This paper describes the progress up to June 2000 for thermophilic digestion of wastewater sludge at the Los Angeles, California, Bureau of Sanitation's Terminal Island Treatment Plant. The development of the microorganism culture has followed a course similar to that seen at other successful plants for establishment of a stable, wellbalanced thermophilic culture in a large digester, but at an accelerated pace. This study began with rapid heating, increasing the temperature of the 4500 m 3 (1.2 mil. gal) digester to the target temperature of 55°C at approximately 3°C/d. A method of feeding to maximize the rate of culture development was used as feeding accelerated to approximately 400 m 3 /d (0.1 mgd). An initial rise of acid concentration (primarily acetate) was seen. Within two weeks, acid concentration declined and stabilized, indicating that acidogenic and methanogenic microbial communities came into balance. Coliform data indicate that digester disinfection was stably effective from the middle of April. The salmonella tests done to date satisfy the U.S. Environmental Protection Agency (U.S. EPA) class A specifi cation. Testing with helminth ova and enteric viruses before and after the digester shows satisfaction of class A standard for those organisms. The present combination of low volatile fatty acids and low hydrogen sulfi de is good news for odor control. The data show increases in volatile solids destruction and estimated gas production, compared with the previous mesophilic operation; however, large uncertainties have been calculated from the data. As the digester is now operating successfully at the current feed rate, there seems to be no barriers to processing the entire sludge production of the plant. Other results indicate that the U.S. EPA requirements for exceptional quality class A biosolids are likely to be achieved. Water Environ. Res., 74, 494 (2002).

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.

Results from several recent studies of UV disinfection of reclaimed wastewater led to the conclusion that disinfection effectiveness is likely to be reduced at low flow velocities. Considerations from basic hydraulics make this likely because the formation of boundary layers along welled surfaces in lamp banks reduces the UV dose absorbed by most of the water. Boundary layers grow thicker with decreasing mean flow velocity through the lamp bank. This analysis of disinfection is supported by two reports of hydraulic tests and a study that changed flow rates to change the estimated dose per bank of lamps and found that the proportion of viruses killed in each bank was independent of flow velocity over the observed range. Additional laboratory and computational work is suggested to test this hypothesis. If. as expected. results confirm the hypothesis. then arrays of small vanes, similar to those used in aeronautical engineering, may provide a useful countermeasure.This recommendationcan be tested by the same methods used to test the hypothesis. Water Environ. Res., 71, 114 (1999).

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A study of oxygen transfer efficiency (OTE) in aeration basins, using measurements of oxygen depletion in offgas collected from them, was carried out over a period of several years by collaborations between the University of California, Los Angeles and the Bureau of Sanitation Research Group of the City of Los Angeles. Measurements were taken of dissolved oxygen (DO), water temperature, oxygen depletion, and air flux at each sampling location as part of the process to obtain standardized OTE. Field instruments, permanently located near the tanks and galleries, are connected to the control room that automatically records return activated-sludge flow, influent flow, tank DO sensor readings, and air flow. Data from the control room and field instruments were collected for the times of the samples to provide context and some degree of quality control for the samples obtained by the measurement team.The combined air flux and OTE measurements not only agree with the familiar inverse relationship between OTE and air flux but have allowed detection of strong evidence that serious leakage has developed in a few weeks in the air distribution systems of tanks 4 and 5, which was indirectly confirmed by observation of water ejection from the air release valves on the tanks. However, the detail provided by OTE measurements allows more specific understanding of the magnitude of the problem and its significance for the operation of these tanks. Analysis of the OTE measurements made at the site since 1991 also indicates that the diffusers in some of the tanks may now be in need of cleaning or replacement. Water Environ. Res., 72, 363 (2000).

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The operating costs of thermophilic and mesophilic operation of an anaerobic sludge digester at a wastewater treatment plant are estimated from a system of mass balance equations modeling the operation of the plant's biosolids handling, including digestion, screening, dewatering, and wet-cake storage. The effect of recycling heat from the outgoing to the incoming sludge is also considered. The result from a cost comparison based on data from the Los Angeles Bureau of Sanitation's Hyperion Treatment Plant clearly indicates that a thermophilic operation can be economically preferable to a mesophilic operation. This advantage would be enhanced by successful implementation of methods to increase gas production in digesters, which are also discussed. The cost model in this paper incorporates only the most quantifiable and typical aspects of biosolids handling, and thus provides a stringent test of the advantage of adopting thermophilic operation. If a plant can achieve parameters beyond the breakeven point as computed by this model then the additional economic or environmental benefits as described are likely to make thermophilic operation even more favorable.