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
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.
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 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 the cost of sedimentation Skilled supervision is required Large quantity of sludge is produced
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
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 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
Sludge Sludge is a semi-liquid Sludge produced by plain sedimentation Sludge produced by chemical precipitation Trickling filter Activated sludge
Methods of Disposal Disposal on land Drying on drying beds Dumping into the sea Heat-drying Incineration Lagooning or ponding 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.