From December 1994 issue of Interface
During the past year the Division's dewatering group at North Ryde has
been working on finding ways of treating and disposing of alum-based
sludges which could offer substantial savings for the water treatment
Raw water is contaminated with many impurities that must be removed
before it is safe to consume. A particular problem is the suspended solids
- usually of inorganic clays and various organic compounds.
The most common treatment method is based on the use of alum to
coagulate the suspended solids particles. This resultant sludge, separated
from the cleaned water, contains around 0.1-0.5% solids and requires
further treatment before disposal.
There are many options for this, but one of the most common is to
further concentrate the sludge using thickeners and settling ponds,
sometimes followed by centrifuges or filters. From there the material can
be transported to a landfill site for disposal.
Alum-based sludges, however, are very fine and extremely difficult to
dewater. Conventional dewatering equipment typically produces very wet
sludges: a liquid content of 75% is not uncommon. Therefore a lot of money
is often spent on disposing of materials that are mainly water. In
locations where transport or landfill costs are significant, there is
considerable financial incentive to improve dewatering and hence reduce
The CSIRO group has been working for the last year on a collaborative
project with the University of New South Wales, under the auspices of the
CRC for Waste Management and Pollution Control, and has identified several
techniques for improving conventional pressure filtration including
electrodewatering and introducing innovative additive regimes.
Electrodewatering in which an electric current is passed through the
cake of a pressure filter to provide an extra driving force for moisture
removal, has produced a sludge with a final solids content of up to 40%.
New additive regimes introduced to straight pressure filtration and also
pressure filtration electrodewatering have led to further increases in
filtration rate and reductions in electrodewatering power consumption.
Electrodewatering is now being developed from laboratory to pilot-scale
membrane filter press.
A preliminary cost benefit study shows that the improvements achieved
in the laboratory could represent significant cost savings for operating
water treatment plants that are disposing of their sludges by landfill.
Of further interest is that preliminary tests using electrodewatering
to extend the work to wastewater treatment (sewage) sludges, has produced
results which are even more encouraging. Sewage sludges in many locations
face similar dewatering issues to the water treatment sludges and are just
as difficult to dewater. Also sewage treatment, because it presents a much
larger problem in terms of tonnage, could offer relatively greater savings
through improved dewatering.
The Division has excellent facilities for dewatering research and Chris
Veal, the project manager, looks forward to its expanded use on other
waste sludges as more companies become aware of the unit's potential.
Contact: Dr Chris Veal
Tel: (02) 9490 8770
Fax (02) 9490