Water treatment washes up well in Bolton

Greater Manchester Waste's thermal recovery plant in Bolton is benefiting from a water treatment system which filters out impurities and aids cleaner emissions

Greater Manchester Waste, a company owned and controlled by the Greater Manchester Waste Disposal Authority, has just completed a multi-million pound improvement programme to its Bolton thermal recovery facility. An integral part of Greater Manchester's integrated waste management programme, the facility handles over 125,000 tonnes of Bolton's waste each year converting it into energy by incineration.

The heat from the combustion is used to produce steam in a 40bar water tube boiler - this drives a turbine and generates a maximum of 11.3MW of electricity, most of which is exported to the national grid. The low pressure steam leaving the turbine is condensed in a water-cooled condenser and then returned to the boiler.

The condenser is currently cooled by evaporative cooling towers, although plans are being considered for a district heating scheme that could eventually recover this heat to provide local residents with space heating and hot water, reducing consumption of fossil fuels even further.

Recovery does not stop there. Ferrous metals are recovered from the bottom ash from the incinerator prior to reusing much of it for construction products. Fly ash is disposed of as a hazardous waste although reuse opportunities are always being reviewed.

The water required for cooling tower make-up comes from the local River Croal but, to protect the heat exchange surfaces from fouling, the river water has to be filtered to reduce the suspended solids content to less than 10mg/l. The water treatment system, now six years old, was designed and built by ELGA Process Water.

Pump action
Pumps deliver up to 40m3/h of river water into a 15m3 break tank that provides storage for filter cleaning. A coagulant chemical is added to the flow to improve filtration and the water is then filtered using two double filtration in series units, each capable of handling 50% of the required flow. The double filtration in series is a two-stage, deep bed sand filter in which the first filter vessel provides a roughing duty and the second stage operates as a polisher.

The boiler requires about 3m3/h of make-up water with conductivity less than 0.2µS/cm and silica less than 20µg/l, and the deionisation plant that produces it is fed with town mains water. This is first pre-treated by duty and standby combined organic scavenging and softening units, each containing a layer of cation softening resin and macroporous anion scavenging resin.

This reduces the organics level at the same time as softening the water to prevent fouling of the downstream reverse osmosis unit. The two resin layers are regenerated at the same time with brine. The water is de-chlorinated by activated carbon filters and then the pre-treated water is passed through 5 micron filters and then on to a Compact III RC304HF reverse osmosis unit for primary deionisation. The Compact III uses thin film composite membranes to produce 4m3/h of permeate with conductivity less than 10µS/cm at a recovery of 75%.

The permeate is fed directly into a H060 CEDI unit. CEDI is short for continuous electro-deionisation, an electrically driven membrane process capable of producing deionised water with conductivity typically less than 0.2µS/cm. CEDI requires no regeneration chemicals because it is continuously regenerated by an electrolytic process.

The treated water from the CEDI unit is pumped to the boiler feed tank via C940 mixed bed polishing cylinders as the final stage of treatment, to ensure that the silica specification of less than 0.02 mg/l SiO2 is maintained at all times. Due to the light ionic loading on these cylinders, they are replaced relatively infrequently.

Quality output
The combination of reverse osmosis, CEDI and mixed bed polishing cylinders produces make-up water of around 0.06µS/cm that, as Greater Manchester Waste's assistant operations manager, Steve Robinson, points out, is "good enough for a nuclear power station".

The water treatment plant is built in containers that minimise site installation time. Constructing the plant in two standard 40' ISO containers, one for the double filtration in series system and one for the deionisation plant, meant that they could be completely prefabricated and tested at ELGA Process Water's manufacturing site. On site, all that had to be done was the connections to the water supply and drain. And the containers act as their own plant rooms.

As is the case with most boiler systems, the make-up plant is relatively small in comparison with the boiler capacity and, after a maintenance drain-down, it would take a several days to refill the boiler circuit. "ELGA Process Water has helped us there," says Robinson. "They hired us a trailer-mounted, mobile deionisation plant which filled the system ten times quicker than our make-up plant would."

ELGA Process Water
www.elgaprocesswater.com

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