Sewage Treatment: Screening and Pre aeration tanks

Screening

Screening is the first line of treatment at the entrance to the wastewater treatment plant where six new fine screens, arranged in parallel channels, intercept solid material in the influent wastewater.

The fine screens replace the old-technology (19 millimetre bar screens) and have a stainless steel mesh with apertures of three millimetres. The drum-shaped screens are not static pieces of equipment but are large revolving mechanisms, constantly rotated by hydraulic drives. The screens break up the raw sewage flowing into the plant and extract material such as paper, fruit and vegetable pulp, plastic, wood and sanitary items.

Banks of water jets within the rotating screens constantly blast the debris from the mesh. The debris or screenings (up to eight tonnes per day) are extracted by screw conveyors, washed and dewatered and conveyed to a large waste skip which is trucked daily to an off-site landfill.

 Pre-aeration tanks

The 12 grit removal tanks, also known as pre-aeration tanks, are

14 metres x 12 metres with a water depth of 4.6 metres. Each tank has a volume of 703 cubic metres and is partially divided into two sections (north and south) with one air sparge pipe and one grit ejector in each section.

Air, pumped from a perforated pipe running along the side of the tank floors, generates a swirling motion which reduces the effective density of the wastewater. This encourages the inorganic material (finer than three millimetres), namely, sand, silt and fine gravel to settle out. The aeration process also adds oxygen to the wastewater which, by the time it reaches the treatment plant, can be oxygen deficient.

The organic solids remain in suspension. The settled grit is collected in a hopper at one end of the steeply sloping floor. Here grit pumps automatically extract the grit and transfer it through a pipe network to grit washing facilities above the truck loading bay. It is then removed by water ejection to a washing tank and fed into another hopper by a screw conveyor where it is dewatered. The extracted grit is trucked off-site for disposal in landfills