Gee kalic dosing replaces lime at Elan Valley WTW

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To achieve improved security and quality of reagent supply, Welsh Water’s operating partner, United Utilities Operational Services, has switched from hydrated lime to kalic dosing for pH control at its Elan Valley Water Treatment Works in Mid-Wales. Key to the change has been the installation of a larger capacity silo and dosing system, specially designed and constructed by Gee & Company.

Elan Valley WTW, situated near the Welsh town of Rhayader, is fed from the nearby Garreg-ddu Reservoir - better known as the source of water supply to Birmingham. With a treated capacity of about 10 mld, this local works is located in national parkland and is therefore subject to strict planning and operational controls. As part of the changeover requirement, it was essential that disruption to buildings and the surrounding site should be kept to a minimum.

Why make the change at all? The existing 5 tonne lime storage and preparation system had originally been installed to produce milk-of-lime solution but, in common with many such plants, had a significant maintenance requirement and was prone to stoppage due to blockages and inconsistent reagent strength. Additionally, Welsh Water had identified problems with infrequent lime deliveries into the existing small reception hopper, particularly during winter months.

The operators’ experience with kalic on other sites has shown it to be a more reliable agent than lime, with fewer operational problems. It was decided to improve pH control by making the change to kalic and, at the same time, increasing storage capacity.

Available space for the installation of new plant inside the Elan treatment works was, however, restricted. At an early stage it was agreed that removal of the redundant 5-tonne lime storage and preparation system would leave insufficient footprint for a conventional dosing skid at ground floor level.

Gee proposed a design that would provide a much larger capacity storage silo, re-circulation system and dosing package. This was achieved by elevating the dosing and re-circulation skids above the bund walls and allowed the dosing pump inlet pipe-work to be flushed with re-circulated product at all times, to stop settlement and keep the kalic solution in suspension. All dosing and re-circulation pumps, together with other vital components, are thereby located in areas that afford easy access for maintenance,

The design configuration provides a welded mild steel storage silo, 3.2 metres in diameter, with a conical base that is connected to stainless steel re-circulation pipe-work. It is a configuration that has enabled the kalic storage capacity to be substantially increased to 30 cubic metres. Within the tank, ultrasonic transducers provide analogue data of material level.

Four peristaltic dosing pumps are installed - paired in duty and stand-by configuration for dosing and re-circulation respectively - and providing metering flows within the range of 3 and 43 litres per hour.

The whole plant utilises remote telemetry control to ensure that the complete process – including the addition of chemicals and the process equipment – is run to produce high quality water. In addition, the new kalic system is also remotely linked, so that both the performance of the chemical metering pumps and the maintenance of kalic stock levels can be monitored and controlled.

In order to minimise work on site, Gee manufactured and factory-tested the complete silo unit and delivered it as a one-piece unit. It was designed to be craned into the treatment works through the existing roller shutter doorway – a precise fit that was carried out with only millimetres to spare! Once the new unit was commissioned, a replacement bund was constructed and the new kalic-dosed system for pH correction was brought on stream.

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