Factors Affecting the Quantity of Dry Weather Flow | Waste Management

The following points highlight the four main factors affecting the quantity of dry weather flow through the sewers. The factors are: 1. Rate of Water Supply 2. Population Growth 3. Type of Area Served 4. Infiltration and Exfiltration.

Factor # 1. Rate of Water Supply:

It is evident that a considerable part of the water supplied to the public through public water supply system emerges as domestic or sanitary sewage. As such the quantity of domestic or sanitary sewage produced depends on the rate of water supply. The quantity of domestic or sanitary sewage produced will usually be slightly less than the quantity of water supplied.

This is due to the fact that some quantity of water is lost in consumption, evaporation, etc. However, besides the water supplied to the public through public water supply system, sometimes water from private sources such as domestic wells etc., may also be used by the public for their domestic needs.

This additional quantity of water used by the public will also produce certain amount of domestic or sanitary sewage which will also enter the sewers. The additional quantity of domestic or sanitary sewage may be assumed to be approximately equal to the quantity of water lost in consumption, evaporation, etc.

With this assumption it may be considered that the rate of domestic or sanitary sewage produced is nearly equal to the rate of water supplied to the public through public water supply system. However, in actual practice the quantity of domestic or sanitary sewage may be assumed to be equal to about 70 to 80% of the quantity of water supplied to the public through public water supply system.

From the above discussion it is thus clear that if the rate of water supply through public water supply system is known, the probable rate of domestic or sanitary sewage produced can be estimated. The rate of water supply through public water supply system is, however, not constant but it increases with the increase in population.

The increase in the rate of water supply will result in an increase in the rate of domestic or sanitary sewage produced. Table 3.1 shows the variation in the rate of water supply and the corresponding rate of domestic or sanitary sewage produced with population for normal Indian conditions. The rate of domestic or sanitary sewage produced has been considered as approximately 80% of the rate of water supply.

Factor # 2. Population Growth:

Alike water supply project a sewerage project is also designed to serve not only the present population but also the prospective or future population which may occur at the end of a reasonable period usually termed as design period. The prospective or future population for which the sewerage project is designed is termed as design population.

For arriving at the design population various methods of forecasting or estimating the prospective or future population are adopted as indicated below:

i. Arithmetical increase method

ii. Geometrical increase method

iii. Incremental increase method

iv. Changing rate of increase method

v. Graphical method

vi. Comparative method or Curvilinear method

vii. Decreasing rate of growth method or Declining growth method

viii. Logistic method

ix. Zoning method

x. Ratio and correlation method

However, these methods can be used to predict the future population only if the population data for the past few decades is available. If such information on population is not available then the densities of population as suggested in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Engineering Organisation may be adopted.

In cities where Floor Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed by the local authority the same may be used for working out the population density. FSI or FAR is the ratio of total floor area (of all the floors) to the plot area.

The densities of population on this concept may be worked out as indicated by the following example:

Assume that a particular development plan rules provide for the following reservations for different land uses –

Design Period:

Sewerage projects are normally designed to meet the requirements over a period of 30 years after their completion. In other words a design period of 30 years is normally considered for a sewerage project. However, the period of 30 years may be modified in respect of certain components of the project depending on their useful life or the facility for carrying out extensions when required and rate of interest, so that expenditure far ahead of its utilisation is avoided.

Factor # 3. Type of Area Served:

The quantity of sewage or wastewater produced from an area would depend on whether the area to be served is residential, commercial or industrial. As indicated earlier the quantity of sewage or wastewater produced from a residential area depends on the quantity of water supplied to the public through public water supply system, and it may be assumed to be equal to about 70 to 80% of the quantity of water supplied to the public through public water supply system.

The quantity of industrial sewage or wastewater produced from an industrial area depends on the type of industries and the corresponding manufacturing or industrial processes. A careful study of the processes involved in different industries is therefore required to be made to determine the quantity of industrial sewage or wastewater. A similar study is required to be made to determine the quantity of sewage or wastewater produced from commercial undertakings.

