2Outline Objectives Outline of treatment process (flow diagram) Preliminary treatmentPrimary treatment (sedimentation)Secondary treatment (activated sludge, filtration)Sludge treatmentDisinfectionTertiary treatment
3Objective of Sewage Treatment To prevent pollution of the receiving waterTo prevent offensive odour in the waterTo prevent the destruction aquatic lifeIf the sewage has to be disposed of on land, the soil will become sewage sick after some time and cannot take any more sewage
4Outline of Sewage Treatment Preliminary treatmentPrimary treatmentSecondary treatmentDisinfectingTertiary Treatment
5Preliminary 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.
6Primary Treatmentconsists of mainly the sedimentation process to remove suspended organic solidsChemicals are sometimes added in primary clarifier to assist in the removal of finely divided and colloidal solids or to precipitate phosphorous
7Secondary or Biological Treatment Activated sludge processFiltration
8Activated sludge process the sewage is biologically treated.
9Filtrationis done in contact beds or intermittent sand filters or tricking filters.It removes finely divided suspended matter.
10Sludge & EffluentThe semi-solid that settles down at the bottom after treatment,is called sludgeliquid to be discharged is called effluent.
11DisinfectingDisinfecting is carried out if necessary by chlorination to kill the bacteria which remain in the effluent of sewage
12Tertiary TreatmentIf 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.
14ObjectivesRemove coarse suspended and floating matter such as rags, plastic, pieces of wood, etc., by means of screeningRemove grit, sand, broken glass, etc., in grit chambersRemove grease, and oil substances by floatation in skimming tanks
15ProcessesScreeningSilt and grit removalOil and grease removal
17PRIMARY TREATMENT Primary sedimentation tank (Secondary sedimentation tank) - sedimentation after activated sludge process
18Advantages The advantages of coagulation over plain sedimentation are: Sedimentation by coagulation is more effectiveBOD, colour and turbidity are reducedLess capacity of sedimentation tanksThe process is simple
19Disadvantages The disadvantages are: Chemicals destroy the bacteria that digest the sludgeChemicals increase the cost of sedimentationSkilled supervision is requiredLarge quantity of sludge is produced
25Contact 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.
26Intermittent Sand Filter the filtering medium consists of sand.The action of the filter is due to:Mechanical straining andBacterial action
27Trickling FiltersA trickling filter is an artificial bed of stone or broken brick materialwaste water is distributed an applied in drops, films or sprayA zoogleal film is formed on the surface media.
28ACTIVATED SLUDGE PROCESS is a biological sewage treatment processa mixture of sewage and activated sludge is agitated and aerated.The activated sludge is subsequently separated from the treated sewage by sedimentation
29What is ACTIVATED SLUDGE is that sludge which settles down after the sewage has been freely aerated and agitated for a certain time.
30How 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 solidsPromote coagulation and flocculation and convert the colloidal and suspended solids into settable solids
31Steps in Activated Sludge process Treat raw sewage in the primary settling tankmix sewage with the activated sludge and aerated for 4-10 hoursfinal (secondary) settling tankdispose of effluentreturn some sludge to the aeration tanksdispose of the rest of the sludge
32Advantages Gives clear sparkling treated effluent Effluent free from offensive odourDegree of purity can be varied as desiredCost of installation lesser than for a trickling filterSmall loss of head in the process
33DisadvantagesSkilled supervision and constant check on the return sludge is necessaryWhen there is change in the quality or quantity of sewage, the process is upsetThe process does not work for some industrial wastesLarge volume of sludge increases difficulty in disposal
34Methods of AerationThe success of the activated sludge process depends on the aeration provided.Diffused air aerationMechanical aerationCombination of the above two methods
38Sludge Sludge is a semi-liquid Sludge produced by plain sedimentation Sludge produced by chemical precipitationTrickling filterActivated sludge
39Methods of Disposal Disposal on land Drying on drying beds Dumping into the seaHeat-dryingIncinerationLagooning or pondingSludge Digestion
40Sludge DigestionIt transforms a portion of solids into liquids and gases, thereby reducing the sludge volume to be dealt withIt breaks the organic matter of sludge into simpler compounds by the action of anaerobic bacteria.