Hostel water storage and a gurgling trap
A 120-bed working women's hostel is supplied from a 5,000-litre overhead tank on the roof, filled once a day by a pump from an underground sump. Occupancy is 110 residents and 10 staff; demand is taken at 135 litres per head per day, of which about 45 litres is for flushing. The tank runs dry by about 7 pm, the top-floor bathrooms have no flow between 6 and 8 pm, yet the ground-floor taps flow freely at the same time. Separately, a soil pipe serving a first-floor water closet makes a loud gurgling noise whenever the ground-floor WC is flushed, and a foul smell is noticed in the bathroom afterwards. That WC has a 75 mm P-trap, its branch is 6 m long with no vent pipe, and the 100 mm soil stack terminates 300 mm above the roof with a cowl that is partly blocked by a bird's nest. The plumber proposes to fit a 10,000-litre tank and a 110 mm trap.
1. Check the tank capacity against the daily demand and explain the 6–8 pm failure. Is the plumber's larger tank the right first response?
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Demand
- Total daily demand = 120 heads × 135 L = 16,200 L/day
- Of this, flushing = 120 × 45 = 5,400 L/day
Storage check
- The 5,000 L tank holds 5,000 / 16,200 = 31% of one day's demand, i.e. about 7.4 hours at the average draw-off rate.
- Common practice for an overhead tank is one-third to one-half of the daily demand, that is 5,400 to 8,100 L. So the existing tank is at the very bottom of the acceptable band and the tank running dry by 7 pm is consistent with the arithmetic, not with a freak event.
Why the ground floor still flows. The ground-floor taps clearly draw from the sump or a direct line, not from the overhead tank. So the storage empties through the upper-floor outlets while the ground floor keeps drawing uncontrolled water - a plumbing configuration fault, and it also means the ground floor is drinking unsoftened sump water without the benefit of the tank's settlement.
Judgement on the proposal. A 10,000 L tank (about 62% of the daily demand) would end the evening shortage, but only if it is filled more than once a day. It also adds about 5 tonnes of load to the roof: the slab and beams must be checked for that load before it is ordered. And it does nothing about the uncontrolled ground-floor draw-off.
Recommended first response
- Provide about 8,000 L of overhead storage (or two tanks), and
- Fit an automatic level control so the pump tops up in the afternoon, and
- Feed the ground floor from the overhead tank too, or through a small booster, so that all floors share the stored water fairly.
2. Diagnose the gurgling and the smell. Explain the role of the trap, the branch length and the stack termination, and decide whether the plumber's 110 mm trap will solve it.
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Diagnosis. The gurgling is air being pulled through the trap, which means the trap seal has been lost - induced (or self) siphonage. When the ground-floor WC discharges down the 100 mm soil stack, it compresses air ahead of it and creates a partial vacuum behind it. If the branch cannot draw relief air, the vacuum sucks the water out of the first-floor trap. Once the seal is gone, drain air enters the bathroom - that is the smell.
Contribution of each fault
- 75 mm trap against a 100 mm stack - the seal is small in volume and in depth relative to the discharge. Adequate water seal (typically 50 mm) and a trap of the same nominal size as the branch are needed.
- 6 m unvented branch - in normal practice an unvented branch of 75–100 mm is kept short (roughly 1.5–2 m). At 6 m the branch is far too long to draw relief air, so siphonage is almost certain.
- Blocked cowl on the stack - the stack must be open to atmosphere to act as a vent. A cowl, especially one partly filled with a bird's nest, restricts airflow and can cause downdraught, destroying the vent's purpose. The stack should terminate with a wire-mesh cage, well above the roof and away from windows.
Verdict on the plumber's fix. A 110 mm trap of itself does not cure the problem; a bigger seal is sucked out just as readily if the branch cannot breathe. The correct set of measures is:
- Clear the nest and replace the cowl with a wire cage; carry the stack full size up to the open air.
- Replace the WC trap with a 100 mm anti-siphon (dual) trap, 50 mm seal, or fit a 50 mm relief vent on the branch.
- Shorten or re-route the branch, or increase it to 100 mm with the correct gradient (about 1:60 for 100 mm), and provide a vent pipe connected to the branch or a one-pipe vented system.
Only after the venting is corrected does any resizing of the trap make sense.
3. If the underground sump is to be a rectangular tank 2.0 m wide with 1.5 m effective water depth, what length would one day's demand require? Comment on whether that much storage is desirable and on any siting precaution.
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Size calculation
- One day's demand = 16,200 L = 16.2 m³
- Effective volume per metre of length = 2.0 m × 1.5 m × 1.0 m = 3.0 m³/m
- Required length = 16.2 / 3.0 = 5.4 m
So a sump of 5.4 m × 2.0 m × 1.5 m effective (say 5.5 m × 2.0 m × 1.8 m overall, with 300 mm freeboard) would hold a full day's demand.
Comment on desirability
- A full day's storage is generous. For a building with a daily piped supply, half a day to one day is a reasonable range; the exact figure depends on how reliable the municipal supply is and how often the tanker/pump replenishes the sump.
- Long storage means long residence time. In a warm climate, water held for a day or more loses its residual chlorine and can grow algae and slime; the tank must be covered, mosquito-proofed, and cleaned periodically. Stagnation is a real risk if the sump is oversized relative to actual consumption.
- A better balance is a sump of about half to one day's demand plus an overhead tank of one-third to one-half of daily demand, so that turnover is regular.
Siting precautions
- Keep the sump at least 15 m from any soak pit or septic tank and from the drain line, and never within the building's foundation influence zone.
- Provide a 600 mm inspection cover, an overflow with a mosquito-proof screen, a washout, and a sloped floor draining to the washout.
- Keep the top of the sump above the surrounding ground level and slope the ground away, so that surface runoff and sewage cannot enter.
- Provide a foot valve/ suction line sized so that the pump does not cavitate, and check the structural design of the sump walls for earth and water pressure.