NWW is the last of the four sewerage undertakers currently dumping large quantities of sewage sludge at sea to come forward with proposals to comply with the dumping ban. Thames Water is building two incinerators in east London, Northumbrian Water is expected to put forward revised proposals for incineration after having its initial plans to burn sludge with industrial waste turned down last year (ENDS Report 214, pp 9-10 ), while Strathclyde Regional Council has rejected incineration in favour of sludge treatment and disposal to a variety of land outlets (ENDS Report 217, p 14 ).
Like all the sewerage businesses, NWW is projecting large increases in sludge production, primarily because of the higher standards of sewage treatment required by the 1991 EC Directive on urban wastewater treatment. Annual sludge arisings in its area currently amount to 125,000 tonnes, expressed in terms of dry solids, but the figure is expected to increase to 192,000 tonnes ten years hence.
Much of the increase will occur in the Mersey Valley as the clean-up of the Mersey estuary proceeds. Sludge generation in the area amounts to 64,000 tonnes per year, of which 10,000 tonnes are disposed of to agricultural land or landfill, with the remainder being dumped in Liverpool Bay. The figure is expected to reach 96,000 tonnes by 2001/2.
NWW's proposed sludge processing centre at Shell Green, near Widnes, will have a capacity to handle 86,000 tonnes of dry digested sludge per year. All of this will be pumped to the site via an existing pipeline network which collects sludge from the Manchester to Liverpool corridor and delivers it to Sandon Dock, Liverpool, for sea disposal.
The site is conveniently within 100 metres of the pipeline, but was selected from 50 candidates some 10 kilometres on either side of the pipeline corridor. Consultants Environmental Resources Management (formerly ERL) ranked all 50 sites on environmental grounds. An NWW spokesman said that Shell Green was not the best location in engineering and operational terms, but came out top on grounds of visual impact, communications and allocation for industrial development in local plans. The nearest homes are 500 metres from the site, which is adjacent to Fiddlers Ferry power station, two chemical plants and a waste transfer station.
The main facility at the centre will be a three-stream fluidised bed incinerator with a capacity to burn 53,000 tonnes of dry sludge annually. NWW intends to install only two of the streams initially, with the third expected to follow by 2001. Heat recovered from the plant will be used in a 3.5MW electricity generation unit.
The detailed design of the flue gas cleaning equipment has yet to be finalised, but NWW intends to achieve the air emission standards recently promulgated in Germany, which are considerably tighter than the minimum standards laid down by HM Inspectorate of Pollution (HMIP).
The company's current plan for the flue gas cleaning train is an electrostatic precipitator, followed by a three-stage scrubbing system to remove acid gases and metals, a reheat unit, and finally activated carbon injection to remove traces of dioxins and mercury.
Ash from the incinerator will be equivalent in weight to about 40% of the sludge dry solids inputs. It is likely to go to landfill, although NWW says it plans to investigate the possibility of converting it into building products.
The remaining 33,000 tonnes of sludge delivered annually to the site will also be dewatered, with about 21,000 tonnes going to landfill. Of the remainder, 9,000 tonnes will be applied to agricultural land, 2,500 tonnes will be used in land reclamation projects, and 500 tonnes will be co-composted with "green" waste delivered to civic amenity sites. However, these figures may change because NWW wants to keep its sludge strategy as flexible as possible, and will be looking to maximise sludge reuse in association with local organisations.
The company intends to submit a planning application to Cheshire County Council and the first of three applications to HMIP in July or August after completing local consultations on the project. It is hoping to avoid a public inquiry, and indeed may need to do so if the incinerator is to be operational by the time the sea dumping ban comes into force.