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Anaerobic treatment of a volatile fatty acid (VFA) mixture was investigated under psychrophilic (3 to 8°C) conditions in two laboratory-scale expanded granular sludge bed reactor stages in series. The reactor system was seeded with mesophilic methanogenic granular sludge and fed with a mixture of VFAs. Good removal of fatty acids was achieved in the two-stage system. Relative high levels of propionate were present in the effluent of the first stage, but propionate was efficiently removed in the second stage, where a low hydrogen partial pressure and a low acetate concentration were advantageous for propionate oxidation. The specific VFA-degrading activities of the sludge in each of the modules doubled during system operation for 150 days, indicating a good enrichment of methanogens and proton-reducing acetogenic bacteria at such low temperatures. The specific degradation rates of butyrate, propionate, and the VFA mixture amounted to 0.139, 0.110, and 0.214 g of chemical oxygen demand g of volatile suspended solids−1 day−1, respectively. The biomass which was obtained after 1.5 years still had a temperature optimum of between 30 and 40°C.

Psychrophilic (2 to 20°C) anaerobic treatment of low strength synthetic and malting wastewater was investigated using a single and two module expanded granular sludge bed (EGSB) reactor system. The chemical oxygen demand (COD) removal efficiencies found in the experiments exceeded 90 % in the single module reactor at an organic loading rate up to 12 g COD dm−3 day−1 and a HRT of 1.6 h at 10-12°C ambient temperature using influent concentrations ranging from 500 to 800 mg COD dm−3. When a two module EGSB system was used at the temperature range 10-15°C, soluble COD removal and volatile fatty acids removal of 67-78% and 90-96% were achieved, respectively, and an OLR between 2.8-12.3 kg COD m−3 day−1 and a HRT of 3.5 h. The second module serves mainly as a scavenger of non-degraded volatile fatty acids (VFA) from the first module. The optimal temperatures for substrate conversion of reactor sludge, after it has been exposed to long term psychrophilic conditions, were similar to those of the original mesophilic inoculum. The specific activities of the sludge in the reactor increased in time by a factor 3, indicating enrichment of methanogens and acetogens even at low temperatures. By adapting the process design to the expected prevailing conditions inside the reactor, the loading potentials and overall stability of the anaerobic high-rate process may be distinctly improved under psychrophilic conditions. The results obtained clearly reveal the big potentials of anaerobic wastewater treatment under low ambient (10-12°C) temperature conditions for low strength wastewaters, very likely including domestic sewage.

Anaerobic wastewater treatment is an attractive and generally accepted technology for the treatment of various types of medium- and high-strength wastewaters. So far, this treatment technology is mostly applied at the mesophilic temperature range between 25 and 40°C. However, results of recent research conducted under both psychrophilic (< 20°C) and thermophilic (> 45°C) conditions, reveal that temperature is not a limiting factor in applying anaerobic treatment, provided the appropriate process design is chosen. Temperature has a considerable impact on various biological and physical factors of the anaerobic conversion process. For instance, the biogas production rate is reduced to a minimum at low temperatures, while it can reach extreme values under thermophilic conditions. In sludge bed systems, the biogas production rate determines the degree of mixing between the biomass and the wastewater and should, therefore, be considered in the process design. Other impacts of temperature are related to inhibition effects under thermophilic conditions and to a non-desirable accumulation of non- or partly degradable organic matter under psychrophilic conditions. Obviously, these effects may hamper the utility of the commonly applied single stage reactor systems. However, by adapting the process design to the expected prevailing conditions inside the reactor, the loading potentials and overall stability of the anaerobic high-rate process may be distinctly improved.