Section 34 · Water Supply
Storage & distribution
A ram delivers a small, steady flow around the clock — and people drink in bursts, at breakfast and at supper. The tank is what reconciles the two, and the gravity main is what gets the water to the tap.
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Sizing the tank
Most ram pump guides stop at the delivery pipe. That is where the pump stops and the water supply begins, and it is where a surprising number of systems are lost — a perfectly tuned ram feeding a tank too small to cover the evening, or a tank sited 40 m below the village with no head left to reach a tap.
The ram's one great advantage is constancy
A ram cannot be switched off and throttled; it runs at a nearly constant rate, day and night, rain or shine. That sounds like a limitation and is actually the whole design premise. A constant supply into a tank becomes a demand-shaped supply out of it, perfectly, for free, with no controls and no electronics:
Valid where the ram's daily output meets or exceeds the daily demand; if it does not, the tank will drain and the shortfall is a ram-sizing problem, not a tank-sizing one.
Step 1 — the demand
Estimate from the number of users and a service level, then add livestock and any irrigation honestly. Domestic figures vary enormously with culture and climate; these are conventional planning values, not laws of nature.
| Use | Typical demand | Notes |
|---|---|---|
| Domestic — basic access | 20 L / person / day | WHO's basic-access level: drinking, cooking and basic hygiene at a standpipe |
| Domestic — intermediate access | 50 L / person / day | On-plot supply, so laundry and washing are included. WHO's "optimal" level is 100 L |
| School | 3–5 L / pupil / day | Drinking and handwashing only (Sphere). Far more where there are toilets and showers |
| Health centre | 5 L / outpatient · 40–60 L / inpatient / day | Sphere figures. Laundry and flushing are counted separately |
| Cattle | 40–70 L / head / day | A dry cow sits near the bottom of the range, a lactating dairy cow at the top; heat pushes it higher |
| Sheep, goats | 5–20 L / head / day | Rises sharply in heat and in late pregnancy or lactation |
| Pigs | 15–25 L / head / day | Heavy users, and more again if pens are washed down |
| Poultry | 0.3–0.5 L / bird / day | Roughly 40–50 L per 100 birds |
| Drip or hand irrigation | 2–8 L / m² / day | Equals 2–8 mm of applied water (1 mm depth = 1 L/m²). Highly crop-, soil- and climate-dependent |
Step 2 — days of autonomy
Autonomy is how long the tank can supply the village with the ram stopped. For a domestic supply the useful range is short: the tank's job is to bridge the gap between constant supply and bursty demand, not to be a reservoir.
- One day is the minimum worth building, and handles the daily peaks.
- Two to three days is the usual target for a community supply, and covers a couple of days of intake cleaning or valve repair without taps running dry.
- A dry season's worth is only for irrigation or where the stream genuinely fails, in which case you are really building a reservoir and should size it on the seasonal water balance.
A village of 300 people at 25 L/person/day needs 7.5 m³/day. A ram delivering 0.1 L/s produces 0.1 × 86,400 = 8.6 m³/day — enough, with a little to spare for a school tap. At two days of autonomy the tank is about 15 m³, say a 3 m × 3 m × 1.7 m covered concrete tank. Note how small the margin is: this scheme only works because supply is only about 15% above demand, so any growth in demand or drop in dry-season flow has to be thought about up front.
Siting and building the tank
Elevation is the whole game: every metre of tank elevation above the highest tap is a metre of head you get for free, forever, with no pump. Put the tank as high as the site allows, even at the cost of a longer delivery pipe — the ram pays that price once, and the elevation pays you back every day.
- Above the taps, always. Aim for at least 10 m of head at the furthest standpipe, and more if the main is long.
- Cover it. An open tank grows algae, breeds mosquitoes and collects everything the wind carries. A sealed lid with a screened vent is not optional for drinking water — see water quality.
- Draw from the top. A floating outlet or a high-level draw-off takes cleaner, cooler, better-oxygenated water and leaves sediment on the floor, where a washout drain can flush it.
- Include a washout and an overflow. Both are needed the day the tank is cleaned, and the overflow must not discharge onto the foundations or undermine the tank.
- Budget for materials. Ferrocement and masonry are common where labour is cheap and transport expensive; HDPE and steel are quicker. All three work; the joints and the lid are where they fail.
Getting water to the tap
Downstream of the tank, water flows by gravity, and the design questions change completely. The ram's delivery pipe carries a constant, known, small flow under a head set by the ram. A distribution main carries a variable flow that spikes when everyone draws water at once, and it must still have pressure left at the far end.
Residual head is the number to protect
Every metre of pipe, every bend and every tap eats head through friction. The design constraint is not the total drop but the residual head that remains at the furthest tap — below about 5 m a standpipe dribbles, and people stop using it. Keep 5–10 m in hand at the worst-case tap.
- Size the main on peak flow, not average. If 30 taps all open at once, the main sees a multiple of the daily average. Design for the realistic simultaneous draw, not the total divided by 24 hours.
- Choose a smoother, larger pipe before you raise the tank. Friction falls steeply as diameter rises — doubling the bore cuts friction loss by roughly a factor of thirty at the same flow, which is almost always cheaper than building the tank higher.
- Use the head-loss calculator on Design tools to check a candidate bore before buying pipe.
Break-pressure tanks
Where the village sits far below the tank, the static head at the bottom of the main can exceed the pipe's pressure rating and blow joints apart. The standard fix is a break-pressure tank: a small open tank partway down the hill that spills into a second main, resetting the pressure to atmospheric. A second short main then serves the lower village. It costs a small tank and gains you the option of cheap, low-pressure pipe on the steep section.
Tap stands and drainage
A standpipe is a small piece of civil engineering with a large effect on whether a scheme is adopted. A tap at the wrong height is not used; a tap with no drainage becomes a mud pit within a week and then a health hazard.
- Height and reach: about 0.9–1.0 m to the spout so a jerrycan fits underneath, with a concrete apron wide enough to stand on.
- Soakaway: drain every tap stand to a stone-filled pit. Standing water around a drinking-water point is the classic route back to contamination.
- Washout and air release: fit a washout at every low point so the main can be flushed, and air release at high points where air will otherwise collect and throttle the flow.
- Keep animals separate: a livestock trough downstream of, and separate from, the domestic standpipes.
Size the tank for two days of demand, put it as high as the site allows, keep 5–10 m of residual head at the worst tap, and fit every standpipe with an apron and a soakaway. Then the ram's steady trickle becomes a water supply that behaves like one.
Water that reaches a tap has to be safe to drink — that is a separate problem, dealt with on Water quality. Upstream, the flow you are storing is set by the design.