The safe treatment of wastewater is one of society's most fundamental challenges. Wastewater treatment creates sewage sludge, the processing of which is highly energy-intensive, contributes to greenhouse gas emissions, and is still far from efficient.
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Bringing together 14 partners from industry, academia and government establishments around Europe, the three-year European Union (EU)-funded research project END-O-SLUDG started in January 2011. Its work is still in its relatively early stages, but already it has made notable progress.
In order to reduce the volume of sludge produced, one promising technique END-O-SLUDG scientists are investigating is Dissolved Air Flotation (DAF). Already in pilot trial phase, DAF exploits the tendency of certain materials to bind to air bubbles. By injecting a flow of micro-bubbles into wastewater, DAF offers the possibility of 100% removal of one of the key components of sludge, known as Total Suspended Solids - compared with 65% reduction achieved through existing methods.
A second technique to reduce sludge production focuses on removing the dissolved organic material which is the other main constituent of sludge. Using a variety of components which 'attract' these organic materials, rather as magnets attract iron filings, which can then be physically removed, it is hoped this technique will replace the traditional – extremely energy-intensive - method of using bacteria to biologically convert the organic materials to sludge.
Although reducing the volume of sludge is an important part of the solution, it can never provide the whole answer. Sludge will still be produced and need to be treated. The aim of treatment is to yield the energy the sludge contains, in the form of biogas which can be used for heating and power. But traditional treatment methods only release 45% of the potential biogas available. By contrast, END-O-SLUDG is researching improved treatments, using ultrasonic waves or fine grinding to break down the sludge cell structures, which could release as much as 80% of the potential biogas.
Another key aspect of the END-O-SLUDG project is the pioneering use of probiotic bacteria, added to the treated sludge, to eliminate the E. coli bacteria that it typically contains, and which often re-grow after initial treatment.
At the end of the treatment and processing operation, the consortium has also made significant progress in turning the final sludge-cake into safe, high-quality, easily transportable and spreadable fertiliser products known as Organo-Mineral Fertilisers (OMF). Pilot trials have already shown results either equal to or better than conventional mineral fertilisers in terms of agricultural benefits.
Finally, the consortium has set itself two further objectives which are less 'technical' but certainly no less important – on which it is actively working with regulators and national authorities. One is to reassure the public of the fact, and another to produce the necessary scientific data to demonstrate – according to the respective (national) procedures - that the final products for use on farmland pose no extra environmental concerns or risks to human health, reducing a long-held fear that arises from a lack of understanding of the practice.
The second is to have such sewage products declassified as a waste material. While they remain so classified, they are subject to strict regulations governing their use, something which naturally undermines their practical appeal to farmers. Declassification would represent a crowning achievement for the END-O-SLUDG project – completing the final step in the process from sludge volume reduction through to the transformation of the sludge that remains into viable, marketable and safe end-products that meet stringent agricultural and environmental standards.