Water & Wastewater Treatment

22 | november 2014 | WWT | www.wwtonline.co.uk which water becomes divided into layers with warm, highly oxygenated water on the top and cold, low-oxygen water beneath. Welsh Water com- missioned a report from consultants Black & Veatch into systems that could reduce the effects of this stratification. The two main options were a system using compressed air which works by oxygenating the water from the bottom up, or the ResMix, which is installed on the surface and uses two 'stirrers' with a diameter of 5m each to circulate the water from the top, pushing warmer, oxygenated air into the depths of the reservoir. "The systems all work on the same principle of de-stratifying the reser- voir, but from the report that we had it was deemed that the ResMix would be more effective at doing that than the compressed air systems, and more energy efficient as well," says Long. ResMix has been used in Australia for 20 years, but is relatively novel in the UK: its first application on these shores was by Scottish Water in 2009. Its energy efficiency advantages stem from the fact that it works on a dif- ferent principle to the alternatives, explains John Gillett, Managing Direc- tor at Gurney Environmental, the firm which supplies the system in the UK. "ResMix really is the first system that allows the water utility compa- nies to start to proactively manage their water resources," says Gillett. "Up until now the only options they've had have been compressed air systems, which are high-energy systems and so are only typically switched on once a problem occurs to try and get it to go away. ResMix is different, it's a low energy system using about one-twentieth of the energy requirement that a compressed air system does. It can be run 24 hours a day, 365 days a year; so for the first time, the water companies can economically, cost-effectively and sustainably manage the water quality in their reservoirs." Because of the size and depth of Pontsticill, Gurney recommended the use of the largest available system, the 5000cc, which has not been deployed in the UK before. "From our evaluation, we con- cluded that the installation of a ResMix 5000cc - which is a two-unit system with a 5kw motor on each unit - would be suitable to create uniform conditions in this reservoir, both in temperature and in dissolved oxy- gen," says Gillett. "The Resmix 5000 is a big unit – 5 metres in diameter, and it moves 10 tonnes of water a sec- ond from the surface to the bottom of the reservoir with only 5 kilowatts of energy. It's a very energy efficient way of moving a lot of water around." Installation from the air Welsh Water employed Gurney Environmental to supply and install the system, which is constructed by WEARS Australia. It needed to be shipped 10,333 miles from Toogoola- wah in Queensland, Australia, before being assembled on the banks of Pontsticill reservoir. Due to its size and weight, it had to be installed into the reservoir using a helicopter which positioned it in the water. "We found quite early on that quite a lot of these old reservoirs are quite remote and the roads up to them are fairly narrow," Gillett continues. "Get- ting cranes to them is sometimes not easy to do, and even if you can get a crane in, typically you can't get the reach to the water depth that we need. So helicopters present a way of doing this which is a lot safer - we don't have to bring very heavy machinery up on site and position it on the dam wall, for instance, and it just works very well and very efficiently. "We were on site for about a week and a half assembling the units, and then on the day of installation, the helicopter came in. Within about half an hour both units were in the water, and the helicopter could disappear - it was all very quick." The positioning of the system was a matter for consultation between Gurney and Dwr Cymru's process design and reservoir safety teams. The deepest point of the reservoir is the most efficient for it to operate successfully, but it needed to be posi- tioned where it would not impact on users of the reservoir, which is used for recreation and water sports. Despite the installation taking place in August, Welsh Water have opted not to make it fully operational until late October, when MIB levels naturally decline. "We're due to switch the system on within the next couple of weeks," explains Long, who was speaking to WWT in early October. "We've decided on a commissioning period of a fortnight to get it up and running properly. That's so we can monitor and manage the resource to monitor the effect that it is having on MIB. We'll switch it on at 10%, and review it over a three or four day period, then push it up to 20%, again review over a three or four day period, and so on. "We've had to wait some time The ResMix system was assembled on the banks of the reservoir a er Australia Project focus: Water resources

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