Nielsen, S.1, Cooper, D.J.2, 1Orbicon A/S, 2ARM Ltd(free)
The most common methods of dewatering sludge are centrifugation which typically produces solids content of approximately 15% and filter pressing which can give solids content of 15 to 25%. In England there are over 200,000 tonnes/annum of water works sludge produced. Most of the dewatered sludge still goes to landfill at considerable expense. There are probably just 12 years of landfill capacity left in England. The Hanningfield Sludge Reed Bed System is a new system treating water works sludge. The system not only reduces the capital and operating cost, but also provides the site with an environmentally-friendly operational area. Trial beds have been monitored (2008 – 2012) to examine the dewatering processes of the iron sludge. Based on a total load of 1,275 tonnes ds /year and the test results a new system has been built with a process area of 42,500 m2 and put into operation in October 2012. This system represents an alternative solution to traditional methods of water works sludge treatment and reduces the need for deposition via landfill in favour of an ecologically-friendly, cost effective and clean method of converting the sludge to a product that can viably be deposited on agricultural land.
Keywords: Ferric sludge, Load, Reed Bed System, Sludge dewatering, Sludge Treatment, Water works sludge.
Treatment and disposal of coagulated settled sludge presents the greatest difficulty to the water industry in the UK today. It arises from the clarification and filtration processes at low solids concentrations using coagulants and polyacrylamide polyelectrolye and is then thickened and dewatered. Dewatering characteristics are poor and the sludge is of limited beneficial use. It has been estimated that there are over 200,000 tons of water works sludge produced on a dry weight basis in England per year. The most common methods of dewatering water works sludge are:
The water works sludge is generated when potable water is produced from Surface Water, Reservoir Water or direct abstraction of river water. For the purification process coagulants are used to remove impurities, Aluminum based coagulants such as aluminum sulphate or polyaluminum chloride and Iron based coagulants such as ferric sulphate or ferric chloride are the most common used chemical additions. The nature of the resulting sludge can vary widely from one works to another but all tend to be:
Some works are able to spray sludge which has not been dewatered onto neighbouring land at certain times of the year. Others pump it to a local waste water treatment plant which only transfers the problem of disposal to the waste water treatment plant. In general, however, most dewatered sludge still goes to landfill at considerable and increasing cost, but it has been reported that there are probably only 12 years of landfill capacity left in the UK.