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.
Origin:
World Congress on
Particle Technology 3, Brighton, UK, 7.-9. July, Proceedings p 77, CD No. 143