Why Your Grease Trap Smells — and How to Fix It

Odour is the most common complaint we hear, and it is a useful signal rather than a nuisance. It usually tells you something specific about what is happening inside the unit or the line around it.

Rotten-egg or sulphur smell

This normally means anaerobic breakdown of trapped organic material producing hydrogen sulphide — in plain terms, the contents have been sitting too long. The fix is a full pump-out with the walls, baffles and tees cleaned, not a partial skim, because the settled layer is where much of it comes from.

Sour or rancid smell

A sour edge usually points to the grease cap itself going off, which again means the interval is too long for how much fats, oils and grease your kitchen produces.

Sewer smell that appears only sometimes

Intermittent sewer odour often is not the interceptor's contents at all. A dry trap seal on a floor drain lets sewer gas straight into the room, and it happens most in areas that rarely get water — which is why it comes and goes.

Odour that persists after a full cleaning

• A damaged or missing baffle or tee, so the unit is not sealing as designed.
• A cracked or poorly seated lid or gasket letting gas out at the access point.
• Grease buildup in the line downstream of the unit rather than in the unit.
• A dry floor-drain trap seal nearby, unrelated to the interceptor.
• A venting problem in the plumbing system.

What does not fix it

Deodorisers, bleach down the drain and enzyme additives cover or move the smell without removing what is causing it. Bleach also does nothing useful for the separation the unit relies on. If odour returns within days of a service, the answer is diagnosis, not more product.

A practical order to work through

• Check when the unit was last fully serviced, and whether the whole contents were removed.
• Pour water into nearby floor drains to re-seal any dry traps, then see if the smell changes.
• Have the lid, gasket, baffles and tees inspected for damage.
• If odour remains, look downstream at the line rather than repeating the pump-out.
• Shorten the interval if the smell reliably returns before the next scheduled visit.

Why it matters beyond comfort

Odour reaching a dining room is a customer-facing problem, and staff should not be working in hydrogen sulphide. It is also usually the earliest warning that the unit is overdue, well before the drains slow down.