Phoenix Journal · Ductwork
Choosing grease kit is really a sizing and compliance exercise in disguise. Here is how to specify a system that satisfies the Building Regulations and keeps your drains - and the water company - on side.
Grease management
Specifying a grease management system is not really about picking a box from a catalogue - it is about matching the right equipment to the way your kitchen actually cooks, and being able to prove that you did.
Get it right and you protect your drains, your reputation and your relationship with the water company. Get it wrong and you inherit blocked gullies, emergency call-outs and, increasingly, the threat of enforcement. This guide walks you through what a sound specification looks like - the standards that govern it, the numbers it has to be sized against, and the practical details worth nailing down before anything is ordered or installed.
Before you compare products, get clear on what the law asks of you, what the equipment has to be sized against, and what a good supplier should be able to answer without hesitating. These three checklists cover the ground.
There is no single best grease management system - only the one that suits your menu, your throughput and the space you have under the sink or out the back. A fish-and-chip operation, a coffee shop reheating pastries and a full production kitchen all generate very different volumes and types of FOG, so start from what you actually cook and how much of it, not from a headline price. In practice you are choosing between three approaches, and often combining them.
A passive grease trap, or grease separator, uses nothing more than gravity and retention time. Wastewater slows inside the chamber so that fats float to the top and food solids sink to the bottom, leaving cleaner water to pass on to the sewer. These units are typically floor-standing and range from around 20 to 400 litres, in stainless steel, mild steel or plastic. They need no power and cost little to buy, but they earn their keep only if they are emptied on schedule - neglect one and it will overflow FOG downstream and announce itself with an unmistakable smell.
An automatic grease removal unit, or GRU, does the same separation but then skims the accumulated grease off on a timed heating-and-scraping cycle into a sealed container you empty by hand. GRUs suit busy sites, or ones where getting a tanker to a large passive trap would be awkward. They cut the manual emptying burden and keep separated grease out of the water stream, but they cost more up front, need a power supply, and still need regular servicing to stay reliable.
Biological dosing introduces safe, stable microorganisms into the drain line that digest FOG over time. The Grease Contractors Association - the British Water-administered body that has set and audited best practice in this field since 2018 - recommends biological dosing but warns firmly against enzyme-based products. Enzymes only emulsify grease so it slips past the trap, then let it re-solidify further along the sewer, which moves the blockage rather than removing it. Treat dosing as a support to a separator, not a replacement for one.
The heart of any specification is the BS EN 1825-2 calculation. The nominal size is the peak flow, in litres per second, adjusted by those temperature, density and detergent factors. It matters more than most people expect. Under-size the unit and grease escapes downstream during your busiest service, exactly when you can least afford it; over-size it and you have paid for capacity and floor space you never use. Discharge temperatures above roughly 60°C are a particular enemy of separation, so if you run high-temperature wash processes, flag that to whoever is doing the sizing.
Siting deserves the same care as sizing. The unit should sit as close to the source of grease as the layout allows, on an accessible run, with room to lift lids, draw off waste and carry out servicing without dismantling half the kitchen. Think about ventilation, about how a tanker or engineer reaches it, and about whether a below-ground separator or an under-sink unit fits your building and your volumes. A perfectly sized trap in an unreachable spot will not get maintained, and an unmaintained trap is no protection at all.
Whatever you specify, the specification is only ever as good as the servicing behind it. Agree an emptying schedule at the outset - a passive trap generally needs attention once it reaches around a quarter full, which for many kitchens means at least monthly, sometimes more. Build that schedule into the contract rather than leaving it to memory, because a trap that is emptied only when someone notices a smell has already failed. Keep every waste transfer note, and fold your grease records into your wider food safety management system so that cleaning, servicing and disposal evidence all sit together as due-diligence paperwork you can show an inspector on the spot. If you already run a trap and simply want it emptied, serviced and documented properly, our approach to grease trap management keeps the whole cycle covered without you having to chase it.
Questions
In practice, yes. Approved Document H of the Building Regulations (section 2.21) says drainage serving kitchens in hot-food premises should have a grease separator complying with BS EN 1825, or another effective means of grease removal. On top of that, Section 111 of the Water Industry Act 1991 makes it an offence to let fats, oils and grease interfere with a public sewer, so having no grease management in place leaves you exposed even where a specific trap is not itemised.
Sizing follows BS EN 1825-2, which produces a nominal size (NS) - roughly the peak wastewater flow in litres per second. That figure is adjusted by three factors: temperature, the density of the fats being separated, and the detergents and rinsing agents you use. The standard offers three ways in, based on meals per day, hydraulic load, or meat-processing units, so ask your supplier which method they used and to show the working.
A passive grease trap uses gravity and retention time to let grease float and solids settle, and needs manual emptying. An automatic grease removal unit (GRU) does the same but skims the grease into a container on a timed cycle, cutting the manual work at a higher cost. Biological dosing adds microorganisms that digest grease in the drain line and works best alongside a separator rather than instead of one.
As a rule, a passive trap needs emptying once it is around a quarter full, which for many kitchens means at least monthly and sometimes more frequently. Leaving it beyond that lets grease carry over into the sewer, defeating the point of the trap. Set the interval in your service contract and keep the waste transfer notes from every visit as part of your compliance records.
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