South East’s spring clean
Tackling the risk of Crypto. contamination at a water source in Hants
Thanks to South East Water (SEW) and PCI Membranes residents of Petersfield in Hampshire are secure in the knowledge their water supply, which comes from the nearby springs at Oakshott, undergoes the highest levels of filtration to remove all traces of Cryptosporidium oocysts.
During routine water quality monitoring SEW discovered Oakshott Springs was at risk of Crypto. contamination. As a result SEW instigated an extensive programme of sampling and evaluation to ensure the spring remained a useable water source.
As the result of comparative trials between various membrane manufacturers and system suppliers, Dynamco — a consultancy and subsidiary of SEW — found PCI Membranes to offer the most suitable and cost-effective method of achieving the required water quality. A turnkey contract was awarded accordingly.
Oakshott Springs supply more than 1Ml/d of potable water. The security of the supply has now been greatly upgraded by the installation of PCI Membranes’ ultra filtration (UF) system. At the heart of this new system are
The membrane acts as a Crypto. barrier ensuring the removal of oocysts. This means the process of continuous sampling, as previously required, can be ceased. Inevitably this has a large cost benefit to SEW, figures from the Drinking Water Inspectorate (DWI) currently place the annual cost of continuous sampling at approximately £45,000 per site.
PCI’s new system also reduces the turbidity levels making chlorination of the potable water more effective than it would be if coarser grades of filtration were being used. The system was designed to enable SEW to comply with the forthcoming turbidity regulations by producing a turbidity level of 0.1NTU or lower (<0.05 NTU).
The system utilises a membrane made up of 12,000 fibres encapsulated at each end and set in its own dedicated pressure vessel. The membrane is mounted vertically allowing for the optimal removal of solids from the membrane surface. The fibres, which measure 1.2mm in diameter, are manufactured from PES, which is chlorine and pH tolerant. The pore size in the fibre wall measures 0.02µm, ensuring not only the removal of Crypto. oocysts, but other bacteria and viruses. The feed water passes down the length of the fibre and permeate filters through the fibre wall. Once through the membrane water is driven into a permeate tank, from where it can be pumped to the local reservoir.
After surveying various locations a suitable site for the installation was found at Sheet WTW. Due to the dimensions of the system a great deal of creative design work was required to ensure the unit could be housed within an existing building. These size restrictions, coupled with the need to provide access for both operation and maintenance staff, and the desire to minimise disruption to the local community, provided the PCI Membranes’ design team with an opportunity to demonstrate its expertise in three-dimensional design.
The fully-automated membrane system has now been operational for more than six months. The programmable logic controller (PLC) is responsible for the cleaning and monitoring of the membrane system and alerts the local operations control centre to any problems. The telemetry system employs a Modbus unit to communicate via a dedicated telephone connection, allowing the operators to view the condition of the membrane system on remote terminals at the local depot in Bordon. This has significantly reduced the need for site attendance for routine checks as all of the required information, such as pressures, chlorine levels and turbidity can be viewed remotely.
A local human machine interface (HMI) also allows the operator to adjust the operating parameters of the system. Every hour the membrane system performs a low-volume clean water backwash to cleanse the membrane surface of any solids that may have collected. During times of low rainfall this procedure is performed every 80 minutes, offering a water saving of 25% over the normal regime. Once a month a chemical clean is carried out on the membranes to remove any build up of mineral or
These processes all employ conventional chemicals which are easily neutralised, thus lowering any impact on the environment. The chemical clean most frequently applied uses acid, at a concentration of 700mg/l, to dissolve calcium deposits, formed when limestone particles are removed from the spring water. The waste stream is neutralised with a caustic solution before being discharged into the sewer. The discharge of such wastewater is controlled by the PLC which monitors pH levels via a probe located in the waste tank. Once the desirable pH level has been reached the PLC will open a drain valve, releasing the water.
The whole system also incorporates an integrity test to ensure the membrane is fully functional and not allowing Crypto. oocysts to pervade the potable water. Instead of using chemicals the test utilises pressurised air which, in
addition to being a far less compromising agent for such tests, affords full automation of the system negating the need for consumable chemicals or reagents.
Building on the success of this project and PCI’s ability to deliver high standard products within short timescales, South East Water has
subsequently commissioned PCI to engineer two additional plants for nearby East Meon and Hawkley. Furthermore, SEW and PCI are currently in the process of negotiating another contract for a much larger site situated at Itchel, near Crondall in Hampshire, which is due for completion by September 2003.