Tech Corner - passive grease traps

Tech corner is a regular feature of the project newsletter aimed at providing basic 101 information on aspects of onsite wastewater systems.

Grease traps - what are  they?

Grease traps intercept and collect fats, oils and grease (FOG) discharged from kitchen wastes. They can protect your wastewater system from blockages by eliminating or reducing FOG discharged into pipework and the downstream systems.

There are two main types of grease traps or interceptors: Passive grease traps/interceptors (located outside of buildings) and mechanical grease separators/interceptors. Most marae have passive grease traps although some marae may be better suited to mechanical separators or hydro mechanical grease interceptors which are smaller and require less space. This issue’s Tech Corner describes passive grease traps (e.g., Figure 3).

How do they work?

Passive grease traps are made up of a series of compartments which wastewater from the kitchen flows through. Their typical configuration is shown in Figure 4.  

They work by cooling down the wastewater so that FOG floats to the surface and other solid material, such as food scraps, sink to the bottom.

To be effective, grease traps must retain the fluid long enough for it to cool and for the FOG to float to the surface.

They should have tight-fitting lids, be raised above surrounding ground, or have raised lips around the top to stop rainwater from getting in. 

How big should they be?

To be effective, grease traps must retain drainage water for at least 30 minutes at peak flow (Crites & Tchobanoglous 1998). The peak instantaneous flow will depend on your kitchen (e.g., how many sinks it has, the number and types of appliances). 

The NZ Building Code Acceptable Solutions and Verification Methods for Foul Water (G13 – Foul Water) requires grease traps to have at least twice the capacity of all sanitary fixtures and sanitary appliances discharging to it, and in no case less than 100 litres.

Two common methods of determining minimum grease trap size are the peak flow method (i.e., based on the estimated peak hourly flow) and the fixture unit method. If you are upgrading or building a new kitchen, seek guidance on system requirements from your local authority and design engineers.

What maintenance do they need?

Regular cleaning helps maintain performance of the grease trap and avoid or minimise odours.

Maintenance of the grease trap involves removing grease from the top and sludge/solids from the bottom, and scraping and hosing down the sides and removing all of the contents.

The level and frequency of cleaning and maintenance required can only be determined through regular usage and experience following installation, as it depends on the size and capacity of the installed unit, how often the kitchen facilities are used, and how much FOG is going into It.

Good housekeeping

You can reduce the amount of cleaning and maintenance on your grease trap by following some good housekeeping practices, including:

  • scraping plates, dishes and cooking utensils into recycling or rubbish bins before washing
  • disposing of waste fats and oils (such as from the deep fryer) separately. NEVER put fat and oils down the drain
  • using minimal fat and oil for cooking
  • recycling fats and oils.


Auckland Regional Council Technical Publication 58 (2004) On-site Wastewater Systems: Design and Management Manual; Third Edition ARC Technical Publication 2004.

Crites, R., Tchobanoglous, G. (1998) Small and decentralised wastewater management systems. Boston: WCB/McGraw-Hill.

BS EN-1825-1:2004 Grease separator-Part I: Principles and design, performance and testing, marking and quality control.

Ministry of Business, Inovation and Employment (2014) Acceptable Solutions and Verification Methods For New Zealand Building Code Clause G13 Foul Water (2014), Second Edition, Amendment 5.

Wellington City Council Fats, oils and your food business pamplet and Passive Grease Trap Factsheet.

Figure 3: Lifting the lid - inside a passive grease trap chamber. [NIWA]
Figure 4: Minimum requirements for grease traps up to 150 litre capacity. [Image sourced from Figure 4 - NZ Building Code Acceptable Solutions and Verification Methods for NZ Building Code Clause G13 - Foul Water). [MBIE]