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Aerated Lagoons (AL)
The AL process normally comprises two or three lagoons in a sequence. The first lagoon has surface aerators, which are like large "paddle mixers". The aerators float on the surface of the pond and continuously stir the incoming sewage, serving to maintain oxygen content in the sewage and preventing any solids from settling.
Sewage has an average retention in the first pond of five days. During this time, bacteria consumed the oxygen to breaking down the organic material in the sewage.
Effluent is passed into the second pond where the degraded organic matter and sediments are settle out to form sludge. The effluent may then pass to a third pond for polishing or be discharged to a receiving waterway. The average retention time in the second pond is one day.
Care must be taken in managing the settling pond in warm climates. These ponds can suffer from algal growth and/or odour generated by anaerobic digestion of the sludge at the bottom of the pond. Depending upon their design these ponds must be desludged approximately every 10 years.
Typical figures for ALs are as follows:
|(mg/L)||Raw Sewage||Effluent||DOE Standard B|
|Biological Oxygen Demand||200-400||20-80||50|
There are approximately 40 AL sewage treatment plants in Malaysia.