Sewage Treatment: Primary sedimentation
The 12 primary
sedimentation tanks are each 70 metres long and 12 metres wide, with an average
water depth of 2.8 metres. These are large tanks which are designed to allow
the wastewater to flow slowly through in a smooth motion, free from turbulence
enabling the organic solids to settle to the bottom. Retention time in the
primary tanks is two to three hours.
The sludge is collected
by two parallel, chain-driven flight scrapers. These move continuously along
the sloping floors of the tanks, slowly ploughing the sludge towards the end of
the tank where a cross collector (also chain and flight) moves the sludge into a
deep hopper. From here, it is removed by new centrifugal pumps to a sludge
Scum, which rises to
the surface of the tanks, is directed by fan- shaped water jets to the inlet
end of the tank. Here, it is lifted over a wall and into a trough by rotating
scum collectors and carried into the sludge sump. The sludge and scum from the
Sedimentation tanks are
pumped to the gravity thickeners. After the sludge has been thickened in the
gravity thickeners, it is sent to the gravity belt thickeners for further
thickening before being sent to the digesters. At this stage, over 70 percent
of the suspended solids have been separated from the liquid waste stream with
40 percent of the BOD removed.
After separation in the primary sedimentation tanks, the liquid stream is conveyed via the interstage pump station at a rate of up to nine cubic metres per second to the reactor/clarifiers for secondary treatment. (See the information sheet Secondary treatment - liquid).