Demand of new municipality 2016, 45,000 pop

Survey year 2016; base period 2 years and design period 20 years give the design year 2038, so $n = 22$ years after the survey.

Design population

$$P_{2038} = 45{,}000\times(1.015)^{22} = 62{,}440$$

Water demand

Assumed unit rates for a municipality: domestic 110 lpcd; day scholar 5 l/d; big animal 45 l/d; small animal 5 l/d; office staff 45 l/d (10 staff per office); hospital 340 l/bed/d (IS 1172); losses 15%.

Losses and wastage $= 15\%$ of net demand $= 1{,}106{,}541$ l/day

$$Q_{avg} = 7{,}376{,}940 + 1{,}106{,}541 = 8{,}483{,}481\ \text{l/day} = 8{,}483.48\ \mathrm{m^3/day} = 98.19\ \text{l/s}$$

Fire demand (National Board of Fire Underwriters formula)

$P = 62.44$ thousand.

$$\begin{aligned} Q_f &= 4637\sqrt P\,(1-0.01\sqrt P)\ \text{l/min}\\ &= 4637\times7.902\times(1-0.0790) = 33{,}746\ \text{l/min} = 562.4\ \text{l/s} \end{aligned}$$

For a fire lasting 2 hours, the volume is $Q_f\times120 = 4{,}050\ \mathrm{m^3}$. This volume is kept as fire reserve in the service reservoir; it is not added to the average daily demand, because fire flow is an occasional rate, not a daily consumption.

Design flow with fire

The distribution system is checked for the larger of:

• Maximum day demand + fire flow $= 1.8\times98.19 + 562.4 = 176.7 + 562.4 = 739.1$ l/s
• Peak hour demand $= 2.7\times98.19 = 265.1$ l/s

So the mains are designed for about 739 l/s.

Answer: Design population = 62,440; average daily demand = 8,483 m$^3$/day (98.2 l/s); fire demand = 33,746 l/min (562 l/s), with a 2-hour fire reserve of about 4,050 m$^3$ in storage; design flow for the distribution system (maximum day + fire) $\approx$ 739 l/s.