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Dorset inlet work triumph

Wessex Water's Holdenhurst Sewage Treatment Works in Bournemouth has benefited from an inlet works replacement project carried out by Trant Construction.

WATER ENGINEERING specialist Trant Construction has completed a large inlet works replacement project at Wessex Water’s Holdenhurst Sewage Treatment Works in Bournemouth, Dorset, which serves 160,000 people. Replacement was required because the inlet screens and screenings-handling plant were aging, with reliability issues.

The new works comprised escalator screens, compactors, new washwater facilities and intelligent motor control equipment. The scheme, completed under budget and ahead of programme, was orchestrated through a multi-disciplinary teamwork between Trant Construction and Wessex Water’s in-house civils team Wessex Engineering & Construction Services (WECS).

It was recognised that a successful transition during construction would require close liaison with operations due to the inability to divert the incoming flows during construction.

Key partners on the £2.1M project were:

• WECS and Wessex Water (project management, design management, operations interface, ICA, commissioning)
• WECS – principal contractor, civil demolition, construction
• Trant Construction – detailed design in-house: electrical, ICA, procurement, installation and commissioning of the mechanical & electrical installation

• Trant Systems Electrical (TSE) – manufacture, installation and commissioning of a new intelligent motor control centre (IMCC) for the inlet works and screenings handling

• Grontmij – outline and detailed designers for civil, mechanical and electrical

Eamon Connolly, project manager for Trant Construction, says: "This was a potentially difficult project build, bearing in mind the plant had to be kept fully operational throughout the upgrade. It went well due to detailed advanced planning and close co-operation between project team partners."

Inlet flows to Holdenhurst are separated into two inlet works streams (stream 1 and stream 2). Full flow-to-treatment for streams 1 and 2 are 950 and 475l/s respectively, and on both streams the flow passes through coarse screens, grit removal and fine screens.

The AMP5 scheme improvement provided:

• New replacement fine screens and screenings handling for stream 1

• New coarse and fine screens and screenings handling for stream 2

• Incorporating duty/standby Kuhn screenings compactor units

• Wash water pumping station

• High pressure booster pumping package with automatic filtration

• IMCC - new control equipment to be located in the existing inlet works MCC building.

• Inlet PS wet well repairs. 

Repair/refurbishment work included:

• Isolation of the wet well and incoming flow diversion

• Repairs to damaged concrete, involving the removal of all the existing soft or contaminated concrete, reinstatement of the walls and base with sprayed concrete, and protection of the concrete with a chemical resistant coating to provide a 10-20-year design life.

• Repair isolating penstocks

• Structural strengthening of the wet well through the addition of an internal RC frame

Construction began last April with WECS Civils construction teams starting with clearing the site, advanced civil works and preparation of the existing works for conversion to accept the new equipment. Crucial to the upgrading process was construction planning to keep the plant fully operational throughout the upgrading process.

Trant Construction joined WECS on site in June when the coordination works began in earnest. To keep the plant fully operational the seven screens to be replaced were being taken out one at a time and in a sequence that meant each stream continued in use throughout the entire operation. Early advanced installation included the installation of the screenings compactors on the stream 2 fine screens as the original equipment had failed. The construction sequence required very close cooperation between WECS, WECS Civils, Operations and Trant so that each screen could be removed, channel modified and new screen reinstalled, all whilst the stream was kept online.

Advanced M&E works and the early installation of the new IMCC streamlined the removal and replacement of the screens. On completion of installation, each item of plant was fully connected, tested, and commissioned, then followed a period of process proving before the next screen removal could take place.

A new IMCC, based upon a form 4a Type 2 construction, utilising a certified distribution system, was designed, manufactured and installed by TSEk, Trant Construction’s own MCC panel manufacturer. The mechanical design process encompassed the client’s specification, WIMES 3.01 specification, BS7671 and the requirements of BS EN 60439 (& BS EN 61439) with regards IP rating requirements, clearances and creepages, protection against electric shock, temperature rise and mechanical operations.

The panel comprised a 200A incomer, with surge protection, five 1.5kW VSD screen drives, five 1.5kW DOL brush drives, four 5.5kW reversing DOL compactor drives, two 7.5kW reversing DOL compactor drives, two 1.1kW DOL coarse screen drives, one TP&N 63A feeder for high pressure washwater Panel, one TP&N 100A feeder for low pressure washwater pump MCC, two TP&N 16A feeders, one 16Way SP&N distribution board for washwater valve actuators, and one 8Way TP&N distribution board for knife gate valves and penstock actuators, and an ICA section containing Siemens PLC, Pulsar level controllers and conditioning equipment for the monitoring of the plant status, flow switches, pressure switches. All drives were connected utilising dual Profibus networks to the PLC.

Works continued on site from June through to October, which included the installation of new low pressure wash water submersible pumps into an existing final effluent chamber, the installation of a low pressure wash water delivery main and the installation of a high pressure wash water booster system.

A Bolfilter fully automatic backflushing filter type 6.19 duplex dual basket filter was installed to filter final effluent washwater to 0.5mm in two dimensions, before entering the washwater booster set break tank.

A Grundfos washwater set has been installed to supply the inlet screens and associated launder troughs, screenings handling units and a number of standpipes. 

The washwater system is designed to achieve a duty at the washwater set outlet of 16 l/s at 51m. The set comprises three booster pumps.  The pumps transfer final effluent from the break tank to inlet works equipment and hydrants connected to the washwater mains. 

The pumps are variable speed driven and operate on a duty/assist/standby basis, based on the pressure in the delivery main. An accumulator is utilised to pressurise the washwater main and limit the number of stop/starts of the booster pumps.

The completed works were handed back into the control of Wessex Water’s Operations Division in December 2012.

Topic: Contractors , Treatment
Tags: sewage treatment , construction

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