Download Water Treatment

Water Treatment
• Water treatment
– designed to remove microbes, chemicals and
pollutants
– not meant to sterilize
– Potable water
Steps of water treatment:
1. Coagulation: Sedimentation and Flocculation
2. Filtration: Sand or Coal
3. Disinfection: Chlorine; Ozone; UV light
Sewage Treatment
• Primary treatment
– Screened and Skimmed
– Sedimentation
– Sludge removed
– Effluent (liquid) moves on to 2nd step
• Biochemical oxygen demand (BOD)
– measure of the oxygen required to fully
metabolize organic matter in water
– Directly proportional to amount of waste in
water
– Normally about 10mg/L of oxygen can be
dissolved in water
– Primary treatment removes about 30% of BOD
• Secondary treatment
– Predominantly biological; designed to remove
organic matter and reduce BOD
– Sewage undergoes strong aeration
• Activated sludge
• Trickle filter
• Rotating biological contractor
– Activated sludge system
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microbes oxidize organic matter into CO2 and water
Zooglea form floc
Transferred to settling tank; floc settles out
Bulking may result in local pollution
• Trickling filters spray
sewage over a bed of
rocks or molded
plastic
• Rotating biological
contractor
• Chemical treatment
– Effluent from secondary treatment is
disinfected and discharged
– Usually involves chlorination
• Tertiary treatment
– Sewage treated to a potable level
– Extremely costly
– Removes any remaining BOD including most
nitrates and phosphates
– Removes all microbes
• Sludge treatment
– Sludge pumped into anaerobic sludge digesters
– Encourages growth of anaerobic bacteria
• especially methanogens
– Decrease organic solids
• degrades them into methane and CO2
– Methane harvested
– Undigested sludge may be deposited in a landfill
or used for soil conditioners
Anaerobic Sludge Digester
• CO2 + 4 H2  CH4 + 2 H2O
• CH3COOH  CH4 + CO2
• Areas with low population density often use
septic tanks
– Sewage flows into a holding tank, particulate
matter settles out
– Effluent flows through a system of perforated
pipes into a leaching fields
– Organics decomposed by soil bacteria