Pond Water Treatment

When we think of water quality in our water gardens and garden ponds we usually think of two things: the Water Garden with Waterfallwater's ability to support aquatic life and its clarity. Lack of clarity is usually not a problem for aquatic life, but limits our ability to see and enjoy our koi and goldfish. The pond owner should be concerned with both of these aspects.

One major concern for water garden and garden pond enthusiasts is achieving and maintaining clear water. When water gardens and garden ponds are first constructed there is usually a rush to fill them up with fish and aquatic plants, but it is important to consider the natural process of these ecosystems. In nature a pond has a period of time for all the components to adjust and interact as the pond evolves. Although suspended algae in water gardens and garden ponds is to be expected most people want to keep it to a minimum. Constructed water gardens and garden ponds require help in order to avoid problems with excess algae. One method to control algae is to stock and arrange the pond with this method:

  • Keep rain run-off out of the pond. It carries with it soil, fertilizers, and chemicals which will prevent the pond from balancing.
  • If in full sun, shade 50 - 70% of the pond surface. Do this with the leaves of water lilies and floating plants. If your pond is in less than full sun, less surface coverage is acceptable.
  • Use one bunch of Anacharis per one sq. ft. of surface area for ponds under 50 sq. ft. and one bunch per two to three sq. ft. for ponds over 51 to 200 sq. ft. For ponds over 200 sq. ft. use one bunch for every three to four sq. ft.
  • Use no more than one inch of fish per sq. ft. of surface area initially. After the pond has fully established or with well filtered water, more fish may be added. Feed only what the fish can eat in 5 minutes. Uneaten food feeds algae.
  • Add biological filtration and an ultraviolet sterilizer.
  • Remove any dead organic matter from the pond.

Pond systems stabilize organic material through natural processes involving sunlight, water, nutrients, algae, atmospheric oxygen and bacterial action. Organic matter in the wastewater is broken down by aerobic bacteria and oxygen found in the pond. This process releases carbon dioxide which is used by the algae to produce simple sugars through a process known as photosynthesis. Oxygen is the by-product of this reaction which is used by other aerobic bacteria to stabilize more organic matter. This becomes a continuous cycle.

The process is aerobic and therefore is based on oxygen. Oxygen is used by the microorganisms to stabilize organic matter. More organic matter requires more oxygen. During daylight hours photosynthesis operates and oxygen is produced. This process puts DO in the pond water. At night photosynthesis stops and all the organisms become oxygen consumers which can greatly reduce the DO levels in the pond. DO levels are also dependent upon depth. Surface levels will have higher DO levels and as the depth increases it becomes more difficult for sunlight to penetrate therefore DO levels decline.

Ponds use a multitude of organisms in the treatment process. Bacteria, algae, protozoa, and insects all have a part of the treatment in a pond system. These organisms coupled with sunlight, oxygen, and long detention times do an effective job of reducing fecal coliform and lowering ammonia levels. In the summer blue-green algae tend to dominate with high organic loadings. The nitrogen associated with ammonia in wastewater stimulates the growth cycle of blue-green algae. A large bloom will be evident by an obnoxious, pig-pen odor. While it is disagreeable to smell, it is effective in reducing ammonia concentrations.

Most ponds are somewhat maintenance free. Many are designed with no pumps and motors. All flows are by gravity. This minimizes maintenance and does not require a lot of operating attention. What maintenance is required is mowing of the property and preventing the growth of cattails and other shallow water plants at the water’s edge. Care must be taken to insure nuisance animals such as beavers, nutria rats, and alligators do not destroy the levees.

The pond system can eliminate 80% to 90% of the BOD and reduce bacteria to levels comparable to other accepted oxidation types of treatment. This type of treatment system meets the needs of many small or rural communities due to low construction costs as well minimal operation and maintenance requirements. The down side is the amount of property required for a system of this type.


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