Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | MAY 2014 | 31 Digging deeper "The Environment Agency is trying to improve lake water with the 2015 Water Framework Directive, which will make county councils responsible for improving the ecological status of 'wa- ter bodies' and 'protected areas' within each of the UK's river basin districts". Tony Wynes responsible for improving the ecological status of 'water bodies' and 'protected areas' within each of the UK's river basin districts. Achieving this, however, could be challenging, given financial constraints and the additional complications of recent flooding. Proven, low cost solutions, requiring little energy or maintenance are urgently needed. Aeration has long been the solution of choice to maintain good water quality. Implemented effectively, it will solve all these problems. It enables oxygen exchange at the surface and releases noxious gases such as carbon dioxide, methane and hydrogen sulphide from the various layers of water. Reservoir aeration has the added benefit of naturally improving water quality and reducing the necessity of expensive chemical interventions at the treatment works. Case Study We have maintained and improved aeration systems in Bristol Water's Blagdon man-made 217-hectare reservoir since the 1970s. Originally there was a simple air curtain. The latest im- provement work in 2008 tackled the main water outlet 2m below the reservoir bed. This meant water going to the treatment works had a low DO and in July each year there were large quantities of manganese in the water column. Aquarius Marine's Diving Team airli'ed over 30cu m of silt from the area and stacked rocks and ancient building materials along the sides of the channel before reaching the granite apron. The Aquaerator was bolted down (due to 1.5 knot outflow) 13m from the 3m diameter outflow pipe grille, which is 2-3m below the surface of the reservoir bed mud deposits. Another Aquaerator was laid to improve the water quality through the alternative outflow siphon. This year there was no need to treat the raw water for manganese removal. For further reading please go to wwtonline.co.uk • Problems and solutions Choosing the right solution can be problematic, partly as there are a ra of products all claiming to offer similar health benefits. Nor is it a matter of simply installing your chosen device and sitting back. As a first step, you really need to understand what is going on in the water, and use accurate sampling and analysis, before any solution is deployed. ● Surface aeration: The use of fountains to aerate the surface water is a traditional method to improve conditions and these can also be attractive, an important consideration for urban lakes and water bodies. But, as they only pull the first 0.3 - 0.6 metres of water up to oxygenate it, they cannot handle large areas and require constant amounts of energy. Floating surface aerators, which disrupt the first 0.6 metres of water are also powered by on-shore electricity and are limited to adding oxygen to much more than a 3-metre diameter, o en leaving the bed water unaffected. Paddles work in a similar fashion, agitating the water to add oxygen, but again they use lots of energy, there are moving parts to maintain, and above all they cannot aerate an entire water column. None of these resolves the low dissolved oxygen depletion problem at depth – which is the underlying issue. Subsurface aeration: Subsurface aeration devices seek to release bubbles at the bottom of the water body. Diffuser aeration systems use multiple diffuser discs to produce fine bubbles. These are generally used to maximise oxygen discharge in tanks and small ponds. Many of these devices, however, due to their design, are only able to affect a relatively small cubic volume of water, so large numbers of them must be deployed to aerate a lake adequately. Initial capital costs to cover purchase and installation are therefore high, and ongoing energy costs can be significant. Also the size of the air bubbles rising to the surface is critical and o en quite large and less effective, depending on the depth. ● A low-carbon solution: A new, low energy solution to mix and aerate bed water with powerful plumes spreading out to 9-metres surface diameter (depending on the water depth) is generating significant interest in Scotland and China. This Aquaerator mixes air and near- bed water together to form a buoyant plume which has enough vertical momentum to expand and rotate while bubbles rise to maximise the rate of entrainment of near- bed water immediately above the device. The device differs from others on the market as its 40 small air jets release tiny bubbles at high pressure, engulfing, we believe, the largest amount of water, while requiring less power than other sub-surface solutions. The ability to mix and aerate a large volume of water from a single device, namely 4.5 tonnes per second from 10m depth and increasing to 13.4 tonnes per second from 20m, offers advantages over solutions that mix the water column just above the device. ● Benefits to the environment: With any subsurface solution, it is also very important that the silt bed, which normally traps unwanted heavy metals, remains undisturbed. A solution, which takes the water horizontally from near the bottom, so without disturbing the silt, ensures the ecology of the lake or reservoir bed is not disturbed and that silt is not drawn up into the water column. ● Reducing costs for the water industry: Cost of course is a fundamental issue. Understanding the capital cost as well as the ongoing energy consumption and likely maintenance issues is another aspect that must be considered. A subsurface solution with no moving parts, which is easy to maintain and has no stainless steel manifold to attract the build-up of heavy metals such as manganese and which can entrain large volumes of mixed air and water to reduce bed water density, will be more economical than surface solutions. ●

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