The effectiveness of composting using stabilizing urban sewage sludge compounds

Abstract

The purpose of this investigation was to verify the effectiveness of composting in the sanitization of sludge from urban sewage (SS). The treatments (T) used consisted of SS mixed with sugarcane bagasse (SB), tree pruning residues (TP), poultry litter (PL) or grass clipping (GC) at a carbon/nitrogen ratio of approximately 30/1 and ratios of T1 (SS+SB), T2 (SS+SB+TP), T3 (SS+SB+GC), T4 (SS+SB+PL) or T5 (SS+SB+TP+GC). Temperature was measured daily for over 280 days of composting. Every two weeks, the compounds were analyzed for thermotolerant coliforms and viable helminth eggs using the multiple-tube and Yanko techniques; and once each month to identify Cryptosporidium spp. and Giardia spp. (oo)cysts using the sucrose and zinc sulfate centrifugal flotation technique and nested PCR followed by sequencing. The maximum temperatures measured in the thermophilic phase of the compounds ranged from 55°C to 64.8°C, and at day 280, weight reductions of 27% (T1), 48% (T2), 63% (T3), 66% (T4) and 64% (T5) were observed. The absence of fecal coliforms was observed from day 98 (T4), day 126 (T1), day 196 (T3 and T5) and day 210 (T2). All treatments resulted in helminth egg inviability and the absence of protozoan (oo)cysts. Only T4 and T5 were positive for G. duodenalis according to the sequencing analysis. In conclusion, high temperatures during the composting thermophilic phase made the use of pathogens required by legislation unfeasible; therefore, it is important to investigate the viability of protozoa in compounds to ensure a safe final product for human and animal health.