Grease Traps

If a building is likely to convey food, oil or grease (FOG) in wastewater, it must have a grease trap or similar to maintain sanitary conditions. The purpose of a grease trap is to prevent FOG from getting into the sewerage system. All food premises' sanitary fixtures must direct their waste through their grease trap.

Grease traps are important in Central Hawke's Bay because when FOG is washed down the drain, it frequently sets in the sewage line when it cools. Blockages result in significant costs to Council, and therefore ratepayers. High levels of FOG also inhibit the effectiveness of sewage treatment, so it is important that levels of FOG are controlled to ensure that our treatment systems are efficient and the corresponding discharges are not harmful to their receiving environments.

No food premises should put FOG down the sewer. Solid food wastes can be disposed of via the refuse collection or composted. Waste disposal units are not recommended. Liquid or solid fats can be recycled through tallow melters but anything that goes into the sewer, should go through a grease trap first.

Is there a grease trap onsite already?

Identify whether there is already a grease trap on your premises site and what type it is. Look for lift-out concrete slabs in the yard of your premises or any paperwork detailing the manufacturers specifications for a grease trap and its cleaning recommendations. You can call a 'sump cleaner' to help you. 

Once maintained, a conventional grease trap should be checked at least monthly. If the FOG layer is thick, it's time to clean it out. This frequency will vary between traps and premises depending on the volume of the trap and the amount of FOG put down it.

Select the best grease trap

There are many FOG trap designs available and in use within the food industry. The type selected will depend on each premises' circumstances in regard to amount of FOG disposed of, number of meals, cost and space available.

The minimum dimensions for a food premises' grease trap must allow for at least five litres per seat, plus an extra 25 percent to allow for peak flushes.

Waste disposal units and dishwashers must be connected before the grease trap so that their discharges are also dealt with. Remember to note the extra capacity required for these.

A conventional grease trap allows the wastewater to settle so that FOG can rise to the top of the solution and set, leaving the remaining water to flow on down the sewer. When the FOG layer is allowed to get too deep, it can block the water outlet and cause dysfunction. If clearing the FOG layer is left too long, it will also turn rancid, increase foul odours and attract pests.

Inspection points are available for checking if the FOG layer is thick enough to warrant clearing. In this instance, the whole lid can be removed to lift off the FOG layer and dispose of it at the tip (trade waste cannot be collected via refuse collection). Alternatively, a registered 'sump sucker' can take it away for you.

Alternatives to conventional grease traps

Enzyme grease converters are completely sealed smaller units that can be installed inside but should not be connected to waste disposal units or dishwashers as they will clog quickly due to their smaller size, and chemicals will kill off the enzymes. They require regular dosing with enzyme powder to allow microbiological activity to work at low flow times.

Automatic interceptors are sealed smaller units that can be installed inside, such as under bench units. These mechanical devices periodically warm the trapped solution and skim off the FOG layer into a separate container for disposal at the tip.