•Primary treatment: Sedimentation (43.15 minute)
The sewage is then passed forward to large tanks. Here fine particles sink to the bottom and
form a sludge, which is regularly moved and further treated. On leaving the grit extractor,
the sewage enters the primary settlement tanks. Here approximately 70% of the remaining
solid matter settles out and forms primary sludge which collects in a sump at the bottom of
the tank. Periodically the sludge is pumped away for disposal or for treatment. Above the
sludge lies the wastewater, that now has a greatly reduced amount of SS, and with it about
30% less BOD.
Secondary (Biological) Treatment: Trickling filters or activated sludge and secondary
sedimentation (at 44.12 minute)
At this stage we use bacteria to clean up the liquid sewage. The bacteria help purify the
liquid by actively feeding on it and turning most of it into carbon dioxide water and Nitrogen.
In biological filters we encourage bacteria to grow in deep beds of irregularly shaped stones.
Gaps between the stones circulate air, which lets the bacteria breathe and thrive as the
liquid passes through them. An alternative to biological filter is a process called activated
sludge, where air is bubbled through the sewage to encourage bacteriological growth. The
air can then be pumped or entrained from the surface by rotating paddles. Either way, once
the bacteria have done their work, the liquid goes into f u r t h e r s e t t l i n g tanks to
remove any remaining particles, leaving clean water to flow to the river or if necessary
onward to further treatment.