Water Treatment

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Water Treatment

• Many cities obtain their water from badly
  polluted sources
• not meant to sterilize, only remove
  disease-causing microbes
• Steps of water treatment
• Coagulation
• Settling tank
• Flocculant chemical is added
• Filtration
• through sand or coal
• Disinfection
• Chlorine
• Ozone reactive form of oxygen
• UV light

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Sewage Treatment

• 1st step primary treatment
• Floating materials are screened out
• Skimmers remove oil and grease
• Sedimentation tanks where solid matter settles
  out as sludge
• Sludge is removed effluent (liquid) moves on to
  2nd step

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• Biochemical oxygen demand (BOD) is a measure of
  the biologically degradable organic matter in
  water
• Primary treatment removes about 30 of this
  matter
• Normally about 10mg/L of oxygen can be dissolved
  in water
• BOD values of wastewater may be 20 times that
• If that water enters a lake, bacteria degrade the
  matter and consume all available oxygen

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• 2nd step secondary treatment
• Predominantly biological designed to remove
  organic matter and reduce BOD
• Sewage undergoes strong aeration
• Activated sludge system
• sewage-metabolizing microbes oxidize organic
  matter into CO2 and water
• Zooglea form floc
• Transferred to settling tank floc settles out
• Clear effluent is disinfected and discharged
• Bulking may result in local pollution

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Activated Sludge
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• Trickling filters spray sewage over a bed of
  rocks or molded plastic
• Rotating biological contractor
• Sewage is then disinfected, usually chlorination,
  and released

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• Tertiary Sewage Treatment
• can be treated to a potable level
• Becoming more common in arid areas
• Extremely costly

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• Primary sludge accumulates in sediment tanks
• Sludge also accumulates from secondary treatments
• sludge is pumped into anaerobic sludge digesters
• Encourages the growth of anaerobic bacteria
• Especially methanogens, which decrease organic
  solids by degrading them into methane and CO2
• Methane is harvested and used to power the
  equipment for the treatment plant
• Undigested sludge may be used for landfill or
  soil conditioners

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Anaerobic Sludge Digester

• CO2 4 H2 ? CH4 2 H2O
• CH3COOH ? CH4 CO2

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• Areas with low population density often use
  septic tanks operate similar to primary
  treatment
• Sewage flows into a holding tank, where
  particulate matter settles out must be pumped
  out periodically
• Effluent flows through a system of perforated
  pipes into a leaching fields
• Effluent is decomposed by soil bacteria
• function may be impaired by excessive amounts of
  antibacterial soaps, detergents, drain cleaners
  and medications

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• Oxidation ponds industry small communities
• inexpensive require lots of land
• Deep anaerobic sludge pond and a shallow aerobic
  pond remove most organics from effluent