Removal of helminth eggs and fecal coliforms by anaerobic thermophilic sludge digestion

Anaerobic digestion of two types of waste sludge was applied in order to assess the suitability of thermophilic conditions for the stabilization of organic matter and removal of fecal coliforms and helminth eggs. Feeding sludge was taken from an activated sludge municipal facility (BS) and from an enhanced primary treatment municipal plant (EPT). As an accompanying experiment, mesophilic digesters were also operated. The four digesters (M1, M2, T1, T2) had a 5 litre volume and an egg shape. A highly stabilized material was obtained at both temperatures with BS type of sludge, taking the reduction of volatile fraction of suspended solids (%R_VSS) as indicator (84% for M1 and 74% for T1). In general, EPT sludge was a more difficult substrate, if compared with BS sludge; thermophilic condition was better adapted than mesophilic for this kind of sludge. Satisfactory reductions on counts of fecal coliforms and helminth eggs were achieved under thermophilic digestion for both types of feeding sludge. T1 digester, fed with biological sludge, removed fecal coliforms below 1000 MPN/gTS and helminth eggs down to 0.28 HE_Larval/gTS, at an HRT of 20 days. As a general conclusion, anaerobic thermophilic digestion may be an appropriate option for sludge stabilization, in order to meet EPA Class A biosolids final disposal regulations. However, further research is needed in order to consistently remove helminth eggs and fecal coliforms from waste sludge at shorter hydraulic retention times.