Design Example

Wastewater Quantity Estimation
Fluctuations in Dry Weather Flow

Population Equivalent

Design Periods & Population Forecast
Wastewater Characterization

Wastewater Quantity Estimation

The flow of sanitary sewage alone in the absence of storms in dry season is known as dry weather flow (DWF).

Quantity= Per capita sewage contributed per day x                       Population

Sanitary sewage is mostly the spent water of the community draining into the sewer system. It has been observed that a small portion of spent water is lost in evaporation, seepage in ground, leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.

Fluctuations in Dry Weather Flow

Since dry weather flow depends on the quantity of water used, and as there are fluctuations in rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 2/3 x (average daily flow).

Population Equivalent

Population equivalent is a parameter used in the conversion of contribution of wastes from industrial establishments for accepting into sanitary sewer systems. The strength of industrial sewage is, thus, written as

Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x
                  (population equivalent)

Design Periods & Population Forecast

This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period. It is suggested that the construction of sewage treatment plant may be carried out in phases with an initial design period ranging from 5 to 10 years excluding the construction period.

Design period is estimated based on the following:

  • Useful life of the component, considering obsolescence, wear, tear, etc.
  • Expandability aspect.
  • Anticipated rate of growth of population, including industrial, commercial developments & migration-immigration.
  • Available resources.
  • Performance of the system during initial period.

Population forecasting methods:

The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer.

  1. Arithmetic Increase Method
  2. Geometric Increase Method

Wastewater Characterization

To design a treatment process properly, characterization of wastewater is perhaps the most critical step. Wastewater characteristics of importance in the design of the activated sludge process can be grouped into the following categories:

Colour and Odour
Carbonaceous substrates
Total and volatile suspended solids (TSS and VSS)
Toxic metals and compounds