Restaurant Drain That Blocks Every Two Weeks
A 30-seat chop bar in Cape Coast has a three-compartment sink, a wash-hand basin and a 20 L plastic grease trap. The 100 mm PVC drain runs 12 m to a public sewer connection. Staff report the line blocks about every two weeks, and greasy water backs up into the yard gully. On inspection you find the fall is only 150 mm over the 12 m, the grease trap is installed after the gully rather than before it, the basin waste enters the drain without a water seal, and there is no vent pipe. The owner blames customers for putting rice and oil down the sink and wants you to 'just rod it and add bleach'. You are asked to give a written assessment and a repair plan that will stop repeat blockages.
1. Identify the installation faults and explain how each contributes to the blockages and odour.
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The fall is 150 mm over 12,000 mm, i.e. 1:80. For a 100 mm drain carrying greasy kitchen waste, that is too flat to be self-cleansing; grease and solids settle and the line gradually closes. The grease trap is in the wrong position: it is after the gully, so grease has already entered the drain before reaching the trap. Its 20 L capacity is far too small for a commercial kitchen, so it fills and passes grease through. The basin waste has no water seal, allowing sewer gas and smells back into the room. The missing vent means air pressure changes can pull trap seals and slow the flow, and there is no easy rodding access at the bends. Bleach will not remove grease or correct falls; it only masks odour and can damage seals.
2. Set out the repair sequence and calculate the downstream invert if the upstream invert is 0.45 m below floor and the new fall is 1:60 over 12 m.
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At 1:60, the fall over 12 m is 12,000 ÷ 60 = 200 mm. If the upstream invert is 0.45 m below floor, the downstream invert becomes 0.45 + 0.20 = 0.65 m below floor. That must be checked against the public sewer connection; if the sewer is shallower than 0.65 m, the design needs rethinking rather than simply forcing the pipe to the connection.
The sequence: isolate and rod/clean first; remove the existing grease trap and gully arrangement; install a properly sized external grease trap with baffles and accessible covers before the gully; re-trap the basin waste; re-lay the 100 mm drain at 1:60 with rodding eyes at the head and changes of direction; add a vent pipe; reconnect and test with clean water; then disinfect the yard. The grease trap should be sized for the kitchen by the supplier or engineer, not reused at 20 L.
3. What maintenance and user instructions would you give the owner so the repair actually lasts?
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Scrape plates and wipe pans before washing; do not pour oil or rice into the sink; use sink strainers. Empty and clean the grease trap at least weekly, or more often if it fills, and record the date. Keep the trap covers accessible. Flush the line with clean water after service and report slow drainage immediately rather than waiting for a backup. Do not use bleach as a substitute for grease removal. Agree a short inspection after the first month, because the first month shows whether the staff routine is working.