Sewage Treatment: Object, Degree and Types | Sewage | Sanitary Engineering

In this article we will discuss about:- 1. Introduction to the Treatment of Sewage 2. Object of Treatment of Sewage 3. Degree of Treatment 4. Period of Design of Sewage Treatment Plant 5. Dumping Chutes for Night Soil 6. Effluent Disposal and Utilisation of Sewage 7. Types of Sewage Treatments 8. Location of Treatment Plants 9. Treatment Processes and Other Details.

Introduction to the Treatment of Sewage:

Sewage contains various types of impurities and disease bacteria. This sewage is disposed off by dilution or on land after its collection and conveyance. If the sewage is directly disposed of, it will be acted upon the natural forces, which will convert it into harmless substances. The natural forces of purification cannot purify any amount of sewage within specified time.

If the quantity of sewage is more, then receiving water will become polluted or the land will become sewage sick. Under such circumstances it becomes essential to do some treatment of the sewage, so that it can be accepted by the land or receiving water without any objection. These treatment process will directly depend on the types of impurities present in the sewage and the standard up to which treatment is required.

Object of Treatment of Sewage:

The sewage can be disposed of on land or in water. When the sewage is disposed of by these methods on the land or in the water, the natural purification will immediately start acting over it and the sewage treatment will be done in due course of time. In case of small towns and when the quantity of sewage is not very high it can be directly disposed of without any treatment.

Now if the quantity of sewage is too high or it contains large amount of waste elements, it will not be possible by the land or the body of water to purify it by the natural forces of purification.

The land on which excessive sewage will be applied will become sewage sick and it will create dirty scene at the site as well as nuisance and objectionable unhygienic smells. If the excessive quantity of sewage is mixed with the natural water course, the water will become septic and totally unfit for any other use.

Therefore, it becomes necessary to reduce the quantity or sewage strength and make it such that it can be safely applied to the land or discharged in the natural water courses. This object is obtained by the treatment of the sewage and by reducing its strength.

Thus the main object of the treatment units is to reduce the sewage contents (solids) from the sewage and remove all the nuisance causing elements and change the character of the sewage in such a way that it can be safely discharged in the natural watercourse applied oh the land.

In other words, the sewage treatment plants are mainly employed to supplement the natural purifying powers of land and water courses and help in maintaining their normal uses.

Practically, the treatment of sewage is required in big cities only where the volume of the sewage is more as well as the quantity of various types of solids, industrial sewage etc. is more and porous land or large quantity of water body is not available for the proper disposal of the sewage. In case of small towns there is no such problem due to small quantity of sewage.

Degree of Treatment of Sewage:

The degree of treatment will mostly be decided by the regulatory agencies and the extent to which the final products of treatment are to be utilised. The regulatory bodies might have laid down standards for the effluent or might specify the conditions under which the effluent could be discharged into a natural stream, sea or disposed of on the land.

These regulatory bodies may be the local body or a State Water Pollution Prevention and Control Board. The method of treatment adopted should not only meet the requirements of these regulatory agencies, but also result in the maximum use of end products with economy.

Period of Design of Sewage Treatment Plant:

The treatment plant, like the sewerage system, is normally designed to meet the requirements over a 30 year period after its completion. The time lag between the design and the completion should not normally exceed 2-3 years, and in exceptional cases 5 years. Care should be taken that the plant is not considerably under-loaded in the initial stages, particularly the sedimentation tanks.

The treatment plant should be considered as a part of the main sewerage project and the area to be served is to be decided based on the needs of the main project itself. The main project may not be executed at one stretch, but may be done in stages as the development of the area takes place.

But in any case, the ultimate period of design of the project should be 30 years and to that extent sufficient accommodation should be provided for all the units necessary to cater to the needs of this ultimate population.

In some cases, it may be necessary to combine a number of sewerage systems with a common sewage treatment plant. It may be planned to combine treatment of the waste from a nearby industry along with the community wastes from the urban area.

But in such cases, it should be carefully investigated whether liquid wastes from the industry do not adversely affect either the sewerage system or the treatment process and could be accepted, if necessary with some preliminary treatment, so that it could be combined with the sewage from the community for or unified and economical treatment.

Dumping Chutes for Night Soil:

Detachable dumping chutes are constructed at selected points in the sewerage system for disposing of the night-soil collected manually from individual houses or through vacuum cars from collection wells in towns, which are only partially sewered. Such discharges have the effect of increasing the concentration of solid and BOD. If necessary, sufficient dilution should be provided to prevent the clogging of sewers.

In such cases, the treatment plant will have to provide for the change in the characteristics of the sewage. These dumping chutes should be removed when the sewerage system is expanded to cover the unsewered areas.

Effluent Disposal and Utilisation of Sewage:

The degree of the treatment provided to the sewage is governed by the specific purpose for which the sewage effluent is to be used. The sewage effluent is preferably used for irrigation of crops (sewage farming) with precautions. The effluent can also be used for artificial recharging of ground water or for industries as process or cooling water.

Another use of the effluent may be at the treatment plant itself for purposes, such as flushing and foam control, chlorinator injector water, lawn sprinkling, fire protection and general plant operations.

In case the conditions mentioned above are not suitable for using the effluent water for any of the above purposes, it may be disposed of in a natural body of water such as river, lake, pond, sea or in the irrigation canal using necessary precautions.

Types of Sewage Treatments:

Sewage treatment units can be broadly classified as:

(a) Preliminary treatment,

(b) Secondary treatment, and

(c) Final treatment.

Table 12.1 gives the various units employed under each classification and the operations involved in brief: