Sewage contains minerals, inorganic matters and living organisms. Therefore, sewage which is collected from residential areas, public places, industries shall be treated properly for the following reasons :

  1. As sewage contains pathogenic bacteria, it causes health hazards. Therefore to prevent pollution of water in to which the sewage is let off, sewage shall be
  2. To prevent offensive odour, causing health hazard & irritation to the people living near such water or land where the sewage is disposed
  3. To prevent the destruction of fish & aquatic life if such sewage is disposed off directly in a river or
  4. To prevent offensive odour, polluting the entire atmosphere affecting the neighbourhood by creating a messy scene is the sewage is disposed off on a


  1. Preliminary treatment: – In the preliminary treatment, suspended floating impuritiessuch as rags, wood, metal, plastic etc. are removed. They have to be removed as they interfere with the treatment processes.
  2. Primary Treatment: – It consists of the sedimentation process to remove the suspended organic Sometimes chemicals are added in primary clarifiers to assist in the removal of colloidal & finely divided solid particles.
  3. Secondary treatment: – (Filtration / Activated sludge process) To remove the finely divided suspended matters, filtration is done in contact beds or intermittent or trickling filters. In the activated sludge process, digested sludge is added to the raw sewage together with oxygen. It promotes coagulation of the suspended & colloidal matter. The coagulated matter settles down at bottom which is called as sludge. The sludge is disposed of in dry beds or dumped into sea. The effluent is disposed for sewage farming or let into a natural drain or sea.
  1. Disinfection :- The process of killing the bacteria in the effluent of sewage by chlorination is called as Disinfection.

All the above processes are not always be required. The type of treatment to be given depends upon the quality of effluent required. If the water is used for drinking purposes on the downstream, complete treatment is necessary.

This practice of purifying the water is feasible for large quantity of sewage water. This type of treatments are feasible only when the total drainage network is provided in a town or city.

In some of the cities, if the total networking of drainage is not available due to some constraints; it is necessary to provide septic tanks for smaller townships. But the effluent water from such septic tanks can not be recycled economically by the conventional treatment methods.

But the water from such septic tanks can be treated to a better extent to make it useful for domestic purposes other than drinking.

It has become necessary to recycle the available water by all means, because of the increasing scarcity of good quality water.

The water collected in a septic tank gets partially treated and hence is not much useful. It is required to treat this water by various means to bring it to the portability level.

The norms for deciding the purity of water are based on following criteria:-

  • O.D.:- Biological oxygen demand
  • O.D.:- Chemical oxygen demand
  • Total suspended particles
  • pH values

The standard values for the above criteria for water to be portable are as follows :

Sr. No.PropertiesValues
3.Total Suspended particles Value 

The conventional methods of water treatment are not feasible practically and economically. The conventional plants require more area , space and more time for the treatment. Taking into consideration these limitations new compact and economical ‘Sewage Treatment Plants’ are made available by new techniques.

To attain this level of purity of water, various means are used conventionally. But in the highly populated cities more compact plants with faster purification capacity are required. Therefore the process of removing the organic wastes from the sewage water is achieved with the help of bacterias.

A special compartment with a plastic media is provided in a vessel with the necessary arrangement for aeration. The plastic media, because of its size and shape provides more surface area for increasing the bacterial growth. This helps to treat more quality of water in shorter time. The bacterias eat the organic material from the seepage and thus help to purify the water. This partially purified water is collected inside the conical vessel at center and is collected outside this rector in a storage tank. The water collected in this tank is treated by chlorination.

The quality of this water is not 100% near to the potability but this water can be recycled for many other domestic purposes except drinking.

This recycling process helps indirectly to economize on the purchase cost of water and helps to maintain theenvironment.

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