Regarding digesters, present guidelines assume that the temperature must be kept constant in order to achieve process stability and that there is a drop in gas production between 40 and 50 °C. Nevertheless, observations of full-scale application show that fluctuations in temperature between mesophilic and thermophilic environments is indeed possible without any loss in biogas production performance. This would be particularly favourable because the digester can thus act as a heat storage. In order to validate temperature fluctuations on full-scale digesters the data of two digesters from different wastewater treatment plants (WWTPs) with high temperature fluctuations were analyzed. In addition, chemical oxygen demand (COD) balances for different temperature ranges were conducted in order to evaluate the process stability. The results show that fluctuations between mesophilic and thermophilic conditions can be achieved without a decrease in biogas production. Increasing the temperature above 50 °C leads to an increase in organic acid concentration as described in the literature. Nevertheless, the total concentration of organic acid was still at an uncritical level below 500 mg/L. The COD balance shows no significant difference between 38 °C, 44 °C and 51 °C. The rate of temperature fluctuations per day specifically seems to be a main factor for process stability rather than temperature itself.

A flexible digester temperature enables holistic storage of surplus heat and minimization of heat deficits on water resource recovery facilities (WRRFs). Heatsaving potentials, performance as heat storage, and emissions of CO 2 are considered in four scenarios with increasing flexibility of digester temperature for a model WRRF with 500 000 PE (population equivalent). In comparison to a digester temperature at 37 °C all year, the scenario with seasonal variation of average digester temperatures between 32.5 and 43.6 °C has the greatest annual heatsaving potential and increase of stored heat. Further, the ranges of digester temperatures indicate a great performance as heat storage.

Variation of the digester temperature during the year enables the operation of digesters as seasonal heat storage contributing to a holistic heat management at water resource recovery facilities. Full‐ and lab‐scale process data were conducted to examine the effect of the digester temperature on process stability, sludge liquor quality and the dewaterability. Both full‐ and lab‐scale digesters show a stable anaerobic degradation process with a hydraulic retention time of more than 20 days and organic load rates up to 2.2 kg COD/(m3·d) at temperatures between 33 and 53 °C. The concentrations of soluble COD and ammonium‐nitrogen in the sludge liquor digested at 53 °C are 2.6‐ to 5.8‐ times and 1.3‐times higher, respectively, than in the sludge liquor digested at 37 °C. Dewatering tests show an enhancement of the dewaterability but a clear increase in the polymer demand at increased digester temperature.

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.

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