Researchers at London’s Imperial College have discovered an energy efficient method of transforming sewage sludge into a carbon-based water filter, a discovery that could save water companies up to £500,000 per year.
The process involves heating sewage to high temperatures, but companies have been reluctant to do this because of public concern over possible emissions. The new technique, however, involves sealing the heating process, which converts sewage to activated carbon, and recycling the resulting gases as a fuel.
First used for water purification by the ancient Egyptians, activated carbon absorbs organic materials or pollutants suspended within water or air passed through it.This form of carbon is porous and hydrophobic and ideal for applications such as solvent recovery or wastewater treatment.
It can be formed by removing water from household and industrial waste, before heating it in an oxygen free atmosphere, driving out remaining moisture and leaving a carbon solid, which is then heated to between 800 to 900 degrees C to complete the process.
Alternatively, an activating chemical such as phosphoric acid can be added to the sewage before heating, turning it into pellets with a large surface area.
During the production process, light hydrocarbon gases are released, and the Imperial College team have now discovered a way to collect these by conducting the process in a sealed chamber.
The gases can then be used as a fuel to drive the system, and as a result, not only does the process run off its own energy, it also produces excess heat.
This can be used for localised heating or electricity generation, leading to a net reduction in carbon dioxide emissions.
‘Sewage sludge disposal is a major problem at the moment,’ said research fellow Geoff Fowler, head of the project.
‘However, good quality active carbon is relatively expensive at around £1to £2 per kg for a good-quality product.
‘All water companies use the substance in filter beds, with each spending between £1m and £2m pounds on it every year.
‘By recycling this way I don’t see any reason why water firms could not reduce their bills by a figure of around 25 per cent.’
The college has now formed a partnership with specialist waste disposal firm Compact Power called the Compact Carbon Company, and plans to develop the system for commercial use.