Articles & Case Studies

Water engineering specialist Trant Construction delivers £2.1m inlet works replacement scheme for Wessex Water under budget and ahead of schedule

Posted: Monday 27th May 2013

Installation of new inlet screens, screening handling, wash water and inlet well at Holdenhurst Sewage Treatment Works in Bournemouth, Dorset.

Water engineering specialist Trant Construction has successfully 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.

In summary, the new works comprised escalator screens, compactors, new washwater facilities and intelligent motor control equipment.

In 2010 Trant Construction was appointed as an AMP5 framework contractor to deliver a programme of works for Wessex Water.

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, said: “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.”

Technical details:

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 475 l/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

· Replacement of the concrete cover slab with pitched GRP cover fitted with access hatches

Construction phase:

Construction began in April 2012 with WECS Civils construction teams commencing 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, the M&E contractor, joined WECS on site in June 2012 when the coordination works began in earnest.

In order to keep the plant fully operational the 7 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.

Inlet screens spec:

Stream 1

Replacement of the existing step screens with 3 Longwood Engineering Fine Screens

(Escalator, 6mm 2D apertures)

Replacement of 6 screenings screw conveyors (2 per screen) with a combined screenings launder trough

Stream 2

Replacement of the existing coarse screens with 2 Longwood Engineering Coarse Bar Screens (Model - ER, bars at 19mm spacing’s)

Replacement of the existing step screens with 2 Longwood Engineering Fine Screens (Escalator, 6mm 2D apertures);

Compactors spec:

4 no. Kuhn KWP 300/1200 wash press units, each sized to handle 4.5 m3/hr of screenings & 2no. Kuhn KWP 400/1200 wash press units, each sized to handle 8 m3/hr of screening.

Each screenings handling unit is provided with a discharge pipe to deposit screenings into a receiving skip.

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 full connected, tested, and commissioned, then followed a period of process proving before the next screen removal could take place.

IMCC spec:

A new IMCC, based upon a form 4a Type 2 construction, utilising a certified distribution system (Busbars, rated at 400 amps, fault rating 30kA/1 second), 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 consisted of a 200A incomer, with surge protection, 5 no 1.5kW VSD screen drives, 5 no 1.5kW DOL brush drives, 4 no 5.5kW reversing DOL compactor drives, 2 no 7.5kW reversing DOL compactor drives,2 no 1.1kW DOL coarse screen drives, 1 no TP&N 63A feeders for high pressure washwater Panel, 1 no TP&N 100A feeders for low pressure washwater pump MCC, 2 no TP&N 16A feeders, 1 no 16Way SP&N distribution board for washwater valve actuators. 1 no 8Way TP&N distribution bBoard for knife gate valves and penstock actuators. An ICA section containing Siemens PLC, Pulsar level controllers and conditioning equipment for the monitoring of the plant status, flowswitches, pressure switches, etc. All drives were connected utilising dual Profibus networks to the PLC.
TSE worked closely with WECS ICA & Automation team to deliver the project to the client’s requirements.

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.


New FE P/S to feed all inlet works equipment on streams 1 and 2.
9000l Storage tank with duty/standby filtration to provide solids free wash water for both streams.
A Bolfitler 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 Grundfoss 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 @ 51m. The set consists of 3 no. 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, with all staff fully trained in the operation and maintenance of the new equipment.

All partners, WECS, WECS civils, Trant, Grontmij and Operations worked very closely from contract inception through to project completion to ensure a successful outcome.

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