-scale polishing stage consisting of either a Biofilter or carbon adsorption unit. These test results indicated that at the selected ATF locations the odor levels remaining after first stage treatment were too high for direct atmospheric release and that a polishing stage was required. Furthermore, air modeling indicated that only the use of activated carbon as the polishing stage would reduce odor levels to targeted goals.Based on the information gained from these initial tests, a 2-stage odor control system employing bio-trickling filter technology followed by carbon absorption was selected for the City's ATF's. In an effort to improve the BTF performances, a new media manufactured by Taiwan-based Matala Water Technology has been tested. . Under a joint effort between Envirogen Technologies, Inc. and the City of Los Angeles, Matala media was placed inside a BTF manufactured by Envirogen Technology. The installation of this unit, located at the Hyperion Treatment Plant, was completed at the end of May 2008. Testing was performed over the following seven (7) month period. This paper will present the test results of those tests.

The digestion process at the Hyperion Treatment Plant received Class A Exceptional Quality (EQ) Biosolids certification from the EPA, Region 9 on December 27, 2002. Currently, the feed to the digesters is a blend of Waste Activated Solids thickened by centrifuge to 6% and nonthickened Primary Solids. Investigations are underway to determine if the use of Thickened Primary Sludge can be employed to optimize the process. Potential benefits are twofold. Firstly, a reduction of the water content being sent to the digestion process will dramatically reduce the mass heated to thermophilic temperatures. This is significant as the energy needed to raise the temperature from Mesophilic to Thermophilic is approximately three times more at a cost of $300,000 per month. Secondly, the concentrated biosolids being digested effectively increases much needed digester volume necessary to achieve Class A Biosolids through Alternative 1-Time/Temperature. In effect this will increase operational reliability, as more units will be available for maintenance, standby duty or future treatment needs.To that end, the City engaged in large scale pilot testing to determine the viability of primary sludge thickening through the use of centrifugation. During the pilot study, centrifuges from two vendors were tested. Both centrifuges were able to thicken primary sludge to as high as thirteen (±13%) percent total solids concentration. Throughout the tests, both solids recovery and centrate concentrations were found to be satisfactory.Based on this preliminary information, it became apparent that a second set of pilot tests was necessary. These tests were aimed at determining the practical limit of primary solids dewatering relative to the operation of the thermophilic anaerobic digestion process. The investigation was aimed at the determination of two possible queries; determine the maximum (%TS) concentration of biosolids allowable to pump, and determine the maximum concentration of biosolids allowable prior to inhibiting the digestion process.