Environmental Engg.

Lesson-16 Design discharge for sewers

Design Period

The length of time up to which the capacity of a sewer will be adequate is referred to as the design period. In fixing a period of design, consideration must be given for the useful life of structures and equipment employed, taking into account obsolescence as well as wear and tear. Because the flow is largely a function of population served, population density and water consumption, lateral and sub main sewers are usually designed for peak flows of the population at saturation density as set forth in the Master Plan.

Population Forecasting

There are several methods for estimation or forecasting of population which can predict or forecast population for a specific design period, usually three to four decades.

Tributary area

The natural topography, layout of buildings, political boundaries, economic factors, etc., determine the tributary area. For larger drainage areas, though it is desirable that the sewer capacities be designed for the total tributary area, sometimes, political boundaries and legal restrictions prevent the sewers to be constructed beyond the limits of the local authority. However, in designing sewers for larger areas, there is usually an economic advantage in providing adequate capacity initially for a certain period of time and adding additional sewers, when the pattern of growth becomes established. The need to finance projects within the available resources necessitates the design to be restricted to political boundaries. The tributary  area for any section under consideration has to be marked on a key plan and the area can be measured from the map.

Per capita sewage flow

The entire spent water of a community should normally contribute to the total flow in a sanitary sewer. However, the observed dry weather flow quantities usually are slightly less than the per capita water consumption, since some water is lost in evaporation, seepage into ground, leakage, etc. In arid regions, mean sewage flows may be as little as 40% of water consumption and in well developed areas, flows may be as high as 90%. However, the conventional sewers shall be designed for a minimum sewage flow of 100 litres per capita per day or higher as the case may be. Non-conventional sewers shall be designed as the case may be.

The flow in sewers varies from hour to hour and also seasonally. But for the purpose of hydraulic design, estimated peak flows are adopted. The peak factor or the ratio of maximum to average flows depends upon contributory population as given in following Table.

Table. Peak factor for contributory population

 

Contributory Population	Peak Factor
up to 20,000	3.00
above 20,001 to 50,000	2.50
above 50,001 to 7,50,000	2.25
above 7,50,001	2.00

The peak factors also depend upon the density of population, topography of the site, hours of water supply and therefore individual cases may be further analysed if required. The minimum flow may vary from 1/3 to 1/2 of average flow.

Infiltration

Estimate of flow in sanitary sewers may include certain flows due to infiltration of groundwater through joints. Since sewers are designed for peak discharges, allowances for groundwater  infiltration for the worst condition in the area should be made as in Table..

Table. Ground water Infiltration

 

Description	Unit	Minimum	Maximum
Area	litres/ha/day	5000	50000
Length of sewers	litres/km/day	500	5000
Number of manholes	litres/day/manhole	250	500

Once the flow is estimated as per the above Table, the design infiltration value shall be limited to a maximum of 10% of the design value of sewage flow.

Sewage from Commercial institutions

Industries and commercial buildings often use water other than the municipal supply and may discharge their liquid wastes into the sanitary sewers. Estimates of such flows have to be made separately.

STORM WATER

Wherever possible, the storm water is to be collected and conveyed in sewers at proper places for the following reasons:

• Damp conditions are created which are unhygienic as they provide flourishing ground for micro organisms

• Existence of waterpools affects the foundations of structures

• Initial washings of streets by storm water contain organic matter and hence such water requires to be collected and to be taken to the treatment plant

• Low lying areas get flooded and transport system is paralysed. It leads to loss of revenue.

• Stagnant waterpools serve as breeding places for mosquitoes.

 The quantity of storm water, which is known as wet weather flow (WWF) entering the sewer is to be carefully determined.  It involves various factors such as intensity of rainfall, characteristics of catchment area, duration of storm, etc.  Following two methods are generally employed for calculating the quantity of storm water for the purpose of designing sewers:

• Rational method

• Empirical method

Rational method

In this method, the following three factors are combined in the form of an equation:

Q= K I A / 360

Where,

Q = peak runoff in m³ per second

K = Impermeability factor

I = Intensity of rainfall, mm per hour

A = Area in hectares

Catchment area

The catchment area to be served by a storm water sewer is measured directly from the map of the locality.

Impermeability factor

Some quantity of rain water that falls on the ground is absorbed by soil and the percentage of rain water that enters the sewer is known as impermeability factor.  The following table gives the impermeability factors for various types of surfaces.

Type of surface	Impermeability Factor
Water tight roofs and such other covered surface	0.70 – 0.95
Pavements of asphalt or concrete	0.85 – 0.90
Areas with many buildings	0.70 – 0.90
Pavements of bricks, stones or wooden blocks	0.75 – 0.85
Pavements of bricks, stones or wooden blocks with open joints	0.50 – 0.70
Areas with adjacent well build up sections	0.50 – 0.70
Macadam roads	0.25 – 0.60
Residential areas having detached houses	0.25 – 0.50
Areas with few buildings	0.10 – 0.25
Gravel roads	0.15 – 0.30
Open spaces, railway yards and unpaved surfaces	0.10 – 0.30
Gardens, lawns, parks, etc	0.05 – 0.25
Areas with wooden surfaces	0.01 – 0.20
Fertilized lands and forest land	0.01 – 0.20

 

 Intensity of rainfall

The intensity of rainfall can be worked out from the rainfall records of the area under consideration.  Where rainfall records are not available, the intensity of rainfall is obtained by applying suitable empirical formula. 

The general empirical formula adopted to calculate intensity of rainfall is:

\[R=\frac{{25.4\c dota}}{{t + b}}\]

            Where, R = Intensity of rainfall in mm per hour

                        t = Duration of storm in minutes

                        a and b are constants

The values of a and b are as follows:

a = 30 and b = 10 when duration of storm is 5 to 20 minutes

a = 40 and b = 20 when duration of storm is 20 to 100 minutes