Treatment of sewage, Biology

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The sewage treatment methods can be classified into the following heads:

Primary treatment

Secondary treatment

Tertiary treatment

1. Primary Treatment:

The primary treatment methods consist of screens, gritty chamber, oil and grease trap and primary sedimentation for the removal of insoluble matter such as grit, grease, and scum from water.

Screens: Screening is the first operation to be performed on the incoming sewage for removal of the large floating matter such as rages, wood pieces etc. The objective of the screening it's the protection of mechanical equipment such as pumps from damage due to large floating and suspended matter. Usually coarse screen such as bar screens are used for this purpose.

Grit Chamber: Grit chamber are for the removal of inorganic particles in the sewage such as grit and detritus. This is achieved y reducing the velocity of flow in an enlarged channel where the particles will get settled out.

The velocity of flow in the grit chamber should neither be very low not very high because if the velocity is high. The inorganic matters will not get settled out in the chamber and if the velocity is very low. Even the organic matter which has to be removed in the subsequent treatment techniques will be setting in the grit chamber.

Oil and grease Trap:

An oil and grease trap is used for the removal of oil and grease and other floating matter before in reaches the biological treatment processes and impedes the aeration process. Removal of oil and grease in essential for the efficient working of any biological treatment system.

Primary Sedimentation:

Sedimentation is the gravitational settlement of suspended and colloidal particles denser than water. When Sedimentation is done without the addition of any chemical coagulants it is called plain sedimentation. When it is adopted before any biological treatment. Than we call it primary sedimentation. Sedimentation tanks are made either in circular or rectangular shape. Major design parameters for the sedimentation tanks are following:

Surface overflow rate or surface area loading: This is to ratio of the discharge passing through the sedimentation tank to the plan area of tank.

Detention Time: Detention time is the duration for which sewage is retained in the sedimentation tank and it is the ratio of the volume of the tank to the discharge passing through the tank.

Horizontal flow velocity: It should be less than 0.3 m/s in the tank.


2. Secondary Treatment:

The secondary treatment of sanitary sewage essentially consists of biological treatment techniques for the removal of BOD (Organic Matter) The biological treatment technique are classified on the type of organisms involved and thus we have the broad categories of aerobic, anaerobic and facultative treatment system. The Classification of commonly treatment techniques are listed in table 2.3.

                                             Table 2.3 typical convention biological treatment for sewage and their classification





Aerobic suspended

Growth process

Activated   sludge   process


Good efficiency in BOD removal,

High energy requirement.

Aerobic attached

Growth process

Trickling filter and rotating

Biological contactor

High BOD removal.

Low  energy  requirement

Compared to ASP.

Occupies large area.

Aerobic suspended

Growth process

Septic tank

 Incomplete treatment.

No energy requirement.

Produces noxious gases having

Fuel value.

Aerobic attached

Growth process

Anaerobic  filters

Incomplete treatment.

No energy requirement.

Produce noxious gases having

Fuel value.


(ASP) activated sludge process:  activated sludge process is an aerobic suspended growth biological treatment system. In this process the aerobic bacteria present in the aeration tank biochemically oxidizes the organic matter present In the sewage. During the above process the unstable organic matter will be converted to stable end -products such as carbon- dioxide, water, nitrate, suphates, energy etc. and large number of bacteria. The excess aerobic bacteria produced in the aeration tank will be removed in the secondary setting tank as sludge. A portion of the sludge removed from the secondary setting tank will be returned to the aeration tank to maintain an active concentration of bacteria there. It is essential to have large number of active organism in the aeration tank to consume in the incoming organic matter in the sewage.

                 A flow diagram of the activated sludge process is shown in fig .2.10.

Trickling filter process: trickling filter is an aerobic growth biological treatment system. A typical flow diagram and construction details of a trickling filter is shown in fig. (a) and (b). sewage after settling in the primary settling tank passes through the trickling filter media and during this process micro-organisms attached to the trickling  filter media biochemically  oxidizes  the organic matter and a portion of it will  be bioflocculated. The bioflocculated organic matter along with the excess aerobic organisms released from the filter media will be settled in the secondary settling tank. The above mechanism results in BOD removal efficiencies up to 98%.

Septic tank: septic tank is an anaerobic suspended growth biological treatment system. The septic tank usually consists of two or more compartments with the first compartment being the largest one. In the septic tank anaerobic bacteria biochemically oxidizes the organic matter.  Sedimentation of the suspended solids takes place in the same compartment where the digestion also takes place. Septic tanks are usually adopted for small communities of population up to 300. The effluent from the septic tank is usually dispersed in the ground using subsurface dispersion trenches.

