Water & Wastewater Treatment Magazine
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22 | OCTOBER 2017 | WWT | www.wwtonline.co.uk The Mayflower treatment process BoB Taylor OPERATIONS DIRECTOR, DRINKING WATER SOuTh WEST WATER "As a business, we are more than 80% reliant on surface water sources. Waters draining from the moorlands, and upland catchments with agriculture and forestry, tend to be quite highly coloured, and can have high levels of dissolved organics and cryptosporidium risk. "This site is no exception. It takes its water from three sourc- es, the River Tamar, the River Tavy and the Burrator Reservoir, and the water quality can vary quite widely with weather condi- tions. In times of high rainfall, there can be significant turbid- ity and discolouration. So we wanted a treatment process that would be able to cope, in a robust way, with a whole range of circumstances. In particular, we wanted to be more secure and robust in the way we dealt with cryptosporidium. And we also needed the plant to be 'future proof ' to some extent, because these are not 10 or 20-year investments, they need to last for 60 years plus. So that's quite a challenging process to design, given the nature and variability of the water sources, and our desire to maintain our position as one of the best companies for water quality in the UK. "We established a relationship with a Dutch water company called PWN, which has a substantial R&D function, including facilities to test new processes. It was through that relationship, and our involvement in an EU-funded research project which was looking at innovation in drinking water treatment, that we came across this process. "It is entirely different to the conventional surface water treatment process used around the world, which consists of coagulation, followed by rapid gravity filtration, sometimes GAC filtration, and then disinfection. "In this case, we start with a process called SIX – suspended ion exchange – and then we have inline coagulation. It is inline coagulation, because rather than being a whole treatment stage, it is literally just an injection of chemicals ahead of the mem- brane process, so the chemical consumption will be very much lower. "That leads on to ceramic membrane filtration, which is what people o—en refer to as the barrier within the treatment process. The membrane pore size is something like one ten thousandth of a millimetre, so it can remove virtually everything including bacteria, cryptosporidium, giardia, many harmful organisms are removed at that stage. So that means that the work that the disinfection stage has to do is significantly reduced. "At the back end, we have more conventional GAC filtration, and then UV disinfection. Most surface water treatment works in the UK are not using UV - they are using chlorine-based disinfection - but we are using UV as the main disinfectant, with a small dose of chlorine at the end of the process just to present a residual in the distribution network. "So this is a substantially new process, and I think it will rep- resent a quantum change in terms of innovation in surface water treatment, which hasn't changed all that much in many years." • WATER COMPANY VIEW: The Works: Water Treatment • CONSTRUCTION: IN fIgURES Since construction began last year, the Mayflower WTW project team have: • Laid 10,200 cubic metres of concrete – enough to fill 133 shipping containers • Fixed 1,420 tonnes of steel – the weight of 1,168 Mini Cooper cars • Moved 110,000 tonnes of earth – the equivalent of 610 blue whales • Imported 23,000 tonnes of aggregates – more than three Eiffel Towers • Laid 8,900 metres of new pipeline – as long as 121 A380 airbuses nose to tail • Laid 7,200m cable ducts – as long as 80 football pitches end to end • Issued 2,430 drawings and 9,000 documents from Crownhill - and an additional 2km of on-site pipework. A hydroelectric turbine on part of the raw water pipeline, together with solar PV panels at the works, will help generate energy to reduce the running costs of the new works. The project is being delivered by South West Water's H5O engineering delivery alliance, with partners including Arcadis as the main design contractor, Pell Frischmann as consulting engineers, Balfour Beatty as the main construction contractor, and Nomenca and Kier Services as MEICA sub-contractors. The treatment works has been named Mayflower WTW as a tribute to the forthcoming 400th anniversary of the voyage of the settler ship The Mayflower, which started from Plymouth in 1620. Backwash vessels at Mayflower WTW