Anaerobic digester feed stream conditioning for increasing biogas production (Technical report 2)

1. Milestone Achieved In this second report we are presenting results on the characterisation of ultrasonicated Waste Activated Sludge samples and the resulting methane potential of pretreated sludge. We found that the total solid (TS) concentration has a significant effect on COD solubilization. At 1 %TS, we achieved 5,000 mg/L SCOD using up to 40,000 kJ/kg TS, while in the range 4-6% TS sludge, we obtained 8, 10 and 11 g/L SCOD at specific energies of 4800, 9615 and 12360 kJ/kg TS, respectively. These high SCOD concentrations were found to be important to increase the methane potential. In the first case (8 g SCOD/L using 4800 kJ/kg TS) it was found that 19% more methane was produced compared to the raw sludge and 85% of the additional SCOD generated by ultrasonication was converted to methane. For the two other specific energy tested (9615 and 12360 kJ/kg TS) there was an important lag-phase and the ultimate methane potential was similar to the first case. Similar trend was found with the BMP test on TWAS. It was found that the temperature increase during ultrasonication also played a role in COD solubilization. It was found that WAS contained proteolytic microorganisms able to produce proteases and these enzymes were active at thermophilic temperatures. Up to 11,000 mg/L SCOD were obtained using indigenous enzymes. It was found that the optimum temperature for the enzymes was 65°C and that only 5% of the sludge volume needed to be treated with ultrasound. The kinetic data revealed that these enzymes were most active within the first two hours