Ultrasonic disintegration of biosolids – benefits, consequences and new strategies

Huge amounts of waste biosolids are produced from biological processes as waste water treatment, agriculture and food production. The volatile compounds could be utilised as a source for renewable energy in anaerobic processes. However, the anaerobic degradation is slow so that large fermenters are necessary. By ultrasonic disintegration the rate limiting hydrolysis of volatile solids is substituted resulting in a considerable intensification of the anaerobic biogas process. In case of sewage sludge the anaerobic digestion time could be 4-times reduced without losses in biogas yield. Even at reduced fermentation times the anaerobic degradation was enhanced. Therefore ultrasound disintegration has a strong potential to reduce fermentation times and hence to minimise the volume of new anaerobic digesters. This paper reviews new strategies and resulting benefits for biosolids treatment which are opened up by the application of new ultrasound technology. Introduction In biological conversion processes large quantities of biosolids are produced. Main biosolids sources originate from biological waste water treatment and agriculture. In Germany, the municipal waste water treatment is resulting in 60 Mio. m3 sewage sludge each year. In agriculture about 190 Mio. m3 of liquid manure are produced. Further amounts of biosolids are available from forestry and industrial food production. The biosolids predominantly consist of particulate microbial or plant cell material. Compared to forest and agricultural biosolids, waste water biosolids are mainly composed of highly putrescible volatiles. It is necessary to treat the raw sewage sludge biologically assuring a subsequent environmentally safe utilisation and disposal. The standard stabilisation process for waste water solids is the anaerobic fermentation. More than 80% of sewage sludge amounts in Germany are treated anaerobically (Esch & Krüger, 1999). In this process a net reduction of the biosolids mass and volume is realised. A portion of the volatile solids is microbiologically converted into methane and carbon dioxide. This biogas is used energetically. The