Wet weather discharge, combined sewer

In a combined system, the wet weather discharge is the dry weather sewage flow plus the storm runoff.

Given

Population = 500, water supply = 160 lpcd, area $A$ = 1.5 ha, rainfall intensity $i$ = 50 mm/hr. Assume that 80% of water supplied becomes sewage.

Step 1: Weighted coefficient of runoff

$$C=\frac{0.4(0.8)+0.6(0.2)}{1.0}=0.44$$

Step 2: Storm runoff (Rational method)

$$Q_s=\frac{C\,i\,A}{360}=\frac{0.44\times 50\times 1.5}{360}=0.0917\ \text{m}^3/\text{s}$$

Step 3: Sanitary sewage (dry weather flow)

$$Q_d=\frac{500\times160\times0.8}{86400\times1000}=0.00074\ \text{m}^3/\text{s}$$

Step 4: Wet weather discharge

$$Q=Q_s+Q_d=0.0917+0.00074=0.0924\ \text{m}^3/\text{s}$$

Answer: Wet weather discharge is about 0.0924 m³/s (92.4 l/s); storm water is 99% of it, so the sewage part is almost negligible.