High-rate anaerobic co-digestion of agro-industrial wastes combined with ammonia recovery and biogas purification

Screened dairy manure, pressure-sterilized animal by-products, and cheese whey were co-digested in a Plug Flow Reactor (PFR) over a period of 150 days. The mixture was characterized by TCOD = 70±8 g/L, SCOD = 26±6 g/L, NH4-N = 1.5±0.3 g/L, TSS = 20±5 g/L, VSS = 18±6 g/L, TS = 33±5 g/L, VS = 26±2 g/L. The PFR was constructed from plexiglass (20 L working volume) and the operational temperature was maintained at 36±1oC. The PFR process was stable even under a hydraulic retention time of 3 days, corresponding to an organic loading rate (OLR) of 22 g/Ld. Effluent COD remained low (5.6±1.4 g/L) while VFA concentrations were negligible (< 0.5 g/L as COD). The biogas production rate from the PFR ranged from 2.6 up to 7.3 L/Ld. The anaerobic digestate was characterized by high ammonia content (1.7±0.5 g/L); therefore air-stripping was chosen for the effective removal of ammonia. Hydrated lime as a slurry was utilized for the necessary pre-treatment step (pH-raising and the reduction of residual particulates), due to its fast reaction and low cost. Moreover, effective clarification of the anaerobic digestate was obtained at an elevated pH (≥11.5). A temperature of ≥45 °C was also needed for efficient ammonia removal. The final effluent was neutralized by CO2 absorption through biogas injection in a scrubber. Concurrently, the biogas was upgraded since its methane content increased substantially, while H2S was completely removed.