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

Abstract

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.

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

“…From the COD balances, specific methane yields, COD degradation rates and OLR were calculated to describe the performance of the lab digesters. In Table 2, The COD degradation rates varied between 62.6 and 64.1% in period I and between 63.4 and 67.0% in period II, with the highest values at 47 • C. Values of specific methane yields, methane contents and COD degradation rates correspond with values reported in the literature [1,2,39] and complement the previously published data in [10].…”

“…Thus, steady-state conditions and acclimatization to each temperature level are assumed. More information about start-up and digester operation can be found in a previous article by [10]. However, digesters were operated at each temperature level for more than two years before the data evaluation and sampling presented here.…”

“…Contradictory results of mesophilic and thermophilic AD, like specific methane yields and content as well as process stability (partially with different process conditions such as hydraulic retention time (HRT), organic loading rates (OLR) and constancy of the temperature) possibly indicate that neither of the temperature regimes might be ideal. Only a few publications report constancy of specific methane yields, methane yields and process stability of the AD process of sewage sludge in between mesophilic and thermophilic temperature regimes [6][7][8][9][10].…”