A clogged grease trap is one of the most common — and most expensive — operational problems in commercial kitchens. When fats, oils and grease (FOG) accumulate faster than the trap is serviced, the result is slow drainage, sink backups, foul odours and, eventually, a sewer-line failure. This guide covers the five most common grease trap problems Drain Blasters sees across Cape Town restaurants, hotels and food service kitchens, and the practical fixes for each.
Grease trap maintenance is also a compliance issue. The City of Cape Town’s Wastewater and Industrial Effluent Bylaw requires food service premises to install a properly sized grease trap and maintain trade effluent within specified FOG limits. Neglect leads to fines and prohibition orders, not just blocked drains.
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How a Grease Trap Works
A grease trap is a passive interceptor installed between kitchen drains and the sewer line. Wastewater enters the trap, slows down, and FOG floats to the top while solids settle to the bottom. The relatively clean middle layer discharges to the sewer. When properly sized and serviced, this is a near-foolproof system. The problems below all share one root cause: the system is full of FOG and solids the trap was meant to remove.
1. Clogged Grease Trap
A clogged grease trap is the most frequent failure mode. Inlet, outlet and crossover lines can all clog. If the inlet blocks, wastewater backs up into sinks and dishwashers. If the outlet blocks, the trap overflows. Visual indicators include grease standing more than 25% of the trap depth, a hardened FOG cap, and food solids visible at the outlet baffle.
Fix: a full pump-out by a licensed contractor (Drain Blasters supplies the compliance certificate). For severe build-up, the inlet and outlet lines are jetted clean with high-pressure water rather than chemically dissolved — chemicals damage stainless steel traps and shorten their service life.
2. Slow Drainage and Sink Backups
A slow-draining sink is the earliest warning sign of a grease trap problem. The trap is still functioning, but FOG accumulation has restricted the inlet baffle. Ignoring it leads to a full blocked drain downstream — often in the kitchen line under the floor slab, where repairs are far more disruptive than a routine trap service.
Fix: service the trap and inspect upstream sink traps. If the sink P-traps are also clogged, a kitchen drain clearing visit clears the full run.
3. Overflowing Grease Trap
An overflow is the loudest symptom. Causes include: skipped service intervals, an undersized trap, or a downstream sewer blockage forcing the trap to back up. Overflows release FOG into the kitchen floor or yard — a hygiene risk, a slip hazard and a Bylaw contravention all at once.
Fix: immediate emergency pump-out, root-cause analysis (size vs throughput, last service date, downstream condition), and a corrected service schedule. If the trap is undersized for the kitchen’s load, replacement is the long-term answer.
4. Foul Odours
Anaerobic decomposition of FOG and food solids produces hydrogen sulphide — the unmistakable rotten-egg smell that drifts back through kitchen sinks. Common causes are extended service intervals, broken or poorly seated trap lids, and dry P-traps on rarely used sinks.
Fix: service the trap, replace or reseat the lid (a fresh gasket costs less than R200), and ensure all sink traps stay filled with water.
5. Corrosion and Structural Damage
Decomposing FOG generates short-chain fatty acids that attack mild steel and even some stainless grades over time. Soft spots, pinhole leaks and visible rust at weld lines are signals the trap is approaching end-of-life. A leaking trap can release grease into soil, contaminating the property and triggering remediation costs that dwarf a replacement unit.
Fix: inspect annually for visible corrosion; replace the trap when corrosion is structural. Grade 304 stainless steel traps offer the best longevity in commercial kitchens.
How to Prevent Grease Trap Problems
Service Regularly
The standard rule is “the 25% rule” — service the trap when the FOG layer reaches 25% of the working depth. For most kitchens this means weekly to monthly intervals. High-volume kitchens may need fortnightly service.
Train Staff on Disposal
FOG should never be poured down sinks. Used cooking oil goes to a collection drum for recycling; pan scrapings go in the bin. Sink strainers catch the residual fines.
Right-Size the Trap
An undersized trap fills up in days and fails repeatedly. Sizing should match peak hourly throughput, not average daily flow. Drain Blasters can spec a replacement against your actual kitchen output. See our overview of commercial grease trap cleaning for sizing guidance.
Use Biological Treatments, Not Chemicals
Enzyme and bacterial dosing products break down FOG naturally between services. They do not replace pump-outs, but they reduce odour and slow build-up. Caustic and acidic cleaners damage trap walls and gaskets — avoid them entirely.
Proactive Grease Trap Management
The cost of preventative grease trap servicing is a fraction of the cost of an emergency call-out, an inspection failure or a downstream sewer repair. Most well-run commercial kitchens treat grease trap servicing the same way they treat extraction canopy cleaning — a scheduled, non-negotiable line item in the operations budget. If you’d like Drain Blasters to assess your current trap and propose a service schedule, our team is available 24/7 for both routine work and emergencies.
