FULL SCALE THERMOPHILIC DIGESTION OF SLUDGE WASTE AT CITY OF LA's TERMINAL ISLAND TREATMENT PLANT: SUCCESS STORY

This is a report on the progress up to May 3, 2000, of the project for full-scale thermophilic digestion of wastewater sludge at the City of Los Angeles (CLA) Bureau of Sanitation's Terminal Island Treatment Plant. The Bureau previously conducted thermophilic digestion at its Hyperion Treatment Plant, as reported in the historically important papers of Garber and his colleagues. However, those periods were ended by changing economic and regulatory conditions, and present knowledge indicates that the slow heating rates at the startups produced an undesirable culture in the digesters. Furthermore, that work was done mainly to improve dewaterability, but the motivation now is to meet impending regulatory requirements for pathogen reduction to improve safety. The present study has been conducted with rapid heating, using recent experience showing how to feed to maximize the rate of culture development. Four feeding plans were followed during the period covered by the report, providing a gradual increase of the feed rate as the microbial activity level increased. The development of the culture has followed a course similar to that seen at some other plants, with an initial rise of acid concentration as the acidogen activity initially exceeded the activity of the methanogens, and a later decline and approximate stabilization of the acid concentration as the activities of the microbial communities came into balance. Balance was achieved in less than two weeks. The chemical parameters have been stable since early in the startup period, indicating that a stable biological community has been established that has been able to increase in numbers to meet the increases in the feed rate. Likewise, disinfection has been stably effective for several weeks, and the present combination of low volatile fatty acids and low hydrogen sulfide is good news for odor control. The few Salmonella tests done to date comply with the Class A specification, as expected from from CFR 503, Rule 40 (interpretation to guidelines) which quotes previous research showing that Salmonella are rarely detectable when the coliform count meets the Class A standard. Soon we will be testing for enteric viruses and helminth ova to verify compliance with the other pathogen specifications of the USEPA Class A standard for biosolids, since from CFR 503, Rule 40 (interpretation to guidelines) notes that the concentration of these organisms is usually low or undetectable in untreated sludge, and it is also know that they are rapidly killed by thermophilic temperatures. As the digester is now operating successfully at the current feed rate, there appear to be no barriers to carrying out our plan to process the whole sludge production of the plant. We are extremely satisfied with the progress of this project.