We managed over 700,000 tonnes of wastewater sludge in 2008/09 from our regulated business, and believe that sludge recycling to agriculture is the Best Practicable Environmental Option (BPEO) where available. This is in accordance with the EU and UK Government policy. However, to secure sustainable outlets in the North West of England for this sludge over the next 25 years we’re expanding our thermal destruction plant at Shell Green to provide a balanced strategy for sludge management. Consequently the amount of sludge to this disposal route will increase in the next few years.
The practice of recycling sewage sludge (or biosolids) to agriculture is a highly regulated and monitored activity. During 2008/09 we disposed of 100% of our sludge in a compliant manner.
This year we have refurbished the existing streams of our thermal destruction plant at Shell Green. This is part of a larger project to expand the capacity of the plant. Over the next few years our sludge balanced asset programme will install advanced sludge digestion technology at our Davyhulme Wastewater Treatment Works, saving some 32,000 tonnes of carbon dioxide equivalent. You can read more about the sludge strategy in the case study below.
We are keen to use this waste as an energy source. This year alone we generated 92.9 GWh of electricity from combined heat and power (CHP) plants at the wastewater treatment works which we operate in our regulated business. The energy from thermally disposed sludge will also be recaptured and used to generate electricity. This technology is also being used at our wastewater operations in Bulgaria. You can read more about this in the case study below.
We have been trialling a new piece of equipment on our Dwr Cymru Welsh Water contract which can treat sewage sludge without the use of chemicals and produces a dry sludge with lower odour. The electric osmosis dehydrator (ELODE) process originated in Asia and has been tried on a couple of sites with encouraging results. The process also uses less energy than current treatment methods – and because it reduces the volume of sludge transported, it has a double benefit for carbon emissions.
Water Treatment Sludge
Impurities removed by processes at water treatment works, and the chemicals used to remove them, make up the constituents of water treatment works sludge. Water can be removed from this sludge to form cake for recycling to land or use in brick manufacture. Alternatively the sludge can be transferred to wastewater treatment works via sewer or tanker. This year our regulated business recycled 43,604 tonnes of water treatment works sludge to agriculture and 3,547 tonnes to brick manufacturing.
Grit and Screenings
Two other waste products from wastewater are grit and screenings. Grit consists of coarse particles of sand, glass, ceramics etc which settle out during treatment. Screenings are rags, plastics and sanitary items removed at the start of the wastewater treatment process. Screenings are difficult to recycle because of their varied nature and are a key focus area for the future. This year we disposed of 21,418 tonnes of grit and screenings from our regulated business of which 10% was recycled to land.