Final Polishing

Final Polishing

Once the effluent has passed through the solids removal stage and the vertical flow reed bed stage, most of the polluting strength has been removed. There remains however the problem of remaining nitrates and faecal coliforms.

Ponds and Cascades

Ponds and Water Cascades serve the function of removal of remaining faecal coliforms. The greater the retention time or the storage capacity of the pond the greater the reduction of the harmful micro organisms which would otherwise cause people to be sick. The use of a water cascade keeps the water aerated and mixed. This helps to keep oxygen levels high to support a greater range of micro-life forms in the pond which feed off these coliforms.

As an interesting garden feature water cascades are particularly attractive, and should be located in view of the seating area of your garden.


During the growing season willows are excellent plants for the absorption of nitrates and any other nutrients still remaining in the water. Furthermore, depending on the number of these trees and their size, a large proportion of the volume of the discharge will be taken up by these trees. It is during warm weather that small streams are at their slowest and so can offer the least dilution from septic tank discharges and the like. By placing a willow plantation between the final treatment stage and the stream, you give a further degree of protection to the ecology of the stream.


Safety Ponds are not recommended if there are small children in the house. Keeping the depth of water to about 400 mm deep is possibly a good compromise if the children understand the dangers and are responsible. Otherwise one should have these areas fenced off.

Vertical flow reed beds can also present a hazard to young children. The surface of a vertical flow reed bed is regularly dosed with live effluent. Children should be warned to look but not to touch. Herr LTD recommend that you have this area fenced off. These beds are as harmful as the inside of a toilet bowl so children should be supervisen at all times.

Because of the safety hazards outlined above a safety fence should always be built around a system.

Horizontal flow reed beds

Horizontal Flow Reed Beds remove the remaining nitrates by passing the waste water horizontally through the bed. The bacteria that grow on the gravel and the roots of the reeds release the notrogen from the nitrate back into the atmosphere. Without nitrate reduction there would be greater tendency for algae to grow in the otherwise nutrient rich final waters.

In these reed beds the flow passes horizontally through the bed. Oxygen is limited and so nitrates and nitrites are broken down at this stage. Any ammonia present will not get treated.

In both stages the polluting power or "BOD" is reduced. Assuming the area of the reed beds have been adequately sized this pollution or BOD will be low enough to discharge safely. The local authorities will be generally happy with effluent of a clear appearance and low BOD.

Horizontal reedbeds are permanently flooded. The bacteria eating the sewage at this stage work in the absence of air. This completes the basic treatment process. Some 80 to 90% of the polluting power of the sewage will now have been removed. Herr Ltd will attempt to design for high tertiary water treatment standards.