Articles & Case Studies

Key water supply site back in service after complete redesign and rebuild

Posted: Friday 9th August 2013

In a £2.5m project, Trant Construction’s water engineering arm successfully completes redesign and build of Tucking Mill Water Treatment Works, Bath, on behalf of Wessex Water.

Trant Construction’s water engineering arm has successfully completed the redesign and rebuild of one of Wessex Water’s strategic water supply sites in the South West of England.

Tucking Mill Water Treatment Works, located on the southern slopes of Bath, Somerset, operates as a base load station treating spring water collected from various sources.

There is also an associated drought relief plant which treats river water to supplement supply when the spring yield is insufficient.

The works, 1km west of Monkton Combe village, are designed to treat a maximum combined site licence output of 10Ml/d. This is based on spring flows during summer of 3.5Ml/d combined with flows from the drought relief plant of 6.5Ml/d.

Existing disinfection equipment at Tucking Mill was nearing the end of the asset life. Following the introduction of a more stringent disinfection policy for stream and surface water treatment, it was subsequently decided to totally replace the existing contact tank and associated pumping equipment.

Trant, an AMP5 framework contractor for Wessex Water, was tasked to deliver the mechanical and electrical elements.

Work summary: demolition and removal of existing structures. The construction of raw water collection chamber, contact tank, treated water wet well, pumping station dry well, raw water transfer pumps, treated water relift pumps, associated motor control centres and controls, associated telemetry and controls and ancillary works, including pipework and cabling.

Key partners on the £2.5m project were: WECS (Wessex Engineering & Construction Services); Wessex Water (project management, design management, operations interface, ICA and commissioning)

WECS Civils (Wessex Engineering & Construction Services - Wessex Water’s in-house civil team) as principal contractor: civil demolition, construction.

Trant: detailed design in-house: electrical and ICA. Procurement, installation and commissioning of the mechanical and electrical installation.

Grontmij: outline, and detailed designers for civil, mechanical and electrical.

The project, completed on time and under budget, was achieved through close integration of partners both on and off site.

Scott Cherrett, Project Engineer for Trant, which is headquartered in Southampton, Hampshire, said: “This was a confined site and the tight schedule of works meant sub-contractors were in the same areas concurrently. Project programming and site liaison between construction teams was therefore essential to ensure a successful outcome.”

The water treatment works primarily supply the Bathampton Down Reservoir and, when required, Hayeswood Tower reservoir. There is also a manually valved connection to the West Ashton/Englishcombe rising main.

Tucking Mill operates to optimise the availability of water from the various sources and balance the treatment works flow to demand.

The treatment works preferentially utilises the gravity spring sources and then supplements these sources from a pumped Midford spring source as demand rises.

During periods of high demand and or low spring yield, additional flow will be made available from a drought relief plant.

Treated water is delivered to a number of receiving reservoirs supplied via relift pumps. The control system manages the pumps in order to meet demand and also to balance the flow from the raw water sources and maintain a stable flow condition through the treatment process.

Design criteria and process performance

Based on an assessment of the raw water quality, the disinfection process for the spring water and river abstraction water must achieve a minimum ECt (Effective Contact Time) of 15 mg.mins/l under all flow conditions.

The new contact tank design is based on a contact tank outlet chlorine concentration of 1mg/l. The new contact tank is designed to accommodate a maximum flow of 10Ml/d, which is the maximum site output. The super chlorination and dechlorination process is retained within the new works.

The new contact tank comprises a single compartment with a working volume of 235m3. This is based on an effectiveness value of 68.35%, a contact tank outlet chlorine concentration of 1mg/l

and 65% plug flow efficiency. The calculated t10 value at maximum flow condition is 21.95 minutes.

Phosphoric acid is dosed together with chlorine upstream of the contact tank via a common static mixer. The static mixer installed will achieve a CoV of 0.05 at the point of sampling over all predicted operating conditions.

The chlorine dosing and control system ensure that sufficient chlorine can be delivered to provide the disinfection required. The free chlorine residual is controlled to within +/- 0.1 mg/l of the setpoint.

The pumping stations upstream and downstream of the contact tank have sumps of sufficient working volume to balance the intermittent raw and treated water flows with the output of the variable speed pumps and so minimise pump start/stops and ensure flow rate changes are slow enough to allow accurate chemical dosing.

