Water-Saving Retrofit at a Public Toilet Block
A ward-office public toilet block in Pune has 20 Indian-type squatting pans with 10 L single-flush cisterns, 6 wall-hung urinals with continuous flushing (each 0.2 L/min) and 4 wash basins with 15 mm pillar taps. Water is stored in a 5,000 L tank filled twice a day from a 25 mm connection, and the pressure at the fixtures is only 0.3 bar. About 800 people use the block daily, and each pan is used roughly 40 times a day. One cistern in the women's section has been running continuously for a week because the caretaker tied the float arm with wire to stop a 'hissing noise'. The health inspector wants a 25% reduction in water use within one month without reducing the number of fixtures, and the ward office has a small budget.
1. Estimate the present daily water use of the pans and urinals, and evaluate whether replacing the 10 L cisterns with 3/6 L dual-flush will meet the 25% target. Show calculations and state the risk at 0.3 bar.
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Pans: 20 × 40 uses × 10 L = 8,000 L/day. Urinals: 0.2 L/min × 60 × 24 = 288 L per urinal, × 6 = 1,728 L/day. Combined = 9,728 L/day. A 3/6 L dual flush, if one-third of uses are full flush, averages (6 + 3 + 3)/3 = 4 L per use: 20 × 40 × 4 = 3,200 L/day, saving 4,800 L/day - about 49% of the pan-plus-urinal total, well beyond the 25% target. The risk is that at 0.3 bar a 3 L flush may not clear an Indian squatting pan, so users flush twice and the saving disappears. A safer specification is 5/10 L cisterns set to give a 5 L flush with a 15 mm inlet, tested on two pans before ordering 20. Also replace continuous urinal flushing with sensor flushing at about 1.5 L per use; that saving alone is large and independent of flush performance.
2. Explain what is happening with the wired float and the correct repair. Calculate the water wasted per day if the valve passes 0.15 L/s.
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The hissing is the inlet valve passing, and tying the float only holds the valve open - the cistern is overflowing continuously. Causes are a worn jumper washer, grit on the seat, a bent float arm or a float that has lost buoyancy. Repair: close the stopcock, dismantle the float valve, clean the seat, replace the washer or jumper, set the float so shut-off is 20 mm below the overflow, and check that the overflow discharges visibly. Replace a cheap plastic valve with a brass one. Waste: 0.15 L/s × 3,600 = 540 L/h × 24 = 12,960 L/day. That is more than two full 5,000 L tanks. The single running cistern can exceed the whole 25% target, so fixing it is the cheapest and fastest saving and should be done first.
3. The inspector reports a smell from the health faucet, and dirty water once appeared in a wash basin when the tank ran dry. Explain the likely cause and the fittings to be installed.
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This is a cross-connection with back-siphonage. The health faucet and basin inlet are connected to the same supply; when the tank empties or a pump stops, negative pressure draws dirty water back through a submerged outlet or the hose. Correct fittings: a dual check valve or vacuum breaker on the health faucet, a check valve on the basin inlet, and an air gap of at least 25 mm (or the valve's specified air-gap dimension) between the float valve outlet and the maximum water level. The overflow must discharge visibly and must not be sealed. Check also that the hose does not hang into the pan. Explain to the inspector that a non-return valve alone is not enough if the outlet can be submerged; the air gap is the reliable barrier.