2.3 Commercial Grease Interceptors & Oil/Grit Separators

Compliance note: G13/AS2 sizes a grease trap at not less than twice the capacity of connected fixtures and appliances. For restaurants and cafes it is at least 5 L per seat and never less than 100 L. Other occupancies and trade-waste quality limits require network-utility approval; do not substitute a universal retention time.

Commercial Grease Interceptors & Oil/Grit Separators

Commercial and industrial wastewater frequently contains high concentrations of contaminants that cannot be discharged directly into the municipal sewerage network. In commercial kitchens and food manufacturing facilities, wastewater contains hot, emulsified fats, oils, and grease (FOG). When untreated FOG enters municipal sewers, it cools, solidifies, and reacts with calcium to form massive congealed deposits known as fatbergs. These block public mains, choke wastewater pump stations, and cause catastrophic environmental sewer overflows.

Similarly, commercial automotive repair workshops, service stations, and equipment wash bays discharge petroleum hydrocarbons, heavy motor oils, solvents, and abrasive suspended sands. Under the Local Government Act 2002, the Resource Management Act 1991, NZBC Clause G14 (Industrial Liquid Waste), and local network utility trade waste bylaws (such as Watercare in Auckland, Wellington Water, and Christchurch City Council), business owners and certifying drainlayers are legally obligated to install approved pre-treatment devices before discharging to the public sewer.

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1. Trade Waste Classifications & Statutory Obligations

Trade waste is defined as any liquid, with or without matter suspended or dissolved, produced during the course of any commercial, industrial, or trade process, excluding domestic sanitary wastewater and uncontaminated stormwater.

Local network utilities enforce three strict trade waste categories:

1. Permitted Trade Waste: Discharges meeting standard threshold limits (e.g. temperature below 40°C, pH between 6.0 and 9.0, FOG less than 100 mg/L, non-flammable). Requires trade waste registration and pre-treatment.
2. Conditional Trade Waste: Discharges containing higher contaminant loads that require formal permitting, continuous monitoring, metered trade waste charges, and engineered pre-treatment systems.
3. Prohibited Discharges: Highly toxic, corrosive, explosive, radioactive, or concentrated petroleum wastes (e.g. chlorinated solvents, bulk waste cooking oils, petrol). These must never enter the sewerage system under any circumstances.

Discharging non-compliant trade waste into a public sewer without an approved interceptor or in violation of permit conditions carries heavy financial penalties under local bylaws and potential prosecution under the Resource Management Act 1991, with fines exceeding NZ$200,000 for commercial entities.

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2. Passive Grease Interceptor Mechanics & Sizing

Passive grease interceptors (often called gravity grease traps) operate on the physical principle of differential specific gravity governed by Stokes' Law. Animal fats and vegetable oils have a specific gravity of approximately 0.88 to 0.94 g/cm3, making them lighter than water (1.00 g/cm3). When warm, turbulent culinary wastewater is slowed and held in a quiescent (calm) chamber, the buoyant grease globules rise to the liquid surface while dense food solids settle to the floor.

The Retention Time Rule

For effective physical separation, wastewater must remain inside the interceptor long enough for grease droplets to cool, coalesce, and float to the surface. New Zealand trade waste bylaws and NZBC G14/VM1 do not prescribe one universal retention period. Under G13/AS2, capacity is at least twice the capacity of connected fixtures and appliances; restaurants and cafes use at least 5 L per seat and never less than 100 L.

Minimum Tank Volume (Litres) = Peak Flow Rate (L/min) * Retention Time (minutes)

Fixture Flow Rate Sizing Method

When sizing an interceptor based on connected commercial kitchen appliances, evaluate peak discharge rates:

Worked Example: Restaurant Interceptor Sizing

A commercial cafe kitchen contains:

• 1 Double-bowl scullery pot sink (60 L/min)
• 1 Commercial dishwasher (30 L/min)
• 1 Food prep sink (30 L/min)
• 1 Floor waste channel (20 L/min)

1. Calculate Maximum Simultaneous Peak Flow:

   • Applying a simultaneous usage diversity factor of 0.70 across the 4 fixtures:
   • Peak Flow = (60 + 30 + 30 + 20) * 0.70 = 140 * 0.70 = 98 L/min

2. Apply 20-Minute Retention Time:

   • Effective Volume = 98 L/min * 20 minutes = 1,960 Litres

3. Select Standard Precast Tank:

   • Certifying drainlayer specifies a standard 2,000-Litre or 2,500-Litre precast concrete grease interceptor to provide adequate sludge holding capacity and satisfy local network utility trade waste criteria.

