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Trant success in Dorset

Trant Construction has delivered a large inlet works replacement for Wessex Water at Poole STW.

Poole is a large coastal town in Dorset with a population of around 170,000. Sewage treatment is provided at a site located about 1km to the north in Poole Harbour.

The existing inlet screens and screenings handling equipment at the Wessex Water Poole STW was nearing the end of the asset life, causing frequent breakdowns that regularly resulted in effluent compliance problems due to carry over of rag /screenings in to downstream processes (egs BAF, UV disinfection).

The new works consisted of Band screens, compactors, new washwater facilities and Intelligent Motor Control equipment. The equipment would be housed in the existing, odour-controlled, covered inlet works building.

Trant Construction, which was appointed as an AMP5 framework contractor in 2010 to deliver a programme of works for Wessex Water, has completed a large inlet works replacement for the utility at the Poole facility.

The project, completed under budget and on schedule, was achieved through multidisciplinary teamwork and close integration on- and off-site between Trant Construction and the Wessex Engineering & Construction Services (WECS) project management team. Successful project delivery depended on the quality of liaison with the operations team as the inlet flows could not be diverted during construction.

Installation and commissioning had to take place whilst ensuring the works remained fully operational at all times. A detailed changeover programme was generated and agreed with all parties.

Key Partners on the £2.4M project were:

 WECS, Wessex Water – project management, design management, operations interface, ICA and commissioning

 WECS Civil – civil demolition, construction

 Trant Construction –principal contractor, carrying out the detailed design in-house; mechanical, electrical and ICA. It was also responsible for procurement, installation and commissioning of the M&E installation.
– Trant Systems Electrical (TSE) – designing, manufacturing, installing and commissioning the new inlet works and screenings handling Intelligent Motor Control Centre 

John Calnan, contracts manager for Trant Construction, says: “We are delighted to have been fully involved with the AMP5 delivery team on this interesting and challenging project. Timescales were tight and maintaining flow to full treatment was critical to Wessex throughout the duration of the project. We achieved the client target date through excellent teamwork, sheer hard work and a can-do approach on site.”

A new Intelligent Motor Control Centre (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 TSE, Trant Construction’s own MCC panel manufacturer.

The Poole STW site had recently undergone a complete Siemens S7 400H control and monitoring system upgrade, therefore the new control equipment was required to interface directly with this.

The panel consisted of 100A incomer, three 2.2kW VSD Screen drives, three 7.5kW Reversing DOL Compactor drives, two TP&N 32A feeders and an ICA section containing Siemens remote I/O PLC interface, ABB Flow metering, and Pulsar Level Control. The Screen drives, Compactor drives and PLC I/O were connected utilising dual Profibus networks with active terminations to the existing Inlet works MCC. TSE working closely with the WECS ICA & Automation team to deliver the project to the client’s requirements.

The existing plant screened incoming flows with six Andritz 1,500mm wide 6mm aperture 1D screens, situated in three channels, separated in each by a 720mm wide concrete plinth.

Three Ovivo CF200 6mm aperture 2D Band Screens, with a design capacity of 1750l/s per screen, were installed to replace the existing screens, working on a duty / assist / assist basis. By isolating one channel at a time two screens were brought offline, decommissioned and removed.

The replacement screen was then installed in the existing channels which were made wider by the removal of the dividing wall between each pair of old screen channels. Due to load restrictions of the overhead beam crane, the screens were manufactured and delivered to site in two halves to allow erection and installation in-situ.

The replacement screen was then commissioned and performance tested, before moving to the next channel. Using Intelligent Siemens Variable Speed Drives, each new screen operates at two different speeds depending on the flow into the works, 34Hz (normal operation) and 50Hz (high flow operation). Flow monitoring is via a Pulsar ultra twin differential system.

Each screen has a Washwater feed to the internal spray bar and its associated launder channel, each launder channel directs flows to one of two compactors via a manual hand stop arrangement.

Three Kuhn KWP 400/800 wash press units including a hopper were installed at an intermediate mezzanine level. Each screenings handling unit is sized to handle 8m3/hr of screenings. Each screenings handling unit is provided with a discharge pipe to deposit screenings into the receiving skip at ground level.

The new launder troughs are arranged such that, during normal operation, screenings from each inlet screen are transferred to its dedicated screenings handling unit. However, in the event of a screenings handling unit failing, screenings may be diverted to an adjacent screenings handling unit by manual operation of horizontally mounted handstops in the launder troughs.

Each of the screenings handling units is provided with an inlet hopper to allow connection of one or more launder troughs. Each compactor also requires direct Washwater feed at a minimum pressure of 2.5bar; each compactor has its own dedicated local control panel. A duplex dual basket filter has been 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 three inlet screens and associated launder troughs, three screenings handling units and a number of hydrants on a ring main. The washwater system is designed to achieve a duty at the washwater set outlet of 35l/s @ 41m. The set consists of 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.

Trant oversaw the production of operation and maintenance manuals via Arlosh; these are available online and on CD to the operational and maintenance team for ease of access and provide a simplified route to drill down to information required without having to leaf through multiple folders.

Specialised graphics were also produced providing an overview of the inlet works in an easily digestible format for anyone without the detailed knowledge of the workings of the site.

Topic: Contractors , Treatment
Tags: wastewater treatment , water companies , water treatment

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