C14.3 Treating waste water
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What happens when the sewage first arrives at the pumping station?
➢ It goes to be screened at the sewage treatment plant
What happens during screening?
➢ The sewage arrives and the first step is to remove large grit and solid objects - passes through a metal grid
What happens after screening?
➢ It goes into Primary treatment
What happens during primary treatment?
➢ In the 1st circular tank, the solid sediments are settled from the mixture ➢Large paddles rotate pushing the solids called sludge towards the centre of the tank ➢ Then the sludge is piped to a storage tank ➢ The effluent(watery liquid) above sludge contains harmful microorganisms and is stored in the second tank
What happens during the secondary treatment?
➢ In the second tank, useful bacteria feed on the effluent of any remaining organic matter and harmful microorganisms still present, breaking them down aerobically ➢ The tank is aerated by bubbling air through the wastewater
What happens during the Final treatment?
➢ The useful bacteria are allowed to settle out to the bottom of the tank as a sediment ➢ Sediment is either recycled back into the secondary treatment tank or passed into the tank where the sludge is treated ➢ Wastewater is safe enough to be discharged back into rivers
How is sewage sludge treated?
➢ Most can be dried and used as a fertiliser on farmland to improve the soil or used as a source of renewable energy
How can biological treatment be carried out on the sludge?
➢ Can be carried out at about 55 degrees which can take up to 30 days to complete ➢ Breakdown of products include biogas ➢ Biogas can be burned and used to power the sewage treatment plant or provide electricity ➢ Or the sludge can be dried into a crusty solid that can be burnt to generate electricity