Dr. Johannes Müller and Prof. Dr. Jörg Schwedes
Institute of Mechanical Process Engineering
Sewage purification results in a large amount of organic sewage sludge, which consists mainly of microorganisms. The cell membranes of the microorganisms can be destroyed by mechanical disruption. One objective of the disintegration of sewage sludge is the improvement of dewatering. By destroying the cell walls the cell water is set free and can be separated mechanically when dewatering the sludge.
Wet milling in stirred ball mills and the disruption in high pressure homogenizers has been proven to be suitable for breaking up microorganisms that are only a few micrometers in diameter. The duration of grinding, the agitator speed and the size of the grinding beads of the stirred ball mill as well as the pressure of the homogenizer have been varied. The change of the sewage sludge’s characteristics and dewaterabillity have been examined.
Even for short disintegration times significant reduction of the mean particle size was found, because the flake structure of the sludge is destroyed. At a higher energy input the break up of cell walls can be reached. For the examination of the dewatering process the surplus sewage sludge had been centrifuged at high relative centrifugal fields. The disintegrated sludge leads to a more compact sediment and a higher concentration of dry solids than an untreated sludge. A high degree of the inorganic fraction of the sludge can be detected in the sludge cake whereas the centrate consists mainly of the inorganic fraction.
To obtain high concentrations of dry solids in technical dewatering machines the sludge’s have to be coagulated and flocculated. An optimal coagulation was obtained by the determination of the surface charge by measuring the streaming potential in the suspension. The coagulation of disintegrated sludge’s requires larger amounts of coagulants to neutralise the surface charge. Dewatering in a chamber filter press results in similar concentrations of dry solids both in disintegrated and in untreated sludge’s. By using a decanter the dry solids content of the dewatered sludge can be increased by a mechanical disintegration.
World Congress on Particle Technology 3, Brighton, UK, 7.-9. July, Proceedings p 77, CD No. 143