Transcript Waste Water Treatment Sewage Treatment

Waste Water Treatment
(Sewage Treatment)
Outline
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Objectives
Outline of treatment process (flow diagram)
Preliminary treatment
Primary treatment (sedimentation)
Secondary treatment (activated sludge, filtration)
Sludge treatment
Disinfection
Tertiary treatment
Objective of Sewage Treatment
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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
Outline of Sewage Treatment
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Preliminary treatment
Primary treatment
Secondary treatment
Disinfecting
Tertiary Treatment
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.
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
Secondary or Biological
Treatment
• Activated sludge process
• Filtration
Activated sludge process
• the sewage is biologically treated.
Filtration
• is done in contact beds or intermittent sand
filters or tricking filters.
• It removes finely divided suspended matter.
Sludge & Effluent
• The semi-solid that settles down at the
bottom after treatment,is called sludge
• liquid to be discharged is called effluent.
Disinfecting
• Disinfecting is carried out if necessary by
chlorination to kill the bacteria which
remain in the effluent of sewage
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.
PRELIMINARY TREATMENT
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
Processes
Screening
Silt and grit removal
Oil and grease removal
PRIMARY TREATMENT
PRIMARY TREATMENT
Primary sedimentation tank
(Secondary
sedimentation
tank)
sedimentation after activated sludge process
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
Disadvantages
• The disadvantages are:
Chemicals destroy the bacteria that digest
the sludge
Chemicals
increase
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cost
of
sedimentation
Skilled supervision is required
Large quantity of sludge is produced
SECONDARY TREATMENT
SECONDARY TREATMENT
• The secondary treatment converts the
remaining organic matter into stable form
by putting aerobic and anaerobic bacteria
into action
Processes
Filtration
Activated sludge process
Filtration
• Aerobic bacteria need a contact surface to
live and carry out their activities
Filtration
Contact beds
Intermittent sand filters
Trickling filters
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.
Intermittent Sand Filter
• the filtering medium consists of sand.
• The action of the filter is due to:
Mechanical straining and
– Bacterial action
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.
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
What is ACTIVATED SLUDGE
• is that sludge which settles down after the
sewage has been freely aerated and agitated
for a certain time.
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
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
Advantages
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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
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
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
TERTIARY TREATMENT
TERTIARY TREATMENT
• MICROSTRAINERS
• RAPID GRAVITY SAND FILTERS
• UPWARD-FLOW “MEDUIM” SAND
FILTERS
• SLOW SAND FILTERS
• PEBBLE-BED CLARIFIERS
• SETTLEMENT
• GRASS PLOTS
• LAGOONS
SLUDGE TREATMENT AND
DISPOSAL
Sludge
• Sludge is a semi-liquid
• Sludge produced by plain sedimentation
• Sludge produced by chemical
precipitation
• Trickling filter
• Activated sludge
Methods of Disposal
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Disposal on land
Drying on drying beds
Dumping into the sea
Heat-drying
Incineration
Lagooning or ponding
Sludge Digestion
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.