The raw water pumping station is a wet well arrangement of approximately 85m3 working volume to accommodate the variations in flows from the individual spring and GAC treatment plant sources.

Operating and control

Tucking Mill is a base load station and is currently operated continuously. Primarily the site feeds Bathampton Down Reservoir whilst also pumping to Hayeswood Tower as required.

The treatment works will run at a constant and maximum available flow rate as long as the demand from the network (available reservoir storage capacity) and availability of source water will permit. Under normal conditions up to 4.5Ml/d will be pumped to Bathampton Down to provide the base load.

Pumps to Hayeswood Tower will be operated on reservoir level control and will start/stop as required, delivering at a pre-selected rate of up to 1.2Ml/d. The raw water pumps are flow-controlled to match the combined output of the relift pumps.

In the event of low demand, as the reservoirs approach their maximum storage capacity, flow from the raw water sources will be sequentially reduced (stopping Midford pumps first). Flow will stop if there is no more available storage capacity at the receiving reservoirs. When the drought relief plant is not operating, the maximum theoretical output of the works will be 5.7Ml/d (4.5Ml/d to Bathampton Reservoir – limited by the capacity of the main, and 1.2Ml/d to Hayeswood Tower).

The historic total combined flow from the springs (assumed maximum yield) is 6.5 Ml/day falling to 2.1 Ml/day in dry weather.

Intelligent Motor Control Centre (IMCC)

A new IMCC was installed within the existing site control building replacing the existing redundant treatment works MCC.

The new MCC has been manufactured by a Wessex Water framework supplier to BS EN 60439 (Type 2 Form 4) with a bottom cable entry complete with direct online (DOL) and variable speed drive (VSD) starters.

The IMCC includes one Siemens PLC and one 12” colour graphical HMI. The IMCC also incorporates the relocated existing S500 telemetry outstation which provides signals to the Wessex Water Regional Operations Centre (ROC). The telemetry outstation communicates with the PLC controller via a Modbus serial link.

The IMCC control system includes a PACSnet Master Telemetry outstation for site to site telemetry signals to remote sites (reservoirs and Midford PS) via BT private wire connection. The site-to-site telemetry outstation communicates with the PLC controller via a Modbus serial link.

The system utilises “intelligent” starters running on a dual non-redundant Profibus network, where Duty/Standby pairs are on separate networks.

A number of existing MCCs and control panels (CPs) along with the new plant will be incorporated into the new process design.

The following existing control panels were interfaced with under this scheme such that the new control system could utilise the signals for the new process:

· Midford pumping station motor control panel

· Disinfection plant local controls and terminal marshalling box

· Chlorguard system local control panel (LCP)

· Chemical dosing plant LCP

· Standby generator plant LCP

· Drought Relief Plant Motor Control Centre (MCC3) (Phase 2)

· Whittaker Springs turbidity and divert CP

· Surge vessel control system LCP

· ASD panel

New control equipment was also installed and interfaced with, for the following equipment

· Remote PLC I/O and instrumentation control panel (dry well)

· Surge vessels control system LCP(s)

· Castle Spring and Tucking Mill Spring turbidity and divert control panels.

· Ventilation system LCP(s)

· Sampling LCPs

· Crypto sampler CPs

Main motors and drives

New Works MCC 6

Size

Type

Relift pump - 5 WA/Englishcombe

200.00

kW

VSD

Relift pump - 3 Bathampton Res

160.00

kW

VSD

Relift pump - 4 Bathampton Res

160.00

kW

VSD

Relift pump - 6 Hayeswood Tower

18.50

kW

VSD

Relift pump - 7 Hayeswood Tower

18.50

kW

VSD

Raw water pump - 1

9.00

kW

VSD

Raw water pump - 2

9.00

kW

VSD

.

Trant oversaw the production of the operation and maintenance manuals via Arlosh; the manuals were available digitally for draft checking, etc. This reduced waste as paper copies were not required.

Specialised layout drawings of the plant and process were also produced by Arlosh to show the complexities of the site infrastructure. This provided an easily digestible format of the workings of the site.

The completed works were handed back into the control of Wessex Water’s operations division in the summer of 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 closely from contract inception through to project completion to ensure a successful project delivery.




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