Temperature-phased anaerobic digestion (TPAD) to obtain Class A biosolids. A discontinuous study

Thermophilic–mesophilic-phase anaerobic digestion assays were carried out in three discontinuous reactors: R1, R2 and R3, which were in thermophilic conditions (55 °C) during 2, 4 and 6 days, respectively. After the first stage, the temperature was reduced to 35 °C, and at the end of the two-stages, the R2 and R3 systems achieved volatile solid reductions of 45% and 52%, respectively. Faecal coliforms and Salmonella spp. densities below 1000 MPN/g TS and 3 MPN/4 g TS, respectively, were also observed. The highest methane yield and the best dewaterability were obtained for R3: 193 mL CH4 g−1 VSadded and 3 min of time-to filter. For R2 and R1, 133 and 97 mL CH4 g−1 VSadded, and 4 and 9 min, respectively, were obtained. Although the best results were achieved maintaining thermophilic conditions during the 6 days; however, to take Class A-cataloguing product, for agrarian ends, 4 days operating under thermophilic conditions was sufficient.