Temperature-phased anaerobic digestion (TPAD) to obtain class A biosolids: A semi-continuous study

Regulations regarding the land application of sewage sludge require the performance of highly efficient treatments for pathogen reduction, such as temperature-phased anaerobic digestion (TPAD). Several semi-continuous experiments with different solid residence times (SRTs) have been made to find the best combination – TPAD 15/15, TPAD 5/15, TPAD 3/15 or TPAD 3/12. TPAD systems showed better performance in terms of VS destruction, gas production, sludge dewaterability, and process stability at total SRT of 15 days, compared with single-stage mesophilic or thermophilic digestion at the same SRT. Average VS reductions (VSR) using the overall TPAD 5/15 (78%), TPAD 3/15 (87%), and TPAD 3/12 (87%) were significantly higher than mesophilic control operating with an SRT of 15 days (40% VSR). The best results were obtained for TPAD 3/15 with 0.62 LCH4 g−1 VSr, faecal coliform 103 MPN/g TS, and Salmonella spp. 1 MPN/4 g TS. Although both TPAD 3/15 and TPAD 3/12 were efficient enough to obtain Class A biosolids.