Giant sludge incineration plant opens in the Mersey Valley

Derek Workman, head of projects at USF Edwards & Jones, describes North west Water’s new mersey valley incineration plant. along with Thames Water’s Beckton and crossness incinerators, it is one of the UK’s largest.

Faced with new regulations banning sewage sludge disposal at sea, North West (NW) Water’s new Mersey Valley processing centre, which serves a population of 5.3M, had to be conceived on a grand scale.

Dewatering process
The first stage in the process is a group of four reception tanks, into which sludge is pumped from the Mersey Valley main. Here the sludge is mixed prior to conditioning for dewatering. Although only one of many similar plants designed and built by USF around the world, the dewatering facility is the largest single plant installed by the company.
A row of nine overhead beam presses share the workload equally. Each press is over 24m long and has 176 chambers. The choice of filter plates is noteworthy, as the plates are not only the largest produced, at 2m x 2m, but also use the most advanced design, derived from USF¹s long experience in both industrial and municipal waste processing systems.
The plates are made of steel reinforced rubber, moulded with replaceable rubber diaphragms. Based on a carbon steel core, the plates are completely encased in moulded rubber to prevent corrosion and resist abrasion or attack by acids and alkalis. The design of the filter plates and their pneumatically inflated rubber diaphragms allows ease of diaphragm replacement without the need to remove the plates from the press.
The operating sequence begins by drawing homogenised sludge from the reception tanks and pumping it into the filter presses. During this stage, the sludge is dosed with a chemical coagulant to improve its dewatering capability. The sludge enters the filter press chambers under pressure, filling every chamber. Pump pressure then rises as the flow rate decreases.
This is a two-stage process, which uses a progressive cavity pump to fill at high flow and low pressure, followed by a Willett positive displacement pump to consolidate at low flow and high press-ure. At the optimum point in th0e flow cycle, automatic valves close the sludge feed line to the filter presses and the squeeze cycle begins. An air inflation module, containing compress-ors and receivers, injects compressed air behind the filter plate diaphragms, squeezing further moisture from the formed cakes.
At the end of the cycle, the diaphragm squeeze air is exhausted, the press opened and the cake discharged onto conveyors, which transfer the cake to incinerator silos.

Incineration
The cake is incinerated in two fluidised bed furnaces, the waste heat from which is used to generate steam to produce electricity for use on the site.
The gas clean-up system was designed to meet German emission standards and therefore exceed the UK IPR 5.11 regulations. Emissions comply with IPC regulations and NW is confident it will meet the European Directive on air emissions, currently in draft form.


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