Essex & Suffolk Water installed the world’s first natural reed bed system to treat ferric sludge produced during the production of drinking water at its Hanningfield Water Treatment Works (WTW). The utility company worked with ARM Limited and its Danish partner Orbicon between 2008 and 2010 to design and build a new reedbed system to treat the sludge.
Hanningfield WTW near Chelmsford in Essex produces 150 million litres of potable water a day, servicing a large part of Essex and east London. The existing sludge settlement lagoon system was almost full so a new treatment system was required to enable continued water production at Hanningfield.
The initial part of the treatment process involves the introduction of ferric sulphate into the pulsator clarifiers to remove silt and algae contained in the raw reservoir water. This generates between 2–4 million litres of a ferric-based liquid sludge per day. The sludge contains 99.8% of valuable water which, once treated and separated, can be returned back to the reservoir for future use.
Essex & Suffolk Water worked with ARM Limited and its Danish partner Orbicon to design and build the new treatment system.
Trials were initially undertaken to determine if ferric sludge was treatable in reed bed systems and how the reed beds would perform compared to a traditional centrifuge. Historically, reed beds have been used to treat a wide variety of wastewaters from sewage and industrial process waters but never for ferric sludge.
The test system was built with six basins, each of 20m2 with a design comparable to a full-scale plant, and planted with Phragmites australis.
Two years of extensive on-site trials showed that the reed beds could achieve up to 60% dry solids, and 99% of the valuable water held in the sludge could be returned to the reservoir.
Tests showed that the media filter had good filtration capability, the sludge residue on the surface cracked well indicating a good dewatering capability, and reject water was of better quality for return to the reservoir.
These results gave confidence to progress with a full scale system.
The new system was constructed in and consists of 16 basins, covering a total of 4.5ha.
The use of 16 reed bed basins allows loading and resting periods. When it is time to reload the first basin, the water will have passed through the vertical bed leaving a layer of dry sludge residue.
ARM has also installed a SCADA-controlled system linked to Essex & Suffolk Water’s software to manage the load to the reed beds to ensure optimal efficiency.
The £4.5 million construction project is the first of its kind, leading the way in helping to provide sustainable drinking water to the UK’s population.
This system reduces both energy and maintenance costs compared to traditional methods of dewatering involving centrifuges and belt presses and provides a long-term sludge treatment solution.
The use of chemicals, such as polyelectrolyte for thickening sludge, is eliminated and there is no requirement to remove the dried sludge from the system for at least 20 years, thus reducing operational costs.
It was estimated that the reed beds will save 70 tonnes of CO2 emissions a year. Further emission cuts are made as the waste solids will not have to be transported off-site on a regular basis.