Survival evaluation of bacterial indicators and viruses during thermal drying of sewage sludge in a greenhouse

ABSTRACT The aim of the study was to evaluate the thermic drying of sewage sludge in agricultural greenhouse, in order to inactivate bacterial and virus, and to evaluate the behavior of somatic coliphages as indicator of these organisms. Each volume of excess sewage sludge discarded from UASB reactor, drained in a drying bed for 14 days and directed to thermic drying treatment in greenhouse corresponded to a batch (B) of sludge. Between February and November 2015, nine batches (L1 to L9) were treated for an average of 13 weeks. Most batches kept the pH between 5.0-6.0. After about 60 days of treatment, all batches showed humidity below 10%, except B3 and B4 (10-25%). The time required to reach 90% of total solids ranged between 40-50 (B1 to B7), 60 (B5, B6) and 85-100 (B2, B3 and B4) days. On average, levels of E. coli reduced to <1x103 NMP/g TS after 50-60 days of treatment. The detection of somatic coliphages was low and eventual. Tests of inoculation showed rapid decay of virus (Sabin 1 strain poliovirus), but somatic coliphage Phi X174 was more persistent in the three temperatures evaluated (greenhouse, laboratory controlled 30 and 60°C). The thermic drying treatment of sewage sludge in agricultural greenhouse was presented as simple and efficient in obtaining granulated and dried product, and in reducing bacterial contamination levels given the parameter set as safe for agricultural use under the Brazilian law.