Estimating sewage quantity for a city

The quantity of sewage is estimated from the future population, the water supply per capita, and the factors that raise or reduce the flow. For a city in Nepal (e.g. Pokhara, Biratnagar) the method is as follows.

Step 1: Design population

Collect census data (CBS, Nepal) and project the population for the design period (about 20-25 years after completion) using the arithmetic, geometric or incremental increase method:

• Arithmetic: $P_n = P_0 + n\bar{x}$
• Geometric: $P_n = P_0(1+r/100)^n$

Add floating population (tourists, commuters, students) if large.

Step 2: Water supply rate

Use the design water demand of the city (Nepal Water Supply Corporation/NWSC design guidelines): 45 lpcd for rural and 85-135 lpcd for urban areas, with additions for institutional, commercial and industrial demand and leakage.

Step 3: Average sewage flow

About 80% of supply returns as sewage (the rest is lost in gardening, evaporation, leakage and consumption):

$$Q_{avg}=\frac{0.8\,P\,q}{86400}\ \ \text{l/s}$$

Step 4: Peak and minimum flow

• Peak factor 2 to 3 (higher for small towns); $Q_{max}=PF\cdot Q_{avg}$ for sewer design.
• Minimum flow about $\tfrac13 Q_{avg}$ for velocity check at start of the design period.

Step 5: Additions

• Infiltration depending on groundwater table and pipe quality (about 5-10% of DWF in Nepali cities, or 5,000-10,000 l/km/day/cm diameter).
• Industrial wastewater from large industries (from their own records).
• Storm water with $Q_s=\dfrac{CiA}{360}$ if the system is combined or partially separate. The intensity for the Nepal city is taken from the DHM intensity-duration-frequency curves, with $t_c=t_e+t_f$.

Step 6: Design discharge

$$Q_{design} = Q_{max} + Q_{infiltration} + Q_{industrial}\ (+Q_{storm})$$

The sewers are designed for $Q_{design}$ at the end of the design period and checked for minimum flow at the beginning (velocity at least 0.6 m/s).

Example

For $P=60{,}000$, $q=135$ lpcd, PF = 3: $Q_{avg}=0.8\times60000\times135/86400=75$ l/s and $Q_{max}=225$ l/s.