How Grease Trap Sizing Works in Commercial Plumbing
A grease trap works by slowing down hot, greasy kitchen wastewater so it has time to cool down. As the wastewater cools inside the chamber, fats, oils, and grease (FOG) float to the top, food solids sink to the bottom, and cleaner water from the middle zone flows out into the council sewer.
If a trap is sized too small, wastewater rushes straight through the chamber in under 5 minutes without cooling down. Hot liquid fat enters the street sewer, solidifies inside main pipes, and causes massive blockages that lead to heavy council fines or trade waste license revocation.
The Hydraulic Sizing Calculation
Australian water authorities size grease interceptors using peak hydraulic flow rates and a mandatory 20 to 30 minute retention time:
Minimum Interceptor Volume (Litres) = Peak Fixture Flow (L/min) × Retention Time (mins) × Venue Load FactorKey inputs used by hydraulic engineers and plumbers:
- Raw Fixture Flow Rate: Standard discharge allowances per fixture (e.g., Pot Sinks @ 15 L/min, Pre-Rinse Spray Sinks @ 12 L/min, Dishwashers @ 10 L/min, Wok Burner Troughs @ 20 L/min).
- Coincidental Diversity Factor: Recognises that not every sink and tap in a busy kitchen runs at 100% full capacity at the exact same second. For 6 to 12 connected fixtures, a diversity multiplier between 0.50 and 0.65 is applied.
- Venue Fat Load Factor: Adjusts for menu grease intensity. High-grease venues (wok kitchens, fried chicken, burger shops) need higher retention margins (1.3 to 1.5 multiplier) compared to low-grease venues like cafes or bakeries (0.5 to 0.7 multiplier).
Water Authority Baseline Requirements by State
Across Australia, water authorities enforce a minimum baseline trap capacity — almost universally 1,000 Litres for commercial kitchens with 3 or more fixtures. Sizing smaller than 1,000L is generally only approved for low-volume cafes via formal trade waste variance applications.
| State / Region | Water Authority | Baseline Min Size | Approval Process | Typical Inspection Fee |
|---|---|---|---|---|
| NSW (Sydney, Illawarra) | Sydney Water | 1,000 Litres | Sydney Water Tap-in Portal | $600 – $900 / year |
| VIC (Melbourne South-East) | South East Water | 1,000 Litres | Trade Waste Agreement | $500 – $800 / year |
| VIC (Melbourne North-East) | Yarra Valley Water | 1,000 Litres | Trade Waste Portal | $500 – $800 / year |
| QLD (Brisbane, Ipswich) | Urban Utilities | 1,000 Litres | Trade Waste Application | $400 – $700 / year |
| SA (Adelaide & Regional) | SA Water | 1,000 Litres | Trade Waste Permit | $450 – $750 / year |
| WA (Perth & SW) | Water Corporation | 1,000 Litres | Trade Waste Permit | $500 – $850 / year |
| ACT (Canberra) | Icon Water | 1,000 Litres | Trade Waste Consent | $400 – $650 / year |
Worked Example — Sizing a Grease Trap for an 80-Seat Sydney Restaurant
Worked Example: 80-Seat Restaurant in Sydney
Venue Details: Licensed restaurant in Parramatta, NSW. 80 dining seats, peak throughput of 50 meals/hour, operating under Sydney Water trade waste guidelines.
- Kitchen Fixture Count: 3× Pot Sinks (45 L/min) + 2× Pre-rinse Sinks (24 L/min) + 1× Commercial Dishwasher (10 L/min) + 1× Wok Burner (20 L/min) + 3× Floor Wastes (24 L/min) + 1× Combi Oven (6 L/min) = 129 L/min raw flow across 11 fixtures.
- Diversity Factor: 11 fixtures → coincidental factor = 0.50.
- Peak Design Flow: 129 L/min × 0.50 = 64.5 L/min.
- Required Retention Volume: 64.5 L/min × 30 minutes retention time × 1.0 menu multiplier = 1,935 Litres.
- Final Selection: Round up to nearest standard commercial trap size → 2,000 Litre in-ground poly or concrete interceptor.
- Scheduled Pump-out Frequency: Based on 2,000L capacity and estimated grease accumulation, licensed liquid waste contractor pump-out required every 13 weeks (4 times per year).
Key Installation Rules & Compliance Pitfalls
- Connecting All Kitchen Floor Wastes: Under AS/NZS 3500.2, every floor waste gulley located inside cooking, prep, and dishwashing zones MUST drain into the grease trap line before discharge.
- Keep Pipe Distance Short: Install the interceptor as close as practically possible to the kitchen (ideally within 10 to 15 metres). Long pipe runs allow grease to cool and solidify inside the building's underground sewer line before reaching the trap.
- No Domestic Sewage Connections: Toilets, urinals, and hand basins in customer bathrooms must NEVER drain into a grease trap. They connect directly to the sanitary sewer line downstream of the trade waste sampling point.
- Liquid Waste Tracking (EPA Regulations): Grease trap pump-outs must be performed by an EPA-licensed liquid waste transporter. Pump-out dockets must be logged with your local water authority to maintain your operating permit.
Above-Ground vs In-Ground Grease Interceptors
In-ground interceptors (1,000L to 5,000L+) installed outside in carparks, driveways, or service yards are the preferred option across Australia. Where outdoor excavation is impossible — such as high-density CBD tenancies, upper-level shopping center food courts, or heritage basements — above-ground stainless steel grease converters or automatic grease removal units (AGRUs) can be installed inside internal plant rooms with a formal water authority performance solution.