Wessex upgrade aimed at sea change

The IT network is at the heart of the modern STW. A recent project to improve bathing water quality at Weston-super-mare demonstrates the levels of sophistication now required.

Wessex Water has opted for settlement lagoons instead of coarse screening.

Wessex Water has opted for settlement lagoons instead of coarse screening.

Azurix Services Engineering is behind the design and construction of a project to deal with sewage management problems at Wessex Water's Weston-super-Mare STW. A 2.7km tunnel was constructed to carry sewage and flood water from Black Rock pumping station in the town centre to a new STW at Bleadon Levels, on the banks of the River Axe. Wessex Water rejected coarse screening in favour of naturally disinfected settlement lagoons for solids separation and has an integrated sludge-drying facility. In partnership with English Nature, Wessex Water is developing the site as part of a sustainability project intended to promote conservation best practice.

The highly automated STW and remote pumping station are controlled by four MX32 Supervisory Control And Data Acquisition (SCADA) systems and ten A2AS modular programmable logic controllers (PLCs), supplied by Mitsubishi Electric. The PLCs are networked using the company's dedicated high-speed twisted pair network, Melsec Net B.

Automatic choice
After working together for five years, Wessex Water and Mitsubishi Electric have recently renewed a framework agreement for the supply of automation equipment, which includes PLCs, human machine interfaces (HMIs), SCADA and AC drives. This has enabled Azurix to develop standard designs and software solutions using Mitsubishi's Melsec MEDOC Plus IEC 61131-3 compliant PLC programming software and 'Function Block' technology of re-usable standard PLC code. Azurix is now investigating 'smart panel' technology based on one PLC per control panel with Profibus DP connection in multiple drive panel solutions.

Wessex Water's Weston-super-Mare project is part of an investment programme in response to the urban wastewater treatment directive, designed to improve the quality of bathing water. In common with many coastal towns, Weston-super-Mare needs careful management of urban wastewater, as well as land and storm drainage, to preserve its reputation as a seaside resort.

A key problem was that parts of Weston-super-Mare lie below the tidal high water mark. The solution was to pump sewage and wastewater from Black Rock pumping station through the new tunnel to the Bleadon STW. The tunnel runs 8-27m below ground and can carry up to 8000 l/s. During heavy rainfall it can store up to 11,000m3 of storm water.

Natural concerns
Bleadon STW is intended as an environmental showpiece, with the settlement lagoons relying on sunlight for disinfection. These, together with specially created wildfowl lagoons, wet meadow areas and reedbeds, provide new habitats for a wide variety of birds and form part of a nature reserve.

Originally, sludge from the settlement lagoon was treated and dried at Avonmouth before being sold as agricultural fertiliser. Developments at Bleadon include a submerged biological contactor (SBC) and bio-drying facility, allowing onsite sludge treatment.

Bleadon STW and Black Rock pumping station are controlled by ten Mitsubishi Electric A2AS PLCs, networked using Mitsubishi's Net B. Besides the bio-drying and SBC plant, other operations controlled include inlet pumps, inlet screens, dissolved air flotation (DAF) plant and return pumps. A standby arrangement of four MX32 SCADA systems is in place to enhance network reliability.

Bleadon STW returns up to 1000 l/s of treated wastewater, via two 700mm-diameter pipes cast into the tunnel benching, to Black Rock. From there it is discharged through the existing outfall into Weston Bay. The tunnel also carries a dual fibre optic link to handle PLC communications. The PLCs are equipped with Ethernet modules and use Mitsubishi's Net B locally and RS485 communications to link the control systems at Black Rock and Bleadon. Four Mitsubishi MX32 SCADA systems are used to control the entire operation.

Azurix involvement in the Weston-super-Mare project represents a milestone in the development of competitive tendering to Wessex Water. Asked why Mitsubishi automation equipment was chosen, David Barritt, principal engineer at Azurix said: "A key feature for us is IEC 61131-3 compliance. We do a significant amount of testing and modelling using simulation software, and we need to interface easily between our modelling software and products such as Mitsubishi's Melsec Medoc Plus programming software and MX32 SCADA package." He added: "We use a variety of special Wessex and Mitsubishi-developed PLC function blocks, for example, for secure communications with Black Rock Pumping station."

These communication function blocks were developed in conjunction with Mitsubishi and Wessex Water to allow data transfers using CCITT CRC error-checking routines. They were developed in PLC Ladder code then compiled as reusable function blocks. Although the principle of PLC systems using CCITT CRC routines is common, most PLC systems have to use expensive Basic modules. Mitsubishi, however, can use standard Ladder code in the PLC CPU. Other blocks are available from Mitsubishi, which the company believes can help reduce PLC code development time and errors.



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