The biological process

Biological process – activated sludge
The activated sludge process is the principal biological treatment process in the facility, the function of which 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 settlement basins.

The activated sludge process takes place in the facility 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 facility. 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 facility 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 aeration is required by the microbes 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 facility.

The wastewater is kept in the activated sludge process for about 10 hours and is then discharged into the secondary sedimentation process.
 

 Aeration basins in the activated sludge process