Inactivation of Salmonella during anaerobic digestion of sewage sludge

Abstract

The inactivation of Salmonella duesseldorf in sewage sludge during anaerobic digestion was investigated at 35 and 48°C with mean retention periods of between 10 and 20 days. Digesters were fed daily with raw sludge containing added Salm. duesseldorf after removal of digested sludge. During steady operation, the levels of Salm. duesseldorf in the digested and the feed sludge were determined and their specific rates of decay were estimated. The latter were: (i) greater at 48°C than at 35°C for the same retention time; (ii) similar for retention periods greater than 15 d, but lower for 10 d; (iii) greater when the level of salmonellas in the feed was lower. Gas production, a measure of steady state, was gradually lost when the mean retention period was reduced to 6.7 d. In experiments in which a single dose of Salm. duesseldorf was added to digesting sludge, the inactivation appeared to follow first‐order kinetics at 35°C and the decimal decay rate, 1.6/d, was similar to that in the daily feeding experiments (1.4/d) with larger and similar inocula of Salm. duesseldorf. At 48°C, however, the rate of inactivation declined with decreasing time from inoculation suggesting that the culture contained cells differing in thermal resistance. The degrees and rates of inactivation of salmonellas in those experiments were greater than in full‐scale digesters, because the latter seldom operated under conditions ideal for inactivation or because indigenous salmonellas are more resistant.

Occurrence of ‘thermophilic’ campylobacters in sewage and their removal by treatment processes

“…They may be killed, stressed or injured (Ray & Johnson, 1984;Humphrey & Cruickshank, 1985) or converted to viable but non-culturable phase (Rollins & Colwell, 1986), resulting in failure to recover them by the isolation techniques used; or concentrated and removed in the sludge. Bearing in mind the disposal practices for sewage sludge on arable land and pastures (Carrington, 1981;Jones & Watkins, 1985), it would seem that further studies should be made. For example more information is needed on the survival time of campylobacters in sludge applied to land and the hazards that survivors might pose to man and animals.…”

Management of plant health risks associated with processing of plant-based wastes: A review

“…However, many studies on eradication of pathogens from compost and other organic wastes report only the time to extinction, without calculating the rate of decay or reporting the initial pathogen concentration. Carrington et al (1982) obtained decay rates of 3.4 log to 4.4 log for Salmonella duesseldorf in anaerobic digesters at 48°C. Godfree and Farrell (2005) reported reduction rates of Salmonella entiriditis and Salmonella dublin in compost of 6.18 log.…”

The effect of volatile fatty acids on the inactivation of Clostridium perfringens in anaerobic digestion

“…However, many types of pathogens can survive such treatments (Watanabe et al 1997;Carrington et al 1982) and elevated temperatures are often needed to improve the rate of sludge stabilization and to enhance the reduction of pathogens.…”