Mesophilic Two-Phase Anaerobic Digestion of Excess Sludge Pretreated by Ultrasound and Lime

The effects of combined pretreatment of simultaneous ultrasound (40 KHz, 50 W) and alkaline (lime, dosage of 560 mg/L) were examined on the subsequent mesophilic (35plusmn1degC) two-phase anaerobic digestion. The influent organic loading rate (OLR) of the process was held at about 1.1plusmn0.2 g VS/L-d, and the total hydraulic retention time (HRT) was 20 days. The results showed that the pretreatment increased the amount of soluble COD, ammonia and total phosphorus of supernatant. The ammonia concentrations in two phases were further enhanced during the anaerobic digestion. However, there was no significant phosphorus release due to the addition of lime. The volatile solids (VS) reduction increased from 29.6% to 40.8% after pretreatment. The specific methanogenic activity (SMA) and specific INT-electron transfer system (ETS) activity were enhanced in two phases. The settlement performance and dewatering ability of sludge became poor after anaerobic digestion.