Destratification of lakes and reservoirs
Diffused aeration is the ideal means of preventing thermal stratification and high surface water temperatures in lakes, and reservoirs. Aeration and mixing of the water can dramatically improve water quality with only a small amount of energy input. The key is to mix the water column and prevent both thermal and chemical stratification of the water. This in turn stabilises the ecosystem and prevents excessive blooms of algae and zooplankton.
The water quality will improve, the productivity of the system will also improve, and in the case of reservoir water treatment, the water become much more stable and predictable. The zeta potential of the water will drop and the oxidation potential will increase which makes the water much easier to filter by sand or AFM® pressure or RGF rapid gravity filters. Chemically stratified anaerobic water is very difficult to treat and often has a high dissolved organic content, phosphate, bacteria and heavy metals including iron and manganese. Destratification can solve many of the water quality problems, indeed in many cases it is possible to down size the scale of the water treatment system and negate the need to use UVc, ozone, or activated carbon. An ecosystem approach using biology and the environment to our benefit is always a much better approach rather than fighting nature and trying to solve a biological problem by adopting an engineering approach.
How does the system work?
Stratification in terms of temperature and dissolved nutrients in the water can lead to the development of toxic algal blooms and general deterioration of water quality in the water body. Simple aeration at the deepest part of the lake has been shown to be very effective at destratifying very large lakes. The process is remarkable by the fact that a small amount of aeration at one spot in a lake can have such a tremendous impact on water quality over the entire lake or several million cubic meters.
Sizing and design of a system
The type of lake will have an influence on the design of the system. If the lake is shallow, with a depth from 1 to 4 meters with a large surface area, then it is best to spread the air diffusers evenly over the lake. However if the same lake has one point where it is 10 m or deeper, then the deep hole is the only section where you use the aeration. The depth profile will therefore dictate the configuration of the diffusers and amount of air required.
|Water depth meters||Amount of Water moved m3/hr by 1 m3/hr of air diffused|
|3 m||10 m3/hr|
|4 m||15 m3/hr|
|6 m||20 m3/hr|
|10 m||40 m3/hr|
|20 m||80 m3/hr|
|30 m||120 m3/hr|
|40 m||150 m3/hr|
|60 m||200 m3/hr|
In order to size a system
- Determine the dimensions of the lake and surface area
- Determine the maximum water depth and average depth of water
- Determine the approximate volume of water in the lake
The objective is to size the aeration system to mix the entire water content of the lake at least once every week. By way of example, lets assume that the volume of the lake is 2 million cubic meters. The average water depth is 5m, however there is an section along the length of the dam which measures 100 m long and is 20 m deep.
1 m3/hr of air will move approximately 80 m3/hr of water, over the course of 1 week, the air will move 1 x 80 x 24 x 7 = 13440 cubic metres of water.
The amount of air required therefore equals 2,000,000 / 13440 = 148 m3 of air per hour.
Each of our air diffusers, product code 6.2.10 will pass 10 m3/hr of air, so the above system will require a minimum of 14 diffusers. This calculation gives you the minimum number of diffusers and size of system, however if the water is required for drinking water pretreatment or if the lake has a very high nutrient input, then it may be required to increase the mixing ration to once every 2 days as a maximum.
How it all works
The diffusers code 6.2.10 are fitted onto an mdpe pipe using mechanical saddle clamps. The diffusers are in pairs and spaced at approximately 6 m intervals.
A second mdpe pipe is connected bellow the air manifold mdpe pipe. The two pipes are held together by a stainless steel bracket. A chain or a rope is connected to the bracket and a weight suspended. An air supply is connected to the lower ballast pipe. When the water is evacuated by the air, the complete assembly will float. When the air supply is turned off, the whole assembly will sink. The arrangement allows the aeration system to be suspended above the sediment of the lake, and provides a mechanism to easily recover the system for any repair or maintenance.
- Air diffusers, we always used product code 6.2.10.
- Air blower, the selection depends on the air flow and water depth, the table below provides a selection guide
- Blower electrical control panel, this is basically a soft starter, however you may have timers, oxygen and temperature readings and alarm >out-puts. Dryden Aqua can design and provide a panel to your specification
- The blowers can be provided with their own acoustic environmental enclosure. The screw compressors can also be provided in an ISO container. If the screw compressors are used for drinking water reservoir treatment, we use a catalytic converter on the air discharge to actually clean the air to a quality better than normal air. The system needs to be in an ISO container or a small building to protect it from the elements.
- The blower needs to be secured to a concrete pad, and the pipework fitted. The first 6 to 10m of pipe should be in steel, thereafter the pipe may be in mdpe of hdpe (medium or high density polyethylene), the compressed air temperature needs to be reduced to less than 60 °C for plastic pipe, depending upon pressure rating.
- The mdpe pipe, is connected to air hose and the air hose is connected to the diffusers. The mdpe pipe may go around the perimeter of the lake and 1/2″ hose from the mdpe pipe goes into the water with a diffuser on the end. The other option is that the mdpe pipe goes into the lake and is anchored to a lake bed with ballast. 1/2″ hose them comes off the mdpe pipe, the length of the hose should be 1 m longer than the depth of the water. The diffuser is then connected onto the end of the hose, this allows the diffuser to be lifted out of the water without having to lift the pipework.
|Blower type||Code||Max.air flow m3/hr||Max. water depth metres||kW|
|Roots type||160 to 500||6||5.5 to 22|
|Zephyr compressors||Follow the link for information on these units, up to 350 m3/hr at 2 bar||
100 to 500 m3/hr
11 to 44
|Air flow rate in m3/hr||Pipe diameter 10 to 100 m long||Pipe diameter 100 to 500 m long|
|10 to 25||25 mm||32 mm|
|26 to 50||50 mm||63 mm|
|51 to 250||90 mm||125 mm|
|251 to 500||125 mm||160 mm|