12.3 Treating waste water
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What does waste water mean
any water that has been contaminated or used by human activity
Three main sources of waste water
Domestic, agricultural systems, industrial
Where does domestic water go to
sewage treatments
What must be removed from domestic and agricultural waste water
organic matter, harmful microbes
What must be removed from industrial waste water
harmful chemicals
Describe stages involved in sewage treatment
Screening: Remove large solid objects and grit from rest of waste water. Sewage passes through metal grid that traps large objects Primary treatment: Solid sediments allowed to settle out from the mixture. Large paddles rotate, push sludge towards centre of tank and piped to a storage tank. Effluent flows into next tank. • Secondary: Useful bacteria feed on any remaining organic matter and harmful microorganisms still present, breaking them down anaerobically. Tank is aerated. • Final: Last tank, useful bacteria settle out to bottom of tank as sediment. It is either recycled back into secondary treatment tank or passed into tank where sludge is treated. It is now safe enough to be discharged back into rivers
Describe difference between conditions in sludge and effluent tanks
Effluent needed aerobic respiration Sludge - anaerobic repsiration
What can the sludge be used for when produced
fertiliser on farmland or used as a source of renewable energy
what does sewage treatment include (summary)
• screening and grit removal • sedimentation to produce sewage sludge and effluent • anaerobic respiration of sewage sludge • aerobic biological treatment of effluent
what is sludge
The solid, heavy material that settles at the bottom of a sedimentation tank during the primary treatment of sewage.
what is effluent
The liquid, lighter material that remains at the top of the sedimentation tank after the solids have settled.