Financial support: CAPES and State University of Maringá - Brazil.

Keywords: anaerobic digestion, enteric bacteria, enteric protozoa, sewage sludge, stabilisation.

This study was designed to evaluate the performance of a slow rate anaerobic digester in treating secondary sewage sludge received from one local municipal wastewater treatment plant. The digester was fed by secondary sewage sludge without any previous thickening. A series of three independent batch experiments was investigated at an operation time of 60 days. The total solids (TS) in the influent sludge contained a percentage of organic matter of 59, 63 and 54%, a concentration of volatile suspended solids (VSS) of 23.7, 29.2 and 27.8 g L^-1 and a chemical oxygen demand (COD) of 51.8, 32.9 and 65.7 g L^-1 for the three experiments, respectively. The operation of anaerobic digestion was stable, with no noticeable scum or foaming problems. The COD reduction in each experiment reached 29, 21 and 45% in the sludge and 95, 85 and 82% in the supernatant. The microbial indicators were surveyed by sampling the sludge throughout the digester operation and counting the number of bacteria in the sampled sludge. Counted bacteria included the total culturable, the total and fecal coliform groups, Pseudomonas aeruginosa and fecal streptococci. The percentage removal of the indicator bacteria was higher for fecal streptococci (99.9%) than for coliform bacteria (96.3%), which in turn was higher than for P. aeruginosa (95.6%). Parasitological analysis was also performed on multiple sludge samples by determination of protozoa and helminth eggs. Protozoa (Eimeria and Entamoeba), helminth eggs (Ascaris, Trichuris, Toxocara, Hymenolepis) and mites were detected in the influent sludge, and particularly among the helminth eggs, only Trichuris was detected in the effluent sludge.