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Destratification of Lakes and Reservoirs

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Destratification, reasons why ? 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 stabile 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.  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 ?  Diffused aeration is also and ideal technique for preventing stratification of lakes and reservoirs. 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 de-stratifying 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.

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 i best to spread the air diffusers evenly over the lake. However if the same lake has one point where it is 10m 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.

In order to size a system

a. determine the dimensions of the lake and surface area

b. determine the maximum water depth and average depth of water

c. 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 100m long and is 20m deep.

1 cubm/hr of air will move  approximately 80 cubm/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 cubm of air per hour.

Each of our air diffusers, product code 6.2.10 will pass 10 cubm/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 pre-treatment 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.

pdf data sheet on diffusers click here

Components

1. air diffusers, we always used product code 6.2.10.

2. Air blower, the selection depends on the air flow and water depth. The table below provides a selection guide

3. 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

4. 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.

5. 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)

6. 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 1m longer than the depth of the water. The diffuser is then connected onto the end of the hose, this allows

Installation schematics

Type 1 lake.  a large surface area, shallow  ( 1 to 4m) with no deep holes

Type 2 lake, a large surface area with a deep section in which all diffusers are located

Type 3 lake, a quarry type configuration, lake is deep with steep sides

Type 4 lake, similar depth through-out the lake,  but reasonably deep at edges,

mdpe pipe goes around the perimeter of the lake and diffusers come of the pipe via hose connections