Environmental Engg.

Lesson-21 Disposal of domestic waste water in rural and urban areas

The methods of sewage disposal can classified as follows:

Disposal by dilution

Disposal by land treatment

Disposal by dilution

In this process, the raw sewage or the partially treated sewage is thrown into natural waters having large volume.  The sewage in due course of time is purified by what is known as the self-purification capacity of natural waters.  The limit of discharge and degree of treatment of sewage are determined by the capacity of self-purification of natural waters.

Conditions favorable for dilution

Following conditions are favourable for sewage to be disposed off by dilution into natural waters

1. It is possible only to provide primary treatment to sewage i.e., removal of floating matter and settleable solids.

2. Currents of flow of diluting waters should be favourable which means that nuisance should not be caused when sewage is discharged into diluting waters

3. Diluting waters are not used for the purpose of navigation for at least some reasonable distance on the downstream from the point of sewage disposal.

4. Diluting waters should not have habitation or they should not hage been used as source of water supply for at least some reasonable distance on the downstream from the point of sewage disposal

5. Dissolved oxygen content of diluting waters should be high

6. The place is situated near natural waters having large volumes

7. The sewage is relatively fresh and it is possible to bring it to the point of discharge within four or five hours of its production.

Types of natural waters

Following are the natural waters into which the sewage can be discharged for dilution

1. Creeks

2. Estuaries

3. Ground waters

4. Lakes

5. Ocean or sea

6. Perennial rivers and streams

Self purification of natural waters

When sewage is discharged into natural water, its organic matter gets oxidized by the dissolved oxygen content in water.  The oxidation of organic matter converts such matter into simple inoffensive substances.  Deficiency of dissolved oxygen thus created in natural waters is filled up by the absorption of atmospheric oxygen.  Thus, the oxygen of water is consumed by sewage and at the same time, it is replenished by the atmosphere.  This phenomena which occurs in all natural waters is known as self-purification of natural waters.  It is thus seen that natural waters, polluted by sewage, are purified in natural course by the phenomena of self-purification

The rate of self-purification will depend on various factors such as rate of re-aeration type of organic matter present in sewage, temperature, velocity of flow, presence of available oxygen in receiving waters, sedimentation, etc

Disposal by land treatment

Here, the raw domestic waste water (sewage) is applied on the land. A part of sewage evaporates and the remaining portion percolates through the ground and is caught by the underground drains for disposal into natural waters.  The sewage adds to the fertilizing value of land and crops can be profitably raised on such land.  The term sewage farming is also sometimes used for indicating disposal of sewage by land treatment.  The design of a good land treatment system demands the services of environmental engineers, hydraulic engineers, irrigation engineers, agronomists, soil scientist, etc.

Conditions favourable for land treatment

1. The area of land treatment is composed of sandy, loamy or alluvial soils.  Such soils are easily aerated and it is easy to maintain aerobic conditions in them

2. The depth of water table is more even in rainy season so that there are no chances of pollution of underground water sources by land treatment

3. The rainfall in the area is low as it will assist in maintaining good absorption capacity of soil

4. There is absence of river or other natural water sources in the vicinity of disposal of sewage

5. There is demand for cash crops which can be easily grown on sewage farms

6. There is availability of large open areas in the surrounding locality for practicing broad irrigation by sewage

Advantages of land treatment

1. It increases the fertility of land

2. It is cheap where land is available in plenty

3. Application of sewage on land is the best method of supplying manure to the soil

4. Crops grown on land treated with sewage possess high calorific value and more vitamins.

5. Increased fertility of land results in profitable returns of crops

6. The method becomes very much useful at places where disposal of sewage by dilution is not possible.

7. The method does not require costly equipment for its working.

8. The method proves economical and safe where available irrigation water is scarce in quantity

9. The method to some extent charges the underground aquifers

10. Water of irrigation canal is saved when this method is practised.

Disadvantages of land treatment

1. If proper precautions are not taken, nuisance developed by sewage farming may lead to possible dangers to the health of men.  It is therefore, necessary that the sewage farms should be operated under skilled technical supervision

2. Crops grown on sewage farms are generally not liked by ordinary public

3. The method is not applicable for all the seasons of year.  In monsoon, some other arrangement of sewage disposal has to be found out.

4. The method requires large area of land which may not be available in some cases

5. Types of crops grown on sewage treated land are limited in number.

Sewage sickness

If sewage is applied continuously on a piece of land, pores or voids of soil are filled up or clogged.  Free circulation of air is thereby prevented and anaerobic conditions develop.  At this stage, the land is unable to take any further sewage load.  Organic matter decomposes and foul smelling gases are produced.  The phenomena of soil is known as sewage sickness of land.

Preventive measures

In order to prevent sewage sickness of land, the following preventive measures may be adopted

Alternative arrangement:  There should be ample provision of extra land so that land with sewage sickness can be given the desired rest.  Alternatively, sewage should be disposed off by some other method when sewage farms are taking rest

Depth of sewage:  If sewage is applied in excess, the chances of sewage sickness are increased.  The land is unable to receive the excess sewage in a satisfactory way and it ultimately clogs up.  Depth of sewage on land should be carefully decided by keeping in view the climatic conditions, drainage facilities, nature of crops and characteristics of soil.

Drainage of soil:  Subsoil drain pipes should be laid in sufficient number to collect the percolated effluent

Intermittent application:  Sewage should be applied on land at intervals.  The period between successive applications depends on general working of sewage farm and the permeability of soil.  Depending on the nature of the soil, this period between successive applications varies from few hours to few weeks.

Pretreatment of sewage:  sewage should be given some pretreatment before it is applied on land.

Rotation of crops:  It is desirable to grow different types of crops on a piece of land instead of one single crop.  Rotation of crops minimizes the chances of sewage sickness.

Treatment to land:  The land affected by sewage sickness should be properly treated before it is put up in use again.  Clogged surfaces should be broken by suitable equipment.