Monitoring of bacterial and parasitological contamination during various treatment of sludge

Wastewater treatment results in the production of large quantities of sludg e. Agricultural use is an interesting alternative provided that the health risks are taken into account. In France, a decree (1998) defines "sanitized " sludge as sludge that has been treated so that pathogens (Salmonella, ent eroviruses. viable nematode eggs) Lire no longer detectable. The aim of thi s study was to determine the impact of various sludge treatment, on 2 types of pathogens (Salmonella and nematode eggs). We also assessed the levels o f micro-organisms that indicate Caecal contamination (E. coli, Enterococci, sulfite-reducing anaerobes spores). Various treatment processes were analy zed: 4 biological, 3 chemical, I heat, 2 storage and 4 sampling campaigns w ere carried out. Concerning the parasitological contamination of sludge at the entrance of all sites, concentrations of total nematode eggs and viable nematode eggs were highly heterogeneous (respectively, from 2 to 53 eggs/1 0 g DM and from 2 to 45 eggs/10 g DM) with viable eggs present in all sampl es, The total concentration of nematode eggs in the sludge was not greatly affected by treatment or storage, and was in the range < 1 - 66 eggs/10 g D NF However, the situation was different for mean viable egg counts. For fou r treatments, no viable eggs were detected. Mesophilic processes are ineffi cient at eliminating viable nematode eggs. Salmonella were systematically d etected in all sludge samples at the entry to the station, except for limed sludge intended for storage. At the end of four treatments Salmonella had been totally eliminated. For the fecal contamination indicators, the larges t decrease in E. coli contamination was observed following timing and after thermophilic treatment, The mesophilic treatments resulted in very small d ecreases in contamination. A similar pattern was observed with Enterococci. For the spores of sulfite-reducing anaerobe bacteria, with the exception o f heat treatment, which reduced contamination by 3.2 log units, the maximum decrease in contamination was 1.7 log. For storage of sludge, E. coli and Enterococci levels had decreased at first but then increased between 6 and 8 months of storage. (C) 2001 Elsevier Science Ltd. All rights reserved.