The size of a grease arrestor is critical in ensuring the wastewater discharged from a restaurant, café, fast food take away or other grease waste producer has the optimum conditions for efficient suspended solids, oil and grease removal.

Two methods can be used to determine the appropriate size of a grease arrestor and both are based on ensuring the wastewater has a minimum retention time of one hour.

Water Corporation has a minimum grease arrestor size of 500 litres and a maximum size of 2000 litres in effective capacity for any individual grease arrestor. Customers wishing to install larger grease arrestors should seek our approval before installation.

Method 1 – Fixture unit rating method

Add the fixture unit ratings (see Table 1) for all fixtures that feed into the grease arrestor and multiply this by 100L. Check where this calculated volume lies in the ‘Calculated Grease Arrestor Size Range’ (Table 2 below) to determine the corresponding ‘Minimum Grease Arrestor Size’.

Table 1: Fixture unit ratings

Fixture unit ratings
Fixture Fixture Unit Rating Fixture Fixture Unit Rating
Steamer 1 Kitchen sink 3
Wok (per burner) 1 Double kitchen sink 3
Hand basin 1 Pot sink 5
Rinse sink 3 Double pot sink 5
Combi ovens 5    

Table 2: Minimum grease arrestor size

Minimum grease arrestor size
Maximum Number of Fixture Units Calculated Grease Arrestor Size Range Minimum Grease Arrestor Size
7 100L – 700L 500L
13 701L – 1300L 1000L
17 1301L – 1700L 1500L
26 1701L – 2600L 2000L
52 2601L – 5200L 2 x 2000L, 4000L
78 5201L – 7800L 6000L

Method 2 – Peak flow rates

Where the hourly peak wastewater flow rate is known, it can be used to determine the recommended grease arrestor size. Compare the peak hourly flow with the ‘Calculated Grease Arrestor Size Range’ in Table 2 to determine the corresponding ‘Minimum Grease Arrestor Size’.

Shared grease arrestors

In circumstances where we approve businesses to share a grease arrestor the minimum size can be calculated using either Method 1 or Method 2. If it is not practical to connect grease arrestors in series, or the FU loading is too high for a single grease arrestor, then the waste streams are to be split and diverted to individual grease arrestors to accommodate the fixture loadings.

Dishwashers & glass washers

Dishwashers and glass washers are not to discharge into grease arrestors due to their use of detergents, high water temperatures and surge loads, which can liquefy or emulsify oil and grease allowing it to be discharged to sewer or overload the arrestor.

Non–typical grease arrestors

We may accept the use of other approved types of grease arrestors such as those which may include the use of filters. These types of grease arrestors may be subject to specific conditions or restrictions in use and therefore it is recommended prior to the installation of such arrestors customers seek advice from Water Corporation if any conditions or restrictions apply.

Worked examples

Example 1: Fixture unit rating method

If a restaurant kitchen has: 1 Double Pot Sink (5 FU), 1 Single Pot Sink (5 FU) and 1 Hand Basin (1 FU), the maximum hourly flow that could be expected can be calculated as follows:

11 FU x 100L = 1100L

Therefore the recommended size is 1000L (from Table 2).

Example 2: Peak flow rate method

The peak flow rate from a kitchen/production area is known to be 0.5L/sec. The minimum grease arrestor size is calculated as follows:

0.5L/sec x 3600 sec/hour = 1800L/hour

Therefore the recommended size is 2000L (from Table 2).

Example 3: Fixture unit rating method – shared arrestor

Business A – 5 FU, Business B – 11 FU and Business C – 4 FU

When 3 businesses (A, B, & C) share a grease arrestor, the maximum hourly flow that could be expected, and hence the grease arrestor size, is calculated as follows:

20 FU x 100L = 2000L

Therefore the recommended size is 2000L (from Table 2).