Psychrophilic anaerobic treatment of low strength wastewaters

The main objective of this thesis was to design a high-rate anaerobic system for the treatment low strength wastewaters under psychrophilic conditions.

Psychrophilic (3 to 20 °C) anaerobic treatment of low strength synthetic and malting wastewater was investigated using a single and two stage expanded granular sludge bed (EGSB) reactor system. The chemical oxygen demand (COD) removal efficiencies found in the experiments with synthetic wastewater exceeded 90 % in the single stage reactor at imposed organic loading rates up to 12 kg COD m ^-3day ^-1and a hydraulic retention time (HRT) of 1.6 h at ambient (10-12 °C) temperature using influent concentrations ranging from 500 to 800 mg COD dm ^-3. A malting wastewater with an anaerobically biodegradable COD of about 73 %, as determined in the batch bioassays at 15 °C was also used during single stage reactor operation at 16°C. The COD removal efficiencies averaged about 56 %, at organic loading rates (OLR) ranging between 4.4 - 8.8 kg COD m ^-3day ^-1and a HRT of approximately 2.4 h. At 20°C, removal efficiencies were approximately 66 % and 72 %; respectively, at OLRs of 8.8 and 14.6 kg COD m ^-3day ^-1, corresponding to HRTs of 2.4 and 1.5 h.

Psychrophilic (3-8 °C) wastewater treatment was further optimized at the laboratory scale two stage expanded granular sludge bed (EGSB) reactor in series, fed with a VFA mixture (500-900 mg COD dm ^-3). The COD removal efficiencies exceeded 90 % at 8 °C and 4 °C, at organic loading rates of 12 and 5 kg COD m ^-3day ^-1, respectively. Even at 3 °C, COD removal efficiencies averaged 80 %. High rate propionate oxidation was for the first time successfully achieved at such low temperatures. Applying this two stage EGSB system to malting wastewater in the temperature range 10-15 °C, gave removal efficiencies for soluble COD and for volatile fatty acids COD 67-78 % and 90-96 %, respectively, at an OLR between 2.8-12.3 kg COD m ^-3day ^-1and a HRT of 3.5 h. The second stage serves mainly as a scavenger of non-degraded volatile fatty acids (VFA) from the first stage.

The specific activities of the reactor sludge increased by a factor 3 after 300 days of reactor operation, indicating enrichment of methanogens and acetogens even at the low temperatures applied. The homoacetogenic, hydrogenotrophic and acetoclastic specific activities of the sludge at 10 °C, were 1.744, and 0.296 and 0.331 g COD g ^-1VSS day ^-1, respectively. At 30 °C the specific activities were 18.024, 2.732 and 2.204 g COD g ^-1VSS day ^-1, respectively. These high specific sludge activities can be attributed to the good and stable enrichment of methanogenic, acetogenic and homoacetogenic bacteria under psychrophilic conditions. Surprisingly, the optimal temperatures for substrate conversion of reactor sludge, after it has been exposed to long term psychrophilic conditions, were still similar to those of the original mesophilic inoculum. The results of EGSB batch reactor experiments revealed apparent half saturation constants of the acetate and propionate degraders in the range of 39-58 mg COD dm ^-3and 7-14 mg COD dm ^-3, respectively. For butyrate degraders, higher K _m values were found, i.e., 142-243 mg COD dm ^-3. The observed low K _m values are in agreement with the high removal efficiencies of the EGSB reactor during anaerobic treatment of the cold, low strength, wastewater.

By adapting the process design to the expected prevailing conditions inside the reactor, the loading rates 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.