Grease Trap Management: Compliance, Smell and Cost

A tank that only works if somebody manages it

A grease trap is simple physics in a box: slow the wastewater down, let fat float and solids sink, and send the cleaner middle layer on to the sewer. The physics never fails. The management does - a trap sized wrong, emptied rarely, or treated as fit-and-forget quietly stops separating, and everything that follows (blockages, smells, an s111 conversation with the water company) traces back to a tank nobody owned. Managing one well needs three lists, and none of them is long.

The lists

Compliance and smell, handled together

The compliance list

Size to the kitchen, not the catalogue: capacity should follow covers, menu fat load and flow rate - European sizing standards for separators exist for exactly this calculation, and an undersized trap is a compliance device that does not comply.
Empty on a schedule set by fill rate, not by smell - by the time a trap announces itself it has been passing fat for weeks. Many kitchens need monthly or better; heavy fryers more often.
Use a licensed waste carrier for the collected grease and keep the waste transfer notes for two years - trap contents are controlled waste, and the notes are your evidence trail.
Log every empty and inspection alongside the food safety diary; water company officers and EHOs both respond well to a paper trail and badly to a shrug.
Know your s111 position: the trap does not discharge the duty not to block the sewer - it is how you meet it.

The smell list

Site and seal: a trap indoors needs a gas-tight lid and intact gaskets - most "mystery" kitchen odours from below are a warped lid or perished seal, not the trap failing.
Do not let the fat layer over-thicken: decomposing FOG generates the sulphurous smell and, given long enough, acidifies and attacks the tank itself.
Check the vent: separators need one, and a blocked or missing vent pushes odour through every weak seal in the room.
Clean the crumb of the problem too - flies breeding in greased drains survive a trap empty if the pipework around it stays coated.

What good management costs against what it prevents

The running cost of a managed trap is boringly predictable: a scheduled empty by a licensed contractor, occasional gasket and baffle parts, and a few minutes of logging. The cost of the unmanaged alternative arrives in lumps - emergency jetting on a Saturday night, services lost to a dead wash-up area (a bottleneck a busy kitchen feels within one service), water company clearance recharges, and at the far end a prosecution whose fines start where the trap's annual budget ends. Sized once, scheduled honestly and logged, the tank is among the cheapest insurance in the building; ignored, it is a liability with a lid on it. Pubs and venues that cannot easily close for remedial drainage work - the same ones that plan deep cleans around trading - have the most to gain from never needing the remedial work at all.

Give the tank a name

Every well-run trap has one thing in common: a person. Someone owns the schedule, checks the contractor actually came, files the transfer note and walks the drains monthly. In leased and multi-operator buildings, establish whose trap it legally is before the first blockage makes it urgent - the answer lives in the lease, and it is cheaper to read than to litigate. Write the trap into the handover pack too: schedule, contractor, where the notes live, what full looks like. Traps fail most often in the month after the person who quietly managed one leaves - the tank did not change, the ownership did, and the sewer finds out first.

The grease the trap catches at the drain has a twin in the ductwork above. TR19 Grease duct cleaning manages the airborne half - assessed, cleaned and certificated to your usage band.

Frequently asked questions

How often should a grease trap be emptied?

On a schedule set by how fast it fills, not by smell or the calendar alone. A common working rule is emptying when the combined fat and solids layers approach a quarter of the tank's depth; for many kitchens that means monthly, and for heavy frying operations more often. By the time a trap smells, it has usually been passing grease downstream for weeks.

Who can empty a commercial grease trap?

A licensed waste carrier - trap contents and waste cooking oil are controlled waste under the Environmental Protection Act 1990. Keep the waste transfer notes for at least two years; they are the evidence that your grease left through the front door rather than the drain, and the first thing an investigator asks to see.

Why does our grease trap smell?

Usually one of four things: the fat layer is overdue an empty and decomposing; the lid seal or gasket has failed; the vent is blocked or was never fitted; or the pipework around the trap is coated and harbouring the smell independently. Emptying frequency and a gas-tight lid fix most cases - persistent odour after both points at the surrounding drainage.

What size grease trap does a kitchen need?

Sized to flow rate and fat load, not floor space: covers per service, menu type and the number of connected sinks and appliances all feed the calculation, and European standards for grease separators formalise it. An undersized trap passes grease at peak flow - precisely when the kitchen produces most - so if in doubt, size up and empty on schedule.

Does a grease trap remove the risk of prosecution under s111?

It is how you manage the risk, not a legal shield. Section 111 of the Water Industry Act 1991 makes blocking the public sewer an offence regardless of equipment; a maintained, logged trap is strong evidence of a kitchen doing the job properly, while a neglected one alongside a fatberg investigation reads as the opposite.

Grease trap management: compliance, smell and cost