Anaerobic treatment of low-strength municipal wastewater by a two-stage pilot plant under psychrophilic conditions

A two-stage anaerobic treatment pilot plant was tested for the treatment of raw domestic wastewater under temperatures ranging from 21 to 14 °C. The plant consisted of a hydrolytic upflow sludge bed (HUSB) digester (25.5 m3) followed by an upflow anaerobic sludge blanket (UASB) digester (20.36 m3). The hydraulic retention time (HRT) varied from 5.7 to 2.8 h for the first stage (HUSB digester) and from 13.9 to 6.5 h for the second stage (UASB digester). Total suspended solids (TSS), total chemical oxygen demand (TCOD), and biochemical oxygen demand (BOD) removals ranged from 76% to 89%, from 49% to 65%, and from 50% to 77%, respectively, for the overall system. The percentage of influent COD converted to methane was 36.1%, the hydrolysis of influent volatile suspended solids (VSS) reached 59.7% and excess biomass was 21.6% of the incoming VSS. Plant performance was influenced by the wastewater concentration and temperature, yet better results were obtained for influent COD higher than 250 mg/l.