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Lab and Pilot-Scale Tests with Post-Digestion Biosolids to Prevent Indicator and Pathogen Regrowth

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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°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.

Document Type: Research Article


Publication date: January 1, 2005

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