Fecal coliform recurrence has been observed at the City of Los Angeles Hyperion Treatment Plant during pilot-scale experiments with a designated thermophilic battery of six anaerobic digesters, while other digesters were still at a mesophilic temperature. Several lab and full-scale experiments indicated the following possible causes of the growth/reactivation of fecal coliforms in post-digestion: a) contamination of thermophilically digested biosolids with mesophilically digested biosolids; b) a large drop in the biosolids temperature between the centrifuges and silos, which could have allowed the reactivation and/or growth of fecal coliforms. These were resolved by the full plant conversion to thermophilic anaerobic digestion and design modifications of the post-digestion train.

It has been shown in the literature that the introduction of Fats, Oils, and Grease (FOG) into Anaerobic Digesters holds many benefits, namely the increase in efficiency of the digester, an increase in digester gas production, and the diversion of organic matter from landfills (Wimmer 2010). Several municipalities in Southern California, such as in Riverside, Rialto, and Oxnard have been involved in FOG digestion. As a result, the City of Los Angeles' Hyperion Treatment Plant started its own FOG digestion pilot project in 2010. The pilot injected FOG into one 2.5 million gallon thermophilic digester. The plant partnered with Baker Commodities, a grease rendering company, to bring 5,000 gallons of FOG initially and ramped up after that. Most critical digester parameters were monitored and results were analyzed. Although the digester was loaded 15% in Volatile Solids the effect was 30% increase in gas production, which demonstrated a "supercharging" or increase in efficiency effect in the digester.

Several experiments were conducted to investigate the cause of fecal coliform recurrence in biosolids after thermophilic anaerobic digestion. Fecal coliform recurrence has been observed at the City of Los Angeles Hyperion Treatment Plant (HTP) during pilot-scale experiments with a designated thermophilic battery of six digesters, while other digesters were still at a mesophilic temperature. The main results of the present investigation were:-QA/QC plant inspections indicated that the designated thermophilic battery and its post-digestion train likely were isolated from mesophilic operations, but contamination by mesophilically digested biosolids could not entirely be excluded. -High-pressure effluent and Mannich polymer did not contain fecal coliforms, hence, their additions to the post-digestion train could be ruled out as contamination sources. -Laboratory simulations could not confirm the suspected role of dewatering centrifuges in the recurrence of fecal coliforms observed at HTP. -Regrowth tests in the laboratory and the field demonstrated that the minimum temperature for preventing fecal coliform recurrence was about 50 0 C.Based on these results, it was postulated the fecal coliform recurrence observed during the pilot-scale tests could probably be attributed to a combination of contamination by mesophilically digested biosolids and a large drop of the biosolids temperature during post-digestion processing.

Anaerobic digestion of high-strength organic wastes, such as Fats, Oils, and Grease (FOG) has become increasingly popular among wastewater treatment facilities in Southern California [1]. In 2010, the City of Los Angeles’ Hyperion Treatment Plant started running its own FOG Digestion Pilot Project. The project injects processed grease trap wastewater (FOG) into one of 16 anaerobic digesters at the facility. A partnership was formed between the Hyperion Treatment Plant (HTP) and Baker Commodities, Inc., a Grease rendering company located in the City of Vernon. They provide processed grease trap wastewater (FOG) to the pilot project. The plan was to load the digester with increasing amounts of FOG and observe the impact. The parameters monitored in the digester during the test are volatile acids, pH, alkalinity, temperature, and gas production in the digester. The pilot project’s objective was to obtain a 10 to 20% increase in gas production. So far those expectations have been exceeded.

The lack of user friendliness of present Offgas Transfer Efficiency (OTE) measurement equipment has been a great impediment to many of the operating activated sludge plants in establishing an aeration-basin diffuser maintenance program. Commercially available OTE instruments are large, cumbersome, fragile, and staff resource intensive. The basic principle of the existing OTE equipment is to bring an offgas stream released by the basin to an oxygen meter under controlled conditions throughout the measuring event to capture raw data. Several of the mechanisms used to sustain controlled conditions can be replaced with modern and smaller components to fabricate a more user-friendly system. Thus, recent technological advances have presented the opportunity for a smaller and more manageable new design that retains the same reliability as its predecessor. The resulting modified semi-automated OTE equipment and the subsequent testing will be presented in this paper. Plans for design and construction of a fully automated OTE instrument will be discussed.