Characteristics of oxidation-reduction potential, VFAs, SCOD, N, and P in an ATAD system under different thermophilic temperatures

One-stage autoheated thermophilic aerobic digestion (ATAD) can stabilize sludge to meet class A standard. In this study, batch experiments were conducted to investigate the characteristics of oxidation-reduction potential (ORP), volatile fatty acids (VFAs), soluble chemical oxygen demand (SCOD), and nutrients at different temperatures (45, 55, and 65 °C) in the one-stage ATAD. Results showed that the ORP values remained between approximately −350 and −120 mV in the primary 5-day digestion despite of excessive aeration in the digester, indicating that the aeration level could be decreased in an ATAD system to save energy. The pH exhibited a poor correlation (R² < 0.11) with ORP, indicating that some variables synthetically effected on pH. Digestion at 65 °C caused more intercellular compounds released because of the highest concentrations of SCOD, VFA, nitrogen (N) and phosphorus (P) in the solution among three digestion temperatures. The volatile suspended solid (VSS) removal rate for sludge at 55 °C was the highest among three digestion temperatures, reaching 41.4 % on day 13 and meeting Class A standard. VSS removal rate of 30.1 % under 65 °C did not satisfy the effluent standard because of the high soluble content of ammonium nitrogen. The majority of nitrogen and phosphorus left in the sludge supernatant under 65 °C could hinder its further use for land applications. Therefore, the optimal temperature of 55 °C is suitable for the ATAD process.