Abstract Summary
Recent studies on anaerobic digestion highlight the benefits of two-phased systems, which enhance digestate stability and sanitation, increase organic loading rates (OLR), boost methanogen activity for higher methane production, improve volatile solids (VS) reduction efficiencies, and enhance digestate dewatering properties. This method employs a two-stage reactor system with a thermophilic first stage and a mesophilic second stage, optimizing conditions for acidogens and methanogens. The dual digestion pilot plant at Salamanca wastewater treatment plant (WWTP) features two continuously stirred tank reactors (CSTR) in series, operated semi-continuously with independent feed flow, biogas measurement, and temperature control (Figure 1). The first reactor, with a 1500 L capacity, operates at 55ºC with a 2-day hydraulic retention time (HRT), while the second reactor, with a 5000 L capacity, operates at 37ºC with a 13-day HRT. As the pilot plant is fed with WWTP sludge, the OLRs fluctuate throughout the year. For OLRs above or below 10 kg VS/m³d, these variations do not significantly impact the plant's performance. Despite variability in feed composition, the system demonstrates stability over time, both in terms of volatile solids reduction and biogas production and composition. The operation of the dual digestion pilot plant for 9 months showed a stable VS reduction over 45%, with a specific methane production of 0.5 m³ CH₄/kg VS removed at an OLR of 11 kg VS/m³d. The methane concentration in the biogas produced by the dual digestion (66%) exceeds that of the biogas at Salamanca WWTP by 7-10%. Moreover, the digestate met USEPA Class A standards for Salmonella and E. coli and complied with regulations for persistent organic pollutants (POPs) and heavy metals according to Spanish regulations. In conclusion, dual digestion demonstrated superior volatile solids elimination and increased biogas production, with microbiologically sanitized digestate, compared to conventional anaerobic digestion at Salamanca WWTP.
