Upgrading of a co-digestion plant by implementation of a hydrolysis stage

Abstract

An existing co-digestion plant needed to be rehabilitated after a 20 year operational period. This was planned to be done in sequence by halving the digester volume for a period of 1.5 years. The aim of the present study was to improve the performance of the halved co-digestion capacity by implementing an upstream thermal hydrolysis reactor or an ultrasonic pre-treatment of the substrates. The results of the ultrasonic bench-scale batch experiments showed that an ultrasonic pre-treatment of the co-substrates ‘municipal bio-waste suspension and excess activated sludge led to disintegration efficiencies of up to 51%. However, treating kitchen-waste and primary sludge in the same manner was not promising as the disintegration yields were rather low. The results of the hydrolysis bench-scale batch experiments showed that the optimal boundary conditions for the hydrolysis reactor were a hydrolysis temperature of about 42 °C at a retention time of 24 h. The results of the continuous two-stage experiments showed that it was possible to reduce the retention time in the second stage to about 24% and to increase the biogas yield to about 12.8 %, and the methane yield to about 28% as a result of the implementation of the hydrolysis reactor in the existing system. After the rehabilitation of the existing digesters it was possible to raise the daily substrate input to the two existing digesters from 312 to 495 m³ day⁻¹ with an upstream hydrolysis reactor volume of only 474 m³.