School Rainwater Harvesting: Oversized Claims, Small Tank, Silted Pit
A senior secondary school in Patiala has a 300 m² concrete roof drained by four 90 mm downpipes. Annual rainfall is about 650 mm, with roughly 80 per cent falling between July and September; the local body requires rooftop rainwater harvesting for a plot of this size. The school has 220 students, a 20,000-litre underground sump filled by tubewell and a 5,000-litre overhead tank. The management wants to cut tubewell use. A contractor proposes a 5,000-litre storage tank, a first-flush pipe of 1 m of 90 mm pipe, a wire-mesh strainer at the downpipe and a recharge pit of 2 m × 2 m × 2 m filled with boulders. He argues: "the roof is 300 m² and the rain is 650 mm, so we get 195,000 litres, enough for the whole year". The school is closed in June and reopens mid-July.
1. Check the contractor's volume claim and propose a defensible storage-and-recharge split with numbers.
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The 195,000-litre figure is roof area multiplied by rainfall only. It ignores runoff losses and first flush.
Harvestable volume = area × rainfall × runoff coefficient = 300 × 0.650 × 0.85 ≈ 166 m³, about 1,66,000 litres a year. A concrete roof loses 10–15 per cent to wetting, evaporation and spillage.
First flush: 1 mm over 300 m² is 300 litres per event. Across roughly 30 events a season that is about 9,000 litres deliberately dumped - cheap insurance, because that first millimetre carries most of the dust and bird droppings.
Demand check: 220 students at about 5 litres a day for drinking and hand-washing is roughly 1,100 litres a day; over 200 school days that is 220 m³ - more than the entire annual harvest. Rainwater cannot replace the tubewell here, but it can carry toilet flushing or recharge.
Split: 166 m³ arrives in about 12 weeks and cannot be stored for a year. A workable rule is storage for 15–20 days of dry-spell demand; for about 2,000 litres a day of flushing that means 30,000–40,000 litres, not 5,000. Tank overflow must go to the recharge pit, and the bulk of the monsoon surplus should be recharged rather than stored. The proposed 5,000-litre tank is a token size that will overflow on the first good rain.
2. The system is built. After one monsoon the stored water smells, the sump overflows within 20 minutes of heavy rain and the recharge pit is silted solid. Diagnose each problem.
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Three separate design faults, not one.
Smell. Anaerobic bacteria in stagnant water. The tank is far too small for the inflow, so water sits for weeks; the inlet drops from the top and stirs the sludge; there is no bottom drain or cleaning access; light enters through a loose lid and algae grow. Fix: right-size the tank, add a calmed inlet, a bottom drain and a tight, opaque lid, and clean it after the monsoon.
Overflow in 20 minutes. A 90 mm downpipe in heavy rain carries several litres a second, so 5,000 litres fills in minutes. The overflow is probably the same 90 mm line or smaller, so the tank backs up and the roof ponds. Fix: size the tank to the inflow, and provide an overflow at least as large as the inlet, discharging to the recharge pit or a safe drain.
First flush is a gesture. One metre of 90 mm pipe holds π × 0.045² × 1 ≈ 6.4 litres - 0.02 mm over a 300 m² roof, about fifty times less than the 1 mm needed. Fit a proper diverter sized at 0.5–1 mm of roof area with a slow-release or manual dump valve that resets each event.
Recharge pit silting. A few-millimetre wire mesh passes everything. Silt, leaves and grit fill the void spaces between the boulders and the pit seals itself. Sequence must be: leaf screen at the downpipe, then first-flush diverter, then a sand/gravel or cartridge filter, then a silt trap with a removable basket ahead of the pit.
3. What maintenance schedule and water-quality checks will you specify, and how do you handle the question of whether the water is drinkable?
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Potable question - answer in writing. The harvested water is not potable unless it is filtered, disinfected and then tested. For flushing and gardening no treatment is needed. If it is to be used for hand-washing, add a 50-micron and carbon filter plus UV, and test before anyone drinks or washes in it.
Testing: pH 6.5–8.5; turbidity below 5 NTU for non-potable use and below 1 NTU if treated; total coliform and E. coli absent in 100 mL for potable use; residual chlorine if chlorinating. Test at commissioning, before each monsoon and once mid-season, when stagnation shows itself.
Maintenance calendar:
- May–June: clean the roof, clear the downpipes, clean the leaf screen and filter cartridge, empty and flush the first-flush pipe, inspect the tank and lid.
- Each heavy event in July–August: confirm the first-flush dump has reset and clear debris from the screen.
- Monthly in season: check tank level, overflow and any scum; skim.
- October: drain and desilt the tank, clean the silt trap, check the recharge pit water level and remove the top silt layer.
- Annually: inspect the tank for cracks and the lid seal, and repeat the water test.
Also: label every rainwater outlet "NON-POTABLE - DO NOT DRINK", keep rainwater and municipal lines physically separate with no cross-connection, and make sure overflow does not discharge onto a neighbour's plot or a wall footing.