Digesters as Heat Storage – Energetic Assessment of Flexible Variation of Digester Temperature

Abstract

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.

Digesters as heat storage: Effects of the digester temperature on the process stability, sludge liquor quality, and the dewaterability

“…The concept of using digesters as seasonal heat storage in temperate climates has already been considered from an energy perspective by Steiniger et al (2021). In addition, the objective of this study is to assess the effects of digesters operated at temperatures between 33 and 53 C on the biogas quantity and methane content, process stability, sludge stabilization, sludge liquor composition, and sludge dewatering based on full-scale process data and additional lab-scale experiments.…”

“…Digester temperature is one, if not the most, relevant aspect of anaerobic sludge stabilization in water resource recovery facilities (WRRF). Beyond the main objective of sludge stabilization, digesters can contribute to a holistic heat management at WRRFs by acting as seasonal heat storage, increasing the digester temperature to store excessive heat and decreasing the digester temperature to compensate heat deficits (DWA, 2014; Steiniger et al, 2021). Hubert et al (2019) and Loidl (2020) reported seasonal adjustment of the digester temperature between 33 and 53°C for three WRRFs in Central Europe, suggesting that no inactivation of methane‐producing bacteria occurs at temperatures in the mesophilic and thermophilic range when temperature gradients are kept to a minimum.…”

Exploring Anaerobic Digestion from Mesophilic to Thermophilic Temperatures—Operational and Microbial Aspects

“…The operation at different temperature levels between 37 and 53 • C enables the use of digesters both for sludge stabilization and, from an energetic perspective, as a seasonal heat storage. The operation mode involves an increase in the digester temperature with excessive heat in summer to compensate for heat deficits occurring in winter by decreasing the digester temperature [34]. Thus, this is suitable for WRRFs in temperate climate zones with distinct seasons and (heated) digesters with insulation according to the state of the art.…”

“…The investigations in this article are an essential part of the research in the operating concept 'digester as heat storage', in which the digester temperature fluctuates within mesophilic and thermophilic temperatures during the year, as described in [34]. Thus, this article aims to describe the AD process for digesters fed with sewage sludge from a municipal WRRF in Germany at temperature levels between 37 and 53 • C in the context of the process performance, process stability and microbial community structure.…”