How is Sewage discharge evaluated?
The total amount of wastewater generated per day is estimated as the product of the projected population at the end of the design period, taking into account the amount of wastewater per capita and the appropriate peak factor. The amount of wastewater per capita can be considered to be 75-80% of the water supplied per person per day.
Population growth also leads to increased per capita water requirements and thus increased per capita production of wastewater. This increase in water demand is driven by improved living standards, improved economic conditions, changing people's habits, and increased demand for utilities.
Quantity of sewage
The only sanitary drainage stream without storms during the dry season is known as Dry weather flow (DWF).
Amount = Wastewater per person per day x Population
It has been observed that a small portion of the water consumed by the inhabitants is lost due to evaporation, infiltration into the ground, leaks, etc. Normally, it can be assumed that 80% of the water supply reaches the sewer.
Sewage volumes
Sewage volume, including moderate infiltration, is generally assumed to correspond to an average water consumption of 135 litres/person/day.
It depends on the population parameter of the city. This amount is known as Dry Weather Flow (D.W.F). The amount of water flowing through the sewer in dry weather when there is no rainwater in the sewer.
The flow varies over a 24-hour period, usually highest in the morning and very low from midnight to early morning. To determine the channel size, you need to assume that the maximum flow is 3 times the DWF.
Factors Affecting Wastewater
Sanitary wastewater is mainly affected by the following factors:
· Population
· Area Type
· Water supply rate
In addition, it can be affected by people's habits, number of industries, water pressure, etc.
The amount of sanitary wastewater is directly dependent on the population. As the population grows, so does the amount of sanitary wastewater. The amount of water supply is equal to the water supply rate multiplied by the population.
The amount of sanitary wastewater also depends on the type of region, such as residential, industrial and commercial.
The amount of drainage from a residential area depends on the amount of water supplied in the area and is expressed in litres/person/day. This amount is obtained by multiplying the population by this factor.
The amount of wastewater produced by different industries depends on different industrial processes, which vary from industry to industry.
The amount of wastewater from commercial and public places can be determined by examining the development of other such places.
Fluctuations in dry weather flows
Dry weather flows also vary due to the amount of water used and the rate of water consumption. In general, it is assumed that
Maximum daily flow = 2 x average daily flow and
Minimum daily flow = 2/3 x (average daily flow)
Population Equivalent
Population equivalent is a parameter used in the conversion of amount of waste from industrial establishments to be accepted in to sewerage systems. The industrial sewage is, thus, written as
Std. BOD-5 = (Std. BOD-5 of domestic sewage per person per day) x (population equivalent)
Let us understand this with the help of an Example:
A city has a population of 50,000 spread over area of 60 hectare. Calculate design discharge for the separate sewer line by assuming rate of water supply of 250 Litres per capita per day and out of this total supply only 75 % reaches in sewer.
Solution
Given data
Q = 250 lit/capita/day
Sewage flow = 75% of water supply = 0.75* 250 = 187.5 litres per capita perday
Total sewage generated = 187.5*50000/ (24*3600) = 108.50 lit/sec = 0.108 m3/s
Assume peak factor = 2 *Total design discharge = 0.217 m3/s.
Conclusion
This article generally reviews the sewage discharge rate and its calculation as well as some of the affecting parameters.
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