4.5 System Demand, Water Production & Design Basics

4.5 System Demand, Water Production & Design Basics

The Need-to-Know Criteria open the Distribution System Components area with three demand tasks: assess water production (including water restrictions and demand), adjust water production to meet demand, and aid in the design of water distribution projects. Demand is the number every other design decision hangs on — pipe size, storage volume, pump capacity, and when you tell customers to stop watering lawns.

The Four Demand Numbers

The multipliers between them are peaking factors, and they get larger as the system gets smaller. A city of 200,000 averages out thousands of independent users; a system serving 400 people sees everyone shower between 6 and 8 a.m., so its peak hour towers over its average. This is why Class I systems need proportionally more storage than large ones.

Worked example. A system serves 480 people at 100 gpcd.

1. ADD = 480 × 100 = 48,000 gpd
2. MDD at a factor of 2.0 = 48,000 × 2.0 = 96,000 gpd
3. PHD at a factor of 1.8 × MDD = 96,000 × 1.8 = 172,800 gpd, which is 172,800 ÷ 1,440 = 120 gpm

That 120 gpm — not the 33 gpm average — is what the distribution mains and the pumps must actually deliver.

Matching Production to Demand

Demand is not something the operator controls; production is. In practice, tank level is the control signal. As demand draws the tank down, level controls start the well or booster pump; as it refills, they stop it. The operator's daily job is to confirm that the automatic logic is actually keeping up:

• Read the master meter and tank level at the same time each day and compare with the same day last week.
• Start additional pumps or lead/lag stages when the tank is not recovering overnight — the classic early warning that MDD is arriving.
• Adjust flow-control valves to balance zones so one pressure zone does not starve another.
• Watch run hours. A well running 20 hours a day in July has no reserve left for a main break or a fire.
• Document the day's production, demand, and tank behavior in the operator log — this record is what a state sanitary survey reviews to judge whether source capacity is adequate.

Capacity Criteria a Class I Operator Should Know

Design guidance such as the Ten States Standards sets the benchmarks the state reviews against:

• Source capacity must at minimum meet the maximum day demand with the largest pump out of service.
• Distribution capacity must deliver maximum day demand plus fire flow while holding at least 20 psi everywhere, or peak hour demand while holding normal service pressure — whichever is more demanding.
• Normal working pressure in the distribution system should be about 35–80 psi.
• Storage covers three needs stacked together: equalization (the daily peak the source cannot follow), fire reserve, and emergency reserve for outages.

Design Inputs the Operator Contributes

You will not stamp drawings, but operators are routinely asked to review distribution project plans, and the criteria are testable:

• Minimum main size — commonly 6 in where fire hydrants are served (8 in in many modern standards); 2-in and smaller mains cannot deliver fire flow.
• Loop the system. Dead ends stagnate, lose residual, and can only be fed from one direction. Where a dead end is unavoidable, it gets a blow-off for flushing.
• Hydrant spacing and valve placement — enough valves that a break isolates a short segment rather than a neighborhood.
• Depth of cover below the local frost line.
• Separation from sewer — 10 ft horizontal, 18 in vertical with water on top.

Water Restrictions and Conservation Staging

When demand outruns supply — drought, a well out of service, a hot dry week — the utility moves through conservation stages defined in its drought or water-shortage plan. A typical ladder:

The operator's role is to recognize the trigger from the data — falling tank levels that do not recover overnight, wells running continuously, pressure sagging at the system periphery — and to report it early enough that management can act. Waiting until storage is empty removes every option except an emergency notice.