Occurrence, removal and seasonal variation of thermophilic campylobacters and Arcobacter in sewage sludge.

The presence of thermophilic campylobacters and Arcobacter was investigated in four types of sewage sludge taken from the treatment plant in Bologna (Italy): primary, activated, thickened and anaerobically digested sludge. Campylobacter jejuni and Campylobacter coli were more numerous during the March-September period and were found only in primary sludge (22.7%) with mean counts of 278 MPN/g dry matter and 1403 MPN/g dry matter respectively. Arcobacter butzleri were found in all types of sludge with frequencies of 80% in activated and thickened sludges and 41% in digested sludges. They were more numerous in the spring/summer period with peaks in April, May, June and September. They were less sensitive to anerobic digestion than fecal bacteria, probably due to their microaerophilic growth properties. However, since they are found in anaerobically digested sludges at mean values of 7649 MPN/g dry matter the land application of digested sludges may cause high risks infection.