United Utilities has gone out to tender with a contract for construction of a new wastewater treatment plant at its existing Davyhulme treatment works – estimated value is between £123 million to £170 million, depending on the solution which is finally selected.

The Contract comprises the design, procurement, construction and commissioning of a new process treatment facility to achieve a lower final effluent (FE) ammonia consent at Davyhulme Waste Water Treatment Works (WwTW), Manchester

The works proposed for the Davyhulme AMP5 water framework directive (WFD) contract are an alternative solution to that required under an earlier Contract Notice (2012/S 97 162177). United Utilities has reserved the right to select either solution – the company said that interested parties would be notified as soon as reasonably practical which solution is selected.

Depending on the type of treatment process selected there may also be an ongoing requirement for the contractor to provide an extended period of training and / or assistance to optimise plant performance.

Davyhulme is United Utilities largest WwTW, treating wastewater from a population equivalent of about 1.1 million from the Greater Manchester conurbation. Davyhulme discharges its final effluent (FE) to the Manchester Ship Canal. The current FE consent is 20mg/l BOD, 30mg/l SS and 3mg/l NH3 (on a 95 %ile basis) and 75 % overall COD removal across the whole WwTW. The future consent will be 20mg/l BOD, 30mg/l SS and 1mg/l NH3 (on a 95 %ile basis) and 75 % overall COD removal across the whole WwTW.

The future FE consent is driven by Davyhulme's inclusion in the National Environment Programme as a result of the receiving Manchester Ship Canal being designated under the Freshwater Fish Directive - the revised consent will also meet the requirements of the Water Framework Directive.

Due to the strategic importance of Davyhulme WwTW as a part of the company's wastewater treatment facilities, United Utilities said that no unauthorised disruption to the existing plant would be allowed throughout the construction and commissioning process - the operation and throughput of the existing plant must be maintained at the current level throughout the construction period.

Key elements the contract is expected to incorporate include:

• Design, construction and commissioning of the new works.
• Primary settlement tanks;
• Activated Sludge Plant;
• Final settlement tanks.
• Mechanical thickening of surplus activated sludge;
• Odour control;
• Works to ensure compliance with the requirements of the EPR permit;
• Internal site roads within the new treatment facility areas;
• New site access road from the new treatment facility to a new site entrance;
• Construction of a new primary sub-station building;
• Modifications to existing site high voltage electrical infrastructure;
• Ancillary and accommodation works - site drainage and return;
• Electrical instrumentation control and automation and a distributed control system;
• Inlet works (FTFT only (optional);
• Refurbishment of the retained existing ASP stream (optional).

Current operation at Davyhulme

Davyhulme WwTW consists of two parallel treatment streams (ASP1 and ASP2) downstream of a common inlet sewer. Both ASP1 and ASP2 streams have preliminary screening and grit removal, primary settlement prior to non nitrifying aeration tanks (provided with surface aeration) and final settlement. The flow split to ASP1 and ASP2 is 45 % and 55 % respectively.

The effluent from the ASP1 and ASP2 final settlement tanks is treated together in a common tertiary biological aerated flooded filter (BAFF) for nitrification, from which final effluent (FE) is discharged to the Manchester Ship Canal. The consented WwTW flow to full treatment (FTFT) and dry weather flow (DWF) are 714Ml/d and 340Ml/d respectively. The site is in an odour sensitive area and has two odour control plants serving the inlet works, ASP1 primary settlement tanks and aeration tanks and raw sludge treatment and storage.

Sludge treatment at Davyhulme is currently undergoing refurbishment and expansion as part of the Sludge Balanced Asset Programme (SBAP) project to treat raw sludge up to 91 000 tonnes dry solids sludge per annum.

Sludge is subject to digestion pre-treatment by thermal hydrolysis followed by mesophilic anaerobic digestion. The digested sludge is either dewatered, using centrifuges and exported to land or transferred to the United Utilities Shell Green incineration facility via the Mersey Valley Sludge Pipeline.

The sludge treatment operations are managed via a permit under the Environmental Permitting regulations - a major variation to the existing licence will be needed to incorporate the new process treatment facility.

Deadline for expressions of interest is 7^th January 2012. Click here to download the full tender document. You must be registered with Waterbriefing (which is free) in order to access the tender.