Effect of Organic Loading Rate and Temperature on Dewaterability of Sewage Sludge at Anaerobic Mesophilic and Thermophilic digestion

Abstract

Sewage sludge is a by-product of the wastewater treatment process. Dewatering of the sewage sludge in the wastewater treatment plants (WWTPs) is a major challenge due to the large amounts of residual sludge that are produced. Anaerobic digestion is commonly used in WWTPs to stabilize sludge produced in primary and secondary treatment and reduce their volume for final disposal. In recent years, research and development have focused on improving the dewatering process to reduce subsequent sludge handling and transport costs. To achieve this, optimization of the temperature and organic loading rate (OLR) in anaerobic sludge digestion at mesophilic and thermophilic conditions were studied in the present study. The influence of anaerobic digestion process temperature at mesophilic (35°C) and thermophilic (52°C) temperatures and three levels of OLR of 0.72, 0.94 and 1.3 kg VS/m3 /day was investigated on the dewaterability using the Time-to-Filter (TTF) parameter. The lowest TTF values under mesophilic and thermophilic conditions were obtained as 200 and 450 s, respectively, at the lowest OLR level (0.72 kg VS/m3 /day). Dewaterability was kept mostly similar at mesophilic level but worsened significantly at thermophilic level and OLR increases. The TTF at 35 and 52°C, respectively, were 230 and 600 s at OLR 1.3 kg VS/m3 /day, much better dewaterability at mesophilic level despite higher VS content. The results showed that mesophilic operating level can provide lower chemical costs for dewatering and keep iths characteristics even at higher OLR despite higher volatile solid reduction in thermophilic digestion. Increase in colloidal substances due to enhanced hydrolysis may be a governing factor for the worsened dewaterability in the thermophilic digestion compared to mesophilic condition.