The biological process
The activated sludge process is the principal biological treatment process in the plant. Its function is to remove dissolved organic materials from the wastewater.
- The process is based on a very high concentration of microbes that decompose the organic material in the wastewater and convert it into carbon dioxide and water. The high microbe count is maintained by re-circulating the microbes into the process from the secondary sedimentation basins (clarifiers).
- The activated sludge process takes place in the plant in three large aeration basins, divided into three secondary basins (see picture).
- One of the basins is an older one that was constructed during the first upgrade of the plant. This basin has an area of 3,720 m² with four mixers and eight aerators having a total output of 540 kW.
- The other two basins were constructed during the second plant upgrade. Each of these has an area of 4,824 m², and each has four mixers and eight aerators having a total output of 1,320 kW.
- The microbes require aeration in order to provide them with oxygen with which they decompose the organic material in an aerobic respiration process.
- The basins are, as noted, aerated with a large number of electrically-operated surface aerators, which operate automatically in accordance with the oxygen levels required in the water.
- Treatment in the activated sludge basins brings about a total reduction of about 98% in the concentration of organic material decomposed biologically (BOD) and a total reduction of about 98% in the concentration of suspended solids in the raw sewage. In other words, the wastewater leaving this process contains only about 2% of the contaminants contained in it prior to its discharge into the treatment plant.
- The wastewater is kept in the activated sludge process for about 10 hours and is then discharged into the secondary clarifiers.
Biological process – activated sludge