Welsh Water chooses to go with the flow
In April 2000 the expansion of Welsh Water's Nash STW was completed. Sited on the banks of the River Usk, a tributary of the Severn, Nash serves the whole of Newport courtesy of a new tunnel and pipeline connecting the western half of the town. The project is of vital importance to Welsh Water, which aims to improve the coastline of the Severn estuary between Chepstow and Newport. The work represents an investment of £89M and will eventually take wastewater not only from the population that borders the Usk but also, via another pipeline, from Chepstow's coastal strip.
The development at Nash also represents the largest single UK application of a sewage pumping technology which does away with variable speed motors by matching pump output to varying inflow rates. At Nash this is very important, because flows vary greatly, with peak storm flow predictions of 285Ml/d. The site's treatment capacity has doubled, and with flows as low as 34Ml/d, inverter drives on the pump motors were the only alternative to the new system.
Engineers at Hyder decided on the Prerostal pumping system after seeing it in use at a small works in the Forest of Dean. "The unique design of Prerostal was found to be beneficial when pumping sewage," explained Hyder Consulting's Lyndon Williams. "The pumps coped with various flow rates and never blocked. Subsequent installations for Welsh Water, at Kymin and Cog Moors, also worked well so we took a look at them for Nash." A benefit of Prerostal pumps discovered during the civil engineering work undertaken at Nash, was that pre-rotation systems could be used as a replacement for Archimedian screws, yet take up a fraction of the space. The inlet works used six Hidrostal Prerostal pumps to introduce raw, unscreened sewage into the works. Two 400mm, guide-rail mounted immersible pumps, each fitted with 110kW motors, were installed in each of the three channels which previously housed Archimedean screws.
Each pump adjusts its output to match the inflow rate, regardless of the flow volume entering the station. This is achieved by rotating the incoming flow using a specially designed GRP basin in which the pump is housed. Each pre-rotation pump cell incorporates a weir and entrance channel to allow a continuous flow to arrive in the basin. Flow matching is achieved by the pumps thanks to a combination of basin design, the pump's impeller and the weir configuration. As the level of the incoming sewage drops, the basin and weir configuration increases the rotational speed of the fluid. The rotation has the effect of reducing the apparent speed of the pump's impeller. The lower the fluid level, the higher its rotational velocity, resulting in a lower pump discharge volume.
Welsh Water wanted to retain the existing inlet works structure, since renewing the old Archimedean screw pump building would have been costly. However, the company also wanted to increase the head at this point. The only way of achieving this using conventional pump technology would have been to construct deeper sumps at the inlet pumping point, which would also have substantial civil engineering costs. By adopting the Prerostal method, no new civil work was needed apart from installing the basins.
Robert Williams, Hidrostal's general manager, explained why Prerostal was developed: "Water companies are very interested in least whole-life cost. This means examining ways to reduce the money spent on installation, which is significant for an Archimedean screw, and the running costs, which can also be high if sophisticated motor control is required for the pumps."
The six inlet pumps, which include one standby unit, use self-cooled motors which rely on circulating oil to dissipate heat. Since water cooling is not required, the pumps do not need to be submerged. They also utilise the screw centrifugal impeller system designed to resist blocking. The Nash STW takes water from a network of existing and new sewers. The Newport tunnel alone, which brings untreated sewage from the soon-to-be abandoned Caerleon works is 6km long and will make redundant 19 existing sewage outfalls which currently discharge into the Usk. Seasonal variations, long periods of dry weather followed by storms, and the strategic importance of Nash mean blocking by debris needs to be avoided. The impeller design copes well with rags, and the Prerostal basins assist in the automatic removal of floating and settled solids normally present when variable speed pumps are used - thereby keeping the sumps clean. The large free passage of the pump installed at the inlet station, a feature of the single spiral vane impeller, allows solids of up to 180mm diameter to pass.
Hyder was keen to involve a single contractor for all of the pumping systems on the site. Hyder favours supply and installation contracts, in which the equipment manufacturer takes overall responsibility for the design, building, installing and commissioning aspects of the job. Hidrostal and Hyder worked closely in the early phase of the site development to ensure each process stage was catered for with the correct pumping system. In all, 17 Hidrostal pumps were commissioned, apart from the six Prerostal units at the inlet works.
Four more 400mm Prerostal pumps, with vertical mounting totally enclosed fan cooled (TEFC) 55kW motors, provide sufficient head at the intermediate pumping station to lift the flows to the new primary settlement tanks. These, like the other units, are controlled via a Profibus fieldbus network linked to programmable controllers and a supervisory control and data acquisition (SCADA) system.
Further down the process, the return activated sludge (RAS) and surplus activated sludge (SAS) flows are controlled using inverter-driven Hidrostal submersible screw centrifugal pumps. Inverter pumps were selected because Welsh Water wanted to control the flow rates at which sludge was pumped forward. This system meant that, irrespective of the fluid level in the sump, flows can be controlled to suit the process requirements.
At the Nash works, thermal drying has been adopted for treating the sludge, a three-tank storage system is used. Tank one contains primary sludge and tank two contains surplus activated sludges. These are thickened in a separate process and the resulting thickened sludge is stored in tank three.
Welsh Water wanted to ensure efficiency of the continuous process was maximised while energy consumption was kept to a minimum. Hidrostal pumps on the primary sludge tank and surplus activated sludge tank, fitted with energy-efficient 15kW motors, ensure the contents remains well mixed and no stratification occurs. On the third storage tank two pumps keep the thickened sludge well mixed and help to provide a consistent forward feed into the sludge treatment centre.
All of Nash's pumps are fitted with an externally adjustable liner which allows the impeller/liner clearances to be maintained to the optimum setting, which greatly aids high pump efficiencies.
Before the Nash upgrade, wastewater discharging into the Usk accounted for almost two thirds of Newport's wastewater. Now, this untreated water is delivered to Nash, ensuring compliance with the urban wastewater treatment directive. Six new storm outfalls have been built, designed to cope with storm overflows without polluting the river, while a new pumping station at Measglas in the west will feed water through a pumping main into the new western pipeline, before finally discharging at Nash. With all this water arriving at the treatment works, the Hidrostal pumps are certain to be put to the test.