Dewatering Alum-Based Sludges

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 industry.

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 disposal cost.

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 8780

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