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3. Internal Anatomy of a Compliant Grease Interceptor

A properly engineered grease interceptor comprises three distinct zones:

1. Upper floating grease blanket (FOG cap, specific gravity approx 0.90)
2. Middle clear-water extraction zone (retention zone)
3. Bottom settled sludge and food solids zone (specific gravity greater than 1.0)

Mandatory Construction Features

• Inlet Baffle: The incoming drain connects to a vertical drop tee or baffle extending at least 150 mm below the normal operating water level. This directs incoming flow downward, dissipating hydraulic velocity and preventing turbulence from breaking up the floating grease layer.
• Outlet Dip Pipe: The outlet must incorporate a submerged dip pipe or baffle extending into the lower third of the water depth, terminating 150 to 200 mm above the floor. This guarantees that water drawn into the discharge line comes strictly from the clear middle zone—never from the top grease cap or bottom sludge.
• Inlet-to-Outlet Invert Drop: There must be a minimum drop of 50 to 75 mm between the inlet pipe invert and the outlet pipe invert to accommodate internal hydraulic head loss.
• Chamber Ventilation: Biological activity in anaerobic grease sludge produces hydrogen sulfide (H2S) and methane. The air space above the liquid must be vented via a dedicated DN80 or DN100 vent extended to open atmosphere above roof level.
• Airtight Access Covers: Heavy-duty, gas-tight cast iron or ductile iron access covers (Class B for footpaths, Class D for heavy vehicular driveways) must be installed over both the inlet and outlet compartments to prevent odor escape and allow vacuum truck suction hoses full access.
• Downstream Sample Eye / Point: An accessible inspection shaft or sampling eye must be installed immediately downstream of the grease trap outlet. This allows council trade waste compliance officers to insert sample bottles and verify that FOG levels remain below 100 mg/L.

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4. Mechanical Grease Interceptors & Automatic Grease Recovery Units (AGRUs)

In high-density urban environments, heritage buildings, or shopping mall food courts, installing an external 2,000-Litre underground tank is frequently physically impossible. In these approved scenarios, Automatic Grease Recovery Units (AGRUs) are installed directly inside the kitchen adjacent to cooking equipment.

• Operation: Wastewater passes through a removable strainer basket to catch coarse food solids. The water enters a compact stainless steel separation tank where internal heating elements maintain water temperature at 40°C to 50°C, keeping grease liquid. A motorized oleophilic (oil-attracting) plastic wheel or rotating belt attracts the free-floating liquid grease. A wiper blade continuously scrapes the oil off the wheel into an external collection caddy, which staff empty daily.
• Trade Limitation: AGRUs require daily staff maintenance, electrical supply, and strict logging. If power fails or staff neglect emptying the caddy, the unit immediately discharges raw grease into the drainage system.

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5. Automotive Oil, Sand & Grit Interceptors

Automotive service workshops, panel beaters, mechanical garages, and commercial car wash facilities discharge a hazardous mixture of motor oil, fuel, grease, coolant, and heavy road grit. Discharging hydrocarbons into a public sewer poses severe fire, explosion, and toxic vapor hazards.

The Triple Interceptor Pit (Sand/Oil/Water Trap)

The standard gravity pre-treatment device for automotive premises is the triple interceptor pit, comprising three interconnected precast concrete compartments:

1. First Pit (Grit & Sand Sump): Heavy gravel, coarse sand, and metal shavings settle to the floor under gravity. The water exits via a submerged baffle.
2. Second Pit (Primary Oil Separation): Slower quiescent flow permits free non-emulsified motor oils and light hydrocarbons (specific gravity 0.75 to 0.85) to float to the surface.
3. Third Pit (Final Polishing & Water Seal): A deep dip pipe draws clean water from the bottom of the chamber, trapping any remaining floating hydrocarbon films.

Coalescing Plate Separators (CPS)

For commercial wash bays and large fleet maintenance depots, gravity triple pits cannot remove tiny, sheared oil droplets. A Coalescing Plate Separator (CPS) is required:

• Wastewater passes through a pack of closely spaced corrugated, oleophilic (oil-attracting) polypropylene plates set at a 45-degree angle.
• Microscopic oil droplets adhere to the plate surfaces, creep upward along the corrugations, and coalesce into large oil globules that break free and float to the surface.
• CPS units achieve effluent quality with total hydrocarbon concentrations below 15 to 30 mg/L, satisfying even the most stringent municipal trade waste permits.

Crucial Wash Bay Drainage Rules

• Stormwater Exclusion: Outdoor vehicle wash pads must be roofed or bunded. Uncontaminated rainwater from yard areas must never drain into an oil/water separator. High storm flows cause hydraulic washout, flushing accumulated oil straight into the public sewer!
• Unroofed Wash Bay Diversion: Unroofed wash pads must be fitted with an automated first-flush sensor valve. When washing occurs, wastewater is diverted to trade waste; during rainstorms, clean roof stormwater is diverted to the stormwater system.

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6. Maintenance, Pumping & Trade Waste Auditing

• The 25% Rule: A passive grease interceptor or oil separator must be completely pumped out and cleaned when the combined thickness of the top grease blanket and bottom sludge reaches 25% of the total liquid operating depth.
• Pump-Out Frequency: Most trade waste permits mandate a maximum pump-out interval—typically every 1 to 3 months for restaurants, carried out by a certified liquid waste contractor holding a waste tracking manifest.
• Water Refilling: After pumping out an interceptor, the contractor or drainlayer must immediately refill the chamber with clean water. If left empty, buoyant groundwater pressure can pop the tank out of the ground (tank flotation), shearing all connected drainage pipes.