Factor # 4. Infiltration and Exfiltration:

Some quantity of groundwater or subsoil water may infiltrate into sewers through defective joints, broken pipe and other similar entry points. The infiltration of groundwater or subsoil water into sewers may take place when the head of groundwater or subsoil water surrounding the sewers is more than the head of sewage or wastewater flowing through the sewers.

On the other hand exfiltration is a term which indicates the leakage of sewage or wastewater from sewers into the ground surrounding the sewers. Exfiltration may occur through defective joints, broken pipe, etc., when the head of sewage or wastewater flowing through sewers is more than the head of groundwater or subsoil water surrounding the sewers. While due to infiltration the quantity of flow through sewers increases, exfiltration results in decrease in the quantity of flow through sewers. Thus exfiltration is reverse of infiltration.

Both infiltration as well as exfiltration are undesirable. The infiltration unnecessarily increases the quantity of sewage or wastewater. The exfiltration pollutes the underground or subsoil source of water, if any. Since infiltration and exfiltration take place mainly due to imperfect joints, these can be prevented to some extent by constructing watertight joints.

From the point of design of sewers infiltration is much more important than exfiltration.

The quantity of groundwater or subsoil water which may enter sewers through infiltration depends on the following factors:

(i) Depth of sewer below the groundwater level (or head of subsoil water)

(ii) Length of sewer

(iii) Size of sewer

(iv) Nature and type of soil through which sewer is laid

(v) Sewer material

(vi) Care exercised in the construction of the sewer.

The rate of infiltration of groundwater or subsoil water into sewers may be considered in the following three ways:

(a) Area Basis:

In this case the rate of infiltration is considered per unit of area served by the sewer and it is expressed as litres per day per hectare. Depending on the above noted factors a flat allowance of 5 000 to 50 000 litres per day per hectare may be made for the amount of infiltration of groundwater into a sewer.

For example – if the rate of infiltration is 10 000 litres per day per hectare and if the sewer serves an area of 15 hectares, the quantity of groundwater that will enter the sewer through infiltration would be 10 000 x 15 = 150 000 litres per day.

(b) Length Basis:

In this case the rate of infiltration is considered per unit length of sewer and it is expressed as litres per day per kilometre. For normal conditions, the rate of infiltration of groundwater into a sewer may be taken as 500 to 5 000 litres per day per kilometre length of sewer.

Thus if rate of infiltration is 2 000 litres per day per kilometre length of sewer and the length of sewer is 100 kilometres, the quantity of groundwater that will enter the sewer through infiltration would be 2000 x 100 = 200 000 litres per day.

(c) Diameter—Length Basis:

In this case the rate of infiltration is considered per unit diameter per unit length of sewer and it is expressed as litres per day per centimetre of diameter per kilometre. This is the most logical way of expressing the rate of infiltration because larger the diameter of sewer greater will be the amount of infiltration of groundwater into the sewer.

Estimation of Industrial Sewage:

The quantity of industrial sewage will vary with the type and size of industry, the manufacturing or industrial processes involved, the degree of water reuse, and the on-site treatment methods that are used, if any. As such a careful study of the various industries is required to be made to determine the quantity of industrial sewage.

However, in general the quantity of industrial sewage may also be assumed to be approximately equal to about 80 to 90% of the quantity of water supplied through public water supply system. Some industries develop their own source of water supply and thus use water other than that is supplied through public water supply system, and may discharge their liquid wastes into sewers.

Estimates of such flows should also be made while determining the quantity of industrial sewage. It may, however, be stated that industrial sewage should be treated to the standards prescribed by the regulatory authorities before being discharged into sewers.

Estimation of Groundwater Infiltration:

The quantity of groundwater which may infiltrate into sewers will depend on workmanship in laying of sewers and level of water table (or groundwater surface). Since sewers are designed for maximum or peak rates of flow, allowance for groundwater infiltration for the worst condition in the area should be made.

The estimates for groundwater infiltration for sewers laid below water table may be made as follows:

The above noted values of maximum and minimum rates of groundwater infiltration are as suggested in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Engineering Organisation.