Waste Water Treatment (Sewage Treatment)

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Waste Water Treatment(Sewage Treatment)
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Outline

• Objectives
• Outline of treatment process (flow diagram)
• Preliminary treatment
• Primary treatment (sedimentation)
• Secondary treatment (activated sludge,
  filtration)
• Sludge treatment
• Disinfection
• Tertiary treatment

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Objective of Sewage Treatment

• To prevent pollution of the receiving water
• To prevent offensive odour in the water
• To prevent the destruction aquatic life
• If the sewage has to be disposed of on land, the
  soil will become sewage sick after some time and
  cannot take any more sewage

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Outline of Sewage Treatment

• Preliminary treatment
• Primary treatment
• Secondary treatment
• Disinfecting
• Tertiary Treatment

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Preliminary Treatment

• Waster water contains floating suspended solids
  such as rags, wood, metal, plastic, etc.
• these suspended impurities have to be removed as
  they interfere with the treatment processes or
  mechanical equipment.

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Primary Treatment

• consists of mainly the sedimentation process to
  remove suspended organic solids
• Chemicals are sometimes added in primary
  clarifier to assist in the removal of finely
  divided and colloidal solids or to precipitate
  phosphorous

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Secondary or Biological Treatment

• Activated sludge process
• Filtration

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Activated sludge process

• the sewage is biologically treated.

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Filtration

• is done in contact beds or intermittent sand
  filters or tricking filters.
• It removes finely divided suspended matter.

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Sludge Effluent

• The semi-solid that settles down at the bottom
  after treatment,is called sludge
• liquid to be discharged is called effluent.

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Disinfecting

• Disinfecting is carried out if necessary by
  chlorination to kill the bacteria which remain in
  the effluent of sewage

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Tertiary Treatment

• If the receiving water provides only little
  dilution or there is a need to preserve a
  particularly high quality of effluent, tertiary
  treatment can be used.

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PRELIMINARY TREATMENT
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Objectives

• Remove coarse suspended and floating matter such
  as rags, plastic, pieces of wood, etc., by means
  of screening
• Remove grit, sand, broken glass, etc., in grit
  chambers
• Remove grease, and oil substances by floatation
  in skimming tanks

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Processes

• Screening
• Silt and grit removal
• Oil and grease removal

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PRIMARY TREATMENT
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PRIMARY TREATMENT

• Primary sedimentation tank
• (Secondary sedimentation tank) - sedimentation
  after activated sludge process

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Advantages

• The advantages of coagulation over plain
  sedimentation are
• Sedimentation by coagulation is more effective
• BOD, colour and turbidity are reduced
• Less capacity of sedimentation tanks
• The process is simple

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Disadvantages

• The disadvantages are
• Chemicals destroy the bacteria that digest the
  sludge
• Chemicals increase the cost of sedimentation
• Skilled supervision is required
• Large quantity of sludge is produced

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SECONDARY TREATMENT
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SECONDARY TREATMENT

• The secondary treatment converts the remaining
  organic matter into stable form by putting
  aerobic and anaerobic bacteria into action

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Processes

• Filtration
• Activated sludge process

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Filtration

• Aerobic bacteria need a contact surface to live
  and carry out their activities

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Filtration

• Contact beds
• Intermittent sand filters
• Trickling filters

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Contact Beds

• A contact bed consists of filtering medium.
• When the sewage is passed over the medium, a thin
  film is produced around the particles.
• The bacteria present in the film oxidise the
  organic matter.

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Intermittent Sand Filter

• the filtering medium consists of sand.
• The action of the filter is due to
• Mechanical straining and
• Bacterial action

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Trickling Filters

• A trickling filter is an artificial bed of stone
  or broken brick material
• waste water is distributed an applied in drops,
  films or spray
• A zoogleal film is formed on the surface media.

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ACTIVATED SLUDGE PROCESS

• is a biological sewage treatment process
• a mixture of sewage and activated sludge is
  agitated and aerated.
• The activated sludge is subsequently separated
  from the treated sewage by sedimentation

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What is ACTIVATED SLUDGE

• is that sludge which settles down after the
  sewage has been freely aerated and agitated for a
  certain time.

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How Activated Sludge works

• Activated sludge contains numerous bacteria and
  other microorganisms. When it is mixed with raw
  sewage saturated with oxygen, the bacteria
  perform
• Oxidise the organic solids
• Promote coagulation and flocculation and convert
  the colloidal and suspended solids into settable
  solids

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Steps in Activated Sludge process

• Treat raw sewage in the primary settling tank
• mix sewage with the activated sludge and aerated
  for 4-10 hours
• final (secondary) settling tank
• dispose of effluent
• return some sludge to the aeration tanks
• dispose of the rest of the sludge

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Advantages

• Gives clear sparkling treated effluent
• Effluent free from offensive odour
• Degree of purity can be varied as desired
• Cost of installation lesser than for a trickling
  filter
• Small loss of head in the process

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Disadvantages

• Skilled supervision and constant check on the
  return sludge is necessary
• When there is change in the quality or quantity
  of sewage, the process is upset
• The process does not work for some industrial
  wastes
• Large volume of sludge increases difficulty in
  disposal

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Methods of Aeration

• The success of the activated sludge process
  depends on the aeration provided.
• Diffused air aeration
• Mechanical aeration
• Combination of the above two methods

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TERTIARY TREATMENT
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TERTIARY TREATMENT

• MICROSTRAINERS
• RAPID GRAVITY SAND FILTERS
• UPWARD-FLOW MEDUIM SAND FILTERS
• SLOW SAND FILTERS
• PEBBLE-BED CLARIFIERS
• SETTLEMENT
• GRASS PLOTS
• LAGOONS

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SLUDGE TREATMENT AND DISPOSAL
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Sludge

• Sludge is a semi-liquid
• Sludge produced by plain sedimentation
• Sludge produced by chemical precipitation
• Trickling filter
• Activated sludge

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Methods of Disposal

• Disposal on land
• Drying on drying beds
• Dumping into the sea
• Heat-drying
• Incineration
• Lagooning or ponding
• Sludge Digestion

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Sludge Digestion

• It transforms a portion of solids into liquids
  and gases, thereby reducing the sludge volume to
  be dealt with
• It breaks the organic matter of sludge into
  simpler compounds by the action of anaerobic
  bacteria.