The best way to clean up flooded lakes and reservoirs

6 min read
Craigshead resevoir with Aquaerators in action.
Craigshead resevoir with Aquaerators in action.

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Agricultural run-off, sewage discharge and, more recently, floodwater are all compromising water quality in lakes, rivers, reservoirs and coastal waters. High concentrations of manganese, iron, aluminium and phosphates, which contribute to algal bloom and noxious gases are affecting our aquatic animals and flora. Unhealthy, polluted water limits leisure activities and expensive chemical treatments are required to treat reservoir water – neither scenario is desirable, or even necessary.

Floodwater has been running off the land into rivers and lakes, carrying with it all sorts of waste, including raw sewage. Tests from Reading University recently showed floodwater contained 60-70 times the amount of bacteria recommended by the World Health Organisation.

But this is only compounding a growing problem. A lack of both management and investment, together with an increase in agricultural chemicals, has seen a steady decline in the health and ecology of many rivers, lakes and reservoirs, with a consequent impact on fish species, animals, plants and micro-organisms that make up a balanced and healthy ecosystem.

Thermal stratification

One of the main causes is thermal stratification in the summer months, a common condition affecting bodies of water with little or no flow— the effect of natural heating by the sun. The result is layers of unmixed water at different temperatures with extremely poor levels of dissolved oxygen in the deeper water and a consequent disruption to the normal water ecology.

This anoxic layer is bad for fish and marine life. Fish in particular have to stay nearer the surface to breathe and become easy victims for fishermen and birds. Lack of oxygen is also bad for the other organisms, which are crucial for the normal breakdown of organics deposited in the lake. If this continues, anaerobic plants will grow on the bed taking up even more of the depleted oxygen and making it impossible for fish to live near the bed. Studies also show that low levels of dissolved oxygen in deeper waters, is linked to the development of toxic blue green algal blooms. At the same time heavy metals such as manganese and iron, all normally locked in the bed sediments become readily soluble, requiring costly treatments.

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 ‘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 pollution complications produced by recent flood waters - potentially a new seasonal occurrence given the prevailing weather patterns. A relentless rise in energy costs doesn’t help. Proven, low cost solutions, requiring little energy or maintenance, are urgently needed.

Current aeration solutions for water improvements

Simple aeration, blowing bubbles, has long been the solution of choice to maintain reasonable water quality. Implemented effectively, it will reduce all these problems. It enables oxygen exchange at the surface and releases noxious gasses such as carbon dioxide, methane and hydrogen sulphide from the lower 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.

However, choosing the right solution can be problematic, partly as there are a raft 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, and then sample and test regularly. Mapping the water body using echo sounding might also be required to understand the topography of the lake bed and where the deepest areas lie.

Surface aeration

The use of fountains to aerate the surface waters is a traditional method to improve conditions and these can also look attractive, an important consideration for urban lakes and water bodies. But, as they only pull the first 0.3 to 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 not much more than a 3-metre diameter, often 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 methods resolves the low dissolved oxygen depletion problem at depth - which is the underlying issue - so literally they are just scratching the surface.

Subsurface aeration

Subsurface aeration devices which release air bubbles at the bottom of the water body make far more sense - at least then there is a chance of improving the whole water column. Diffuser aeration systems use multiple diffuser discs to produce fine bubbles but these are generally only suitable to maximise oxygen discharge in tanks and small ponds.

Furthermore, due to their design, most of these devices 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 ever increasing size of the air bubbles rising to the surface is critical. The larger they become the less effective they are, because there is no reaction which is mixing the water and air, simply a rising momentum. Diffuser systems are also prone to clogging over time and will need maintenance.

A low carbon solution

A new, low energy solution, scientifically proven to mix and aerate bed water with powerful plumes spreading out to 9 metres in surface diameter (depending on the water depth) is generating significant interest in Scotland, China and Paraguay. The “Aquaerator” mixes air and near-bed water together, thus reducing its density and forming a turbulent 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 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, approximately 4.5 tonnes per second from a 10 metre depth and increasing to 13.4 tonnes per second from 20 metres, offers significant advantages over solutions that simply effect the water column just above the device. From a cost stand point alone far fewer devices are needed to aerate a given volume, so capital cost is lower and energy consumption is reduced.

Silt bed disturbance

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, thus 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.

Fishermen will tell you that as the ecology is left undisturbed you will find the fish living around an Aquaerator as the highly oxygenated water grows their food faster. In fact when International Fishing Competitions are in progress they are switched off in order to ensure that competition is completely fair for visiting entrants. Reducing costs for the water industry

Plenty of options

So there are plenty of options, but all said and done, cost is going to be the underlying issue for the water industry. Capital expense will be influenced by the number of devices required so selecting a proven solution which is able to aerate and mix the largest volume of water from a single device will make a very considerable saving - the difference in some cases between an obstacle course of devices on the reservoir or lake floor, to just a handful of Aquaerators, for example.

A whole raft of factors can influence cost, but as an example a 200 hectare lake with depths of 10 metres would only require around seven Aquaerators, with a unit cost in the region of £6,000 each, to deliver an effective and lasting solution to maintain water quality. Running costs would involve a single 37 Kw compressor feeding all seven Aquaerators during the summer months, and if necessary to remove ice during the winter. Of course consultancy, installation and monitoring costs also need to be factored in, but to achieve the same results with a comparable solution could require as many as 100 individual diffusers involving considerably more capital outlay, and a very large energy hungry compressor.

Ongoing energy consumption

Understanding the ongoing energy consumption required and likely maintenance issues is another aspect that you will need to factor in. Sub surface solutions will always be more effective because they act on the whole water column and rely on naturally rising air bubbles so use less energy. Experience has shown that some aeration products and diffusers are not efficient enough to oxidise heavy metals and manganese and return them to the silt bed.

Consequently filter plates have quickly clogged, requiring frequent removal and cleaning to maintain efficiency. A sub surface solution with no moving parts, designed so as not to attract the build-up of heavy metals such as manganese oxide will suffer less maintenance downtime. Combined with ability to entrain large volumes of mixed air and water to reduce bed water density, the Aquaerator provides the best option for improving reservoir water quality and limiting unwanted chemical intervention.

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