Residues from the dairy industry as co-substrate for the flexibilization of digester operation

Water resource recovery facilities (WRRF) can make an important contribution to increase the share of renewable energies in Germany. In this context, it is important to utilize unused digester capacities on WRRF. In addition, a demand-orientated biogas production could synchronize electricity demand and electricity generation and improve the overall energetic balance of the WRRF. As part of the project "Water Resource Recovery Facilities in interaction with the waste and energy industry: A German-Austrian Dialogue - COMITO," the influence of residues from the dairy industry on the digestion process was examined as well as the suitability for the flexibilization of digester gas production. Four reactors were fed with different amounts of flotation sludge from the dairy industry for several months. The difference in the feed resulted in organic loading (OLR) rates between 3.2 kg COD/(m³  day) and 6 kg COD/(m³  day). The reactors were fed with a daily shock load. The investigations showed that volumetric loads up to 4.4 kg COD/(m³  day) did not lead to an accumulation of organic acids. Organic loading rate of 6 kg COD/(m³  day) showed a significant accumulation of organic acids higher than 2,500 mg/L oHAc. Nevertheless, the reactor could be operated with a degradation rate of 71% with a corresponding biogas yield with a methane content of 71%. With increasing flotation sludge content, a higher concentration in ammonium of up to 2.000 mg/L NH₄ -N could be detected in the effluent of the digester. Despite higher phosphorus concentration in the flotation sludge, the concentration of PO₄ -P remained constant for all reactors fluctuating between 20 and 40 mg/L PO₄ -P. Dewatering worsened significantly with increasing levels of flotation sludge. PRACTITIONER POINTS: Main purpose of the research is to flexibilize digester operation on WRRF using flotation sludges from the dairy industry. Flexibilization of the digester using flotation sludge is possible up to an organic load of 6 kg COD/(m³  day). Higher NH₄ -N concentration in the effluent of the digester must be accepted when using higher amounts of flotation sludge. Phosphate concentration in the effluent of the digester remained on a low level despite higher phosphorus content in the flotation sludge. High levels of organic acids (mainly acetic acid) can be tolerated and can be recovered within a short time after reducing the load.