Reservoir Mixing System Improves Water Quality

The Resmix reservoir mixing system installed in a Scottish reservoir earlier in 2009 has completed its first summer with encouraging results, according to Gurney Environmental. The Resmix technology, developed by Water Engineering and Research Solutions, uses a slowly rotating impeller to skim a layer of water from the surface of the reservoir and then 'push' it down to the bottom. The advantage of top-to-bottom exchange is that dissolved oxygen (DO), which is plentiful at the surface, can be made available where it is required at the bottom.

As a result the quality of water is improved and maintained. The Resmix system is claimed to have proven its effectiveness in some of the harshest conditions imaginable. In its first UK application, a Resmix 3000 was installed to reduce the levels of manganese and other metals arriving at the reservoir's treatment works. Using approximately 1kW of energy the Resmix system has been designed to maintain uniform conditions in the 3,700 mega litre reservoir reducing summer peaks of manganese to levels treatable by the existing treatment works and therefore negating the need for an expensive upgrade.

David Orme of Gurney Environmental said: 'Manganese release occurs in reservoirs and water impoundments due to the natural process of stratification occurring during the summer months. 'The surface layer of the reservoir is warmed by sunlight and becomes more buoyant than the water below, preventing the underlying water from reaching the surface. 'Trapped beneath the warm surface layer, the main body of water becomes depleted of DO. 'The resulting low concentration of DO causes iron, manganese and other metal salts to leach from sediments leading to discolouration, odour and bad taste. 'Stagnation of the water also encourages the growth of algae in the surface layer.

Traditional methods of mixing reservoirs, typically either compressed air diffuser systems or mechanical mixers, bring poor quality water from the bottom, and mix it into the entire water column. This requires large amounts of electrical energy and can in some cases even exacerbate water quality problems. Andy Muir from Scottish Water Solutions added: 'We have had the Resmix unit in service now for the best part of a year. 'During this time we have monitored both the manganese levels in the raw water and the reservoir oxygen levels and temperature. 'Owing to some technical difficulties with, among other things our temporary power supply, the Resmix unit was not operating for an extended period at the hottest part of the year.

'During this period our monitors showed that reservoir stratification occurred and with it an increase in soluble manganese levels in the raw water. 'Stratification was evident from temperature and oxygen differentials from top to bottom of the water column. 'With the Resmix system back in service there was a marked drop in soluble manganese and the reservoir temperature and oxygen differentials decreased. 'This is a favourable outcome as the cost of running this unit is a fraction of the cost of running a filtration removal plant.

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