Treatment plants meet EU incineration standards

New EU waste incineration directives are driving Yorkshire Water's plans to build four effluent treatment plants for sewage sludge incineration in the North East of England. Peter Ripley, managing director of ACWa Services in the UK describes the project and the process.

New sludge incineration plant in Knostrop, Leeds

New sludge incineration plant in Knostrop, Leeds

ACWa is currently working on a £2.5 million contract to design, build and commission four effluent treatment plants at Yorkshire Water's sewage sludge incinerator sites in Leeds, Bradford, Sheffield and Huddersfield. Designed to treat contaminated scrubbing liquors from four upgraded gas cleaning systems, the new effluent treatment plants are part of a Yorkshire Water initiative to ensure full compliance with the latest European waste incineration directives by December 2005.

New scrubbing systems, with integral liquor re-circulation facilities, are to be installed at all sites except the Knostrop plant in Leeds. The Knostrop plant will be converted from a single pass process to a circulating flow system for connection with the new ACWa effluent treatment plant.

When operational, the effluent treatment plants will satisfy European legislation EC 2000/76 limits, with particular emphasis on the removal of solids and mercury.

The order, placed by main contractor Earth Tech Morrison, includes the supply of automated equipment, mechanical and electrical services, instrumentation and controls. ACWa Services was selected as effluent treatment subcontractor on the basis of a number of similar projects successfully completed in the UK and Scandinavia.

Removal of contaminants
At each site, ash particles will be removed from the fluidized bed incinerator gases by an electostatic precipitator installed before the wet scrubbing system. Re-circulated liquor in the first stage acid quench scrubber will remove dust HCI and HF. Sulphuric dioxide (SO2) and the bulk of remaining dust and hydro halogens will be removed in the second stage packed column alkali scrubber.

To prevent any of the constituents adsorbed by the wash-water from increasing concentration to a level where it would interfere with the efficient operation of the scrubber, blow-down from each scrubber to the effluent treatment plant will be performed at a continuous rate of 4m3/h.

Effluent treatment
The new treatment plants will be capable of operating around the clock, 365 days a year, so as not to inhibit the continuous operation of the incinerators. Each plant will accept blow-down liquor from the scrubbing system, remove contaminants and provide secure discharge to the sewage works drainage system.

The various process steps will include effluent balancing, pH correction and precipitation, caustic soda dosing, up-flow clarification, sand filtration, discharge temperature and flow measurement. Centrifuge dewatering of removed sludge, together with discharge to a skip and the reception and safe storage of all treatment chemicals will be provided at each site.

At the start of the effluent treatment process, blow-down liquors from two scrubbing systems will be pumped to 22m3 and 38m3 capacity polypropylene balance/storage tanks installed at the inlet.

Both tanks will incorporate agitators to maintain fluidity of the effluent and prevent the settlement of solids, whilst monitors indicate liquid levels and control the forward feed pumps. Both tanks will normally operate at a low level to provide four hours storage capacity and effluent will be monitored for pH before being pumped through a common transfer line to the neutralization tank.

Within the polypropylene neutralization/reactor tank, process liquors will be monitored for pH and temperature and dosed with a caustic soda (sodium hydroxide) solution through activated valves. The process will be installed with a high energy mixing unit, sized to ensure full chemical reaction, allowing effluent pH to increase to optimum levels before gravitating to the precipitation tank.

Precipitation tank
In the precipitation tank, the pH will be further adjusted to 8.5 by the addition of sodium hydroxide solution to precipitate the metals. TMT 15, ferric chloride and a polymer will also be dosed into the precipitation tank. The effluent will react with the TMT 15 to precipitate the heavy metals and with ferric chloride and a polymer to flocculate the precipitant and other solids. A slow-speed agitator will be installed to flocculate solids in suspension.

The alkali to be used for the neutralization of the effluent is a solution of sodium hydroxide. Activated valves will be used to dose 47% solution from an existing extending ring main, according to pH levels, in the neutralisation and precipitation tanks.

To provide the correct dilution and make-up of the selected polymer, ACWa Services will install an automated polymer preparation system and two sets of dosing pumps dosing to the precipitation tank and the centrifuge.

Ferric chloride solution will be stored in a 1.5m3 'minibulk' system from which it will be dosed to the precipitation tank by dosing pumps. TMT 15 solution will be delivered in IBCs and pumped by the dosing pump into the precipitation tank.

Chemical and sand filtration
For the removal of solids, flocculated effluent from the precipitation tank will gravitate to a settlement tank, which, to improve efficiency, will be fitted with a fixed-bridge scraper. This tank will allow clarified liquor to overflow to the sand-filter feed tank whilst settled sludge is pumped to a sludge storage tank. From the sand-filter tank, clarified liquor will be pumped to a continuous sand filtration system. Cleaning of the sand-filter media will be achieved by a compressed-air lift, from which backwash water gravitates to the settlement tank prior to being reprocessed through the system.

Filtered water will gravitate to the discharge monitoring point where it is monitored for pH, temperature and flow. An auto-sampler will take either a flow-proportional or time-proportional sample.

Sludge handling
Process sludge is to be stored in a purpose-built tank installed with a thickener/stirrer and a level monitor to control the operation of the centrifuge feed pumps. The sludge tank will incorporate decant facilities to reduce the volume of sludge discharged into the centrifuge. The sludge will then be pumped to a centrifugal decanter where a polymer solution dosed into the feed pipe facilitates the dewatering process. The sludge cake will be discharged to skip for off-site disposal and all centrate will flow to a storage tank before being pumped back to the alkali balance tank.

Contact: ACWa Services
Tel: +44 1924 848702
Web: www.acwa.co.uk

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