Waste Technologies of Australia - Water Recovery & Management

Electro-Dewatering (EDW) - Minimising the Cost and Environmental Impact of Treating Sewage

	- Snapshot - EDW
	- The Problem
	- Electro-Dewatering
	- Advantages

The Problem
Sewage handled by a sewage treatment plant (STP) typically comprises only about 0.1% solids. The STP separates and treats the water so it can be returned to rivers or the ocean, or reused in agricultural or industrial processes. The remaining solids comprise sewage sludge (also referred to as "biosolids"). Each year, STPs around the world produce millions of tonnes of sludge.

To minimise the environmental impact of disposal of sludge and offset the cost of operating STPs, wastewater authorities have devised ways of reprocessing this sludge so that it can be put to good use e.g. as fertiliser or compost.

However, the high water content of the sludge produced by most STPs - typically about 85% - means that the transport and storage adds significantly to the cost of reprocessing.

The movement of large volumes of sludge has an environmental cost. In Sydney Australia, for example, sludge is transportedin large trucks (10-15 tonnes) from STPs located in coastal suburbs across the city for 50 km to reprocessing sites on the western extremity in large.

Some of the sludge is then transported for hundreds of kilometres to farms and tree plantations for reuse as soil conditioners and fertilisers. These journeys create greenhouse gases, contribute to noise pollution, consume fossil fuels, and add to the cost of operating the STP.

Various methods of increasing the solids content of sewage sludge have been tried with little success. For example, thermal drying of sludge is costly because of its high energy consumption, and centrifuges have been unable to reach the desired dewatering levels.