Process reduces sludge from wastewater treatment plants

Biological treatment plus ozone can reduce the amount of sludge coming from wastewater treatment plants by a factor of 10.

The process was developed by the Water Research Institute (WRI) of the Italian National Research Council and was tested and scaled up as part the EU-funded Innowatech project.

Biological processes are said to offer the cheapest way to treat industrial wastewater but pollutants from industries such as leather, textiles and pharmaceuticals are not easily broken down by microbes.

WRI scientists have reportedly developed a technology where the microbial biomass, which breaks down wastes, grows mainly as granules and the process is known as SBBGR (Sequencing Batch Biofilter Granular Reactor).

According to a statement issued by, the granules are trapped in pores between plastic support material in a reactor and the microbes are stressed so less sludge is produced. This is because the microbes are not given suitable conditions to proliferate, so fewer microbes and less waste results.

Removing one kilo of wastewater through a biological system produces half a kilo of sludge, which then needs to be disposed of, according to Antonio Lopez, project co-ordinator at WRI. ‘With this technology, you produce only 50g of sludge.’ A treatment plant using this technology could be 10 times smaller than usual.

Complicating factors can arise, such as what goes into the reactor and biomass concentration, said Christoph Brepols, wastewater expert at Erftverband, the Erft river water association in Germany. If the sludge production is decreased by 10 times, he believes, this would not necessarily mean that the bioreactor volume can be decreased by the same ratio.

Integrating ozone with the reactor allows effluents from leather and textile processing to be treated. Ozone is a costly but powerful oxidiser and can break down most organic compounds. In the new system, ozone does not completely break down the pollutants but transforms them into more biodegradable compounds. This means less ozone is needed, reducing treatment costs.

‘Tannery wastewater is a sort of benchmark in the sector of industrial wastewater,’ said Lopez. This is because its composition is complex and difficult to treat.

At a pilot-scale facility for treating tannery wastewater, the final effluent from the process looked like tap water, the WRI researchers reported. In Italy alone, leather processing has a turnover of around €5bn (£4bn) — around 1,500 companies are involved and about 20,000 workers employed.

Sludge output in Europe is said to be on the rise; 5.5 million tonnes of dry solids came from plants in 1992, increasing to 10 million by 2007, and each tonne of dry solids costs between €350 and €750 to dispose of.