Factors affecting sewage; calculating discharge

Factors affecting discharge

1. Population and its density, and the future growth over the design period.
2. Rate of water supply (sewage is 75-80% of supply).
3. Type of area (residential, commercial, industrial) and standard of living.
4. Private and public water supplies, and water metering.
5. Groundwater infiltration, depending on water table and condition of joints.
6. Unauthorised connections (roof, yard drains).
7. Climate, season and hour of the day.

Calculation of sanitary sewage discharge

1. Design population $P$ at the end of the design period (20-30 years), by arithmetic, geometric or incremental increase method.
2. Average sewage flow:

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

where $q$ = water supply (lpcd) and 0.8 = fraction becoming sewage. 3. Peak (maximum) flow: $Q_{max}=\text{PF}\times Q_{avg}$. The peak factor is 2 to 3 (about 3 for small, 2 for big towns; or by Harman's formula $PF = 1+\dfrac{14}{4+\sqrt{P/1000}}$). 4. Minimum flow is about $\tfrac13 Q_{avg}$ (used to check the self-cleansing velocity). 5. Add infiltration (e.g. 5,000-15,000 l/day/km per cm diameter of sewer, or a percentage) and industrial flows if present.

Design discharge = peak sewage flow + infiltration (+ storm runoff $CiA/360$ for combined systems).