The slow degradation rate of sewage sludge in anaerobic digesters is due to the rate limiting step of sludge hydrolysis. The effect of ultrasound pretreatment on sludge degradability was investigated using ultrasound at a frequency of 31 kHz and high acoustic intensities. Ultrasound treatment resulted in raw sludge disintegration as was demonstrated by increase of Chemical Oxygen Demand in the sludge supernatant and size reduction of sludge solids. Semi-continuous fermentation experiments with disintegrated and untreated sludge were done for four months on a half-technical scale. One fermenter was operated as a control with a conventional residence time of 22 days. Four fermenters were operated with disintegrated sludge and residence times of 22, 16, 12, and 8 days, respectively. In the fermenters operated with identical residence times of 22 days reduction of volatile solids was 45.8% for untreated sludge and 50.3% for disintegrated sludge. The fermentation of disintegrated sludge was stable even at the shortest residence time of 8 days with biogas production 2.2 times that of the control fermenter. Due to ultrasound disintegration a better degradability of raw sludge was achieved that permitted a substantial increase in throughput.

Primary and secondary effluents of three waste water treatment works were analyzed for the size distribution of particle mass, chemical oxygen demand (COD), and phosphorus. Fractionation of suspended solids was done by 0.1-8.0μm polycarbonate filters and 32-125μm sieves, respectively. Primary as well as secondary effluents exhibited characteristic differences in the size distribution curves. Particle size distributions in terms of particle mass could be represented by a power law function. Automatic measurement of particle size distribution by a laser scanning device (CIS-100) was evaluated. Especially low concentrated waste water suspensions proved to be susceptible to aggregation during analysis. Therefore repeated short time analysis of subsamples is recommended.