The application of psychrophilic digestion for municipal sewage treatment

Used in practice biological wastewater treatment systems under aerobic conditions are characterized by high energy consumption and a significant sewage sludge production. Anaerobic wastewater treatment differs from conventional aerobic treatment. The absence of oxygen leads to controlled conversion of complex organic pollutions, mainly to carbon dioxide and methane. Anaerobic treatment has favourable effects like removal of higher organic loading, low sludge production, high pathogen removal, biogas gas production and low energy consumption. Psychrophilic anaerobic treatment can be an attractive option to conventional anaerobic digestion for municipal sewage (and wastewaters with industrial effluents) that are discharged at moderate to low temperature. Most reactions in the biodegradation of organic matter require more energy to proceed at low temperatures than at a temperature optimum of 37ºC. However, same reactions, such hydrogenotrophic methane production, acetate formation from hydrogen and bicarbonate, require less energy. In this work, an upflow anaerobic filter for municipal sewage treatment was used. The investigations were carried out in laboratory scale. The sewage treatment process was operated in psychrophilic conditions in the temperature 15-19°C. The value of hydraulic retention time in anaerobic bioreactor was 48 h. A real municipal sewage was taken from the treatment plant, operated in a full scale. The COD values of raw sewage were between 380 and 895 mg O2/L. The sewage used was additionally polluted with ammonia nitrogen average 81 mg NH4+/L, phosphate - 27 mg PO43-/L as well as sulphates 69 mg SO42-/L. The investigations comprised of three series of different organic loading rates: series 1 - 0.19 kg COD/(m3·d), series 2 - 0.32 kg COD/(m3·d) and series 3 0.26 kg COD/(m3·d). Depending on values of sewage/organic loading rates 46-72% of COD were removal at psychrophilic conditions. The presence of ammonia nitrogen and sulphates did not have a negative impact on the process of organic matter degradation. Anaerobic treatment was effective in removing biodegradable organic compounds, but mineral compounds (NH4+, PO43-, S2-) remained in high concentrations. Therefore, these compounds require removing by an additional post-treatment step