Oxidation ponds: oxidation ponds is a low cost natural  treatment system in which the oxygen required by the bacteria for the biochemical  oxidation of organic matter is provided by the atmospheric wind and algae present in the oxidation  pond. Algae utilize the nutrients and carbon-di-oxide provided by the bacteria for photosynthesis and supplements the bacteria with oxygen, a product of photosynthesis. This symbiotic relationship between the algae and bacteria is a major feature of the oxidation pond. In this process, the depth of the pond must the minimal (around 0.5m) to keep the entire pond aerobic. If the depth of the pond is more then, enough oxygen will not be reaching the bottom portions and anaerobic digestion will start.

3. Tertiary treatment or advanced treatment:

                  The main function of tertiary treatment is to remove the load of nitrogen and phosphorous compounds in the effluents. It also removes toxic substance and disinfects the water.

Its various steps are as follows:

(i)              Phosphorus removal:

For its removal, the effluent received after secondary treatment is tested with calcium oxide which converts the phosphorous compounds into insoluble calcium phosphate. It settles down at the bottom from where it can be separated out.

           Phosphorous can also be removed biologically by a process called enhanced biological phosphorous removal. It can be achieved by using specific bacteria called polyphosphate accumulating organisms which can accumulate large amount of phosphorous within their cells.

(ii)            Nitrogen removal:

A nitrification - denitrification process can be used for the removal of nitrogen from waste water. In the first step, the environment is aerobic (which favours microbial nitrification), hence nitrogen is converted to nitrates


               NH4 (aq.)+ 2O2(aq.)+H2O   1119_food chain.png  NO3 (aq.)+2H3O   (aq.)

                                                Or nitrobactor

The second step needs anaerobic environment and denitrification takes place in the presence of denitrifying bacteria called  pseudomonas, micrococcus . Here the nitrates are reduced to N2 gas in the presence of electron donors like sugars and other organic compounds in the waste water.

(iii)          Disinfection:

It is the process of removing pathogenic bacteria, virus and protozoa from water. For this purpose, the following methods can be used:

(a)boiling: it is could be used  only for small quantities of water for which water  should be boiled for at least 20-25 minutes.

(b) Bleaching powder method: bleaching powder (CaOCI2) is a very good disinfectant.

CaOCI2 + H2O   1119_food chain.png  Ca(OH)2 + CL2

CL2 + H2O  1119_food chain.png  HCL  +HOCL

Germs  + HOCIL  1119_food chain.png Germs are killed.

HOCIL (Hypochlorous Acid)  1119_food chain.png  powerful germicide.



(i)              Increases hardness.

(ii)            It deteriorates during storage.

(iii)          Excess CaOCI2 gives bad taste and odour to treated water.

(c)   Direct chlorination: chlorine (either gas or in concentrated solution form) produces hypochlorous acid which is a powerful germicide.

                                      CI2+H21119_food chain.pngHOCI  + HCL

                          Bacteria  + hocil 1119_food chain.png bacterial  killing.


           (i)            economical and stable.

           (ii)           does not introduce hardness.


           (i)            excess produce irritation on mucous membrane.

           (ii)           excess chlorine produces unpleasant taste and odour.

(d)   by using chloramine (CI-NH2)


                                                      CL2    +   NH3  1119_food chain.png  CINH2     +   HCL

                                                        2             1              chloramine           

Chloramine is more  lasting than chlorine alone, hence it is a better disinfectant.

                                                      CINH2  +H21119_food chain.png HOCI     +   NH3



     (i)       it imparts good taste to treated water.

     (ii)      excess does not produce irritating odour.

(e)  ozonisation:


                                       3O2           1119_food chain.png    2O3

                                                   Electric  discharge

                                       (oxygen)                  (ozone)


Ozone is highly unstable and breaks down liberating nascent oxygen.

                                       O3 1119_food chain.png  O2  +   (O)

                                                               Nascent  oxygen.  (powerful  oxidizing  agent)

Kills all the bacteria and oxidizes the organic matter present in water.


        (i)             excess of ozone is not harmful as it decomposes to give oxygen.

        (ii)            it improves taste of water.

Disadvantages:   very expensive.

        (f) By adding  KmnO4: it is generally used in rural areas where water supplies are from wells. It removes Fe, H2S and Mn from water. The normal  dosage of KmnO4 is to 2 mg/ within a period of 4 to 6 hours.

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