Food, oil and grease (FOG) treatment for food premises connected to the sewerage system
Regulation: If a building is likely to convey FOG in wastewater, it must have a grease trap or similar, to maintain sanitary conditions. The purpose of the grease trap is to prevent FOG from getting into the sewerage system. All food premises' sanitary fixtures shall 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 the Central Hawke's Bay District Council and therefore the ratepayers.
- High levels of FOG inhibit the effectiveness of sewage treatment. It is therefore important that levels of FOG are controlled to ensure that our treatment systems are efficient and that 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, i.e. gets washed down the sink, should go through a grease trap first.
Selection of a trap
There are a number of designs of FOG traps available and in use within the food industry. The type selected will depend upon each premises' circumstances in regard to amount of FOG disposed of, number of meals, cost and space available.
Minimum dimensions for a food premises' grease trap is an allowance of at least 5 Litres per seat, plus an extra 25% to allow for peak flushes:
|Number of seats||Without dishwasher or waste disposal||With dishwasher or waste disposal|
|Up to 50||350 L||455 L|
|Up to 100||625 L||600 L|
|Up to 150||975 L||1200 L|
|Up toi 250||1575 L||2700 L|
Waste disposal units and dishwashers must be connected before the grease trap so that their discharges are also dealt with. Note the extra capacity needed for these.
A conventional grease trap basically 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 and cause increased foul odours and attraction to pests.
Inspection points are available for checking if the FOG layer is thick enough to warrant clearing. When this is so, 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
An enzyme grease converter: A completely sealed unit that is capable of being installed inside. It is advisable not to connect waste disposal units and dishwashers into these 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: A sealed unit capable of being installed inside. These are a mechanical device that periodically warms the trapped solution and skims off the FOG layer into a separate container for disposal at the tip. These units are smaller and may fit under bench units.
First, you need to identify whether there already is a grease trap on your premises site and what type it is. Look for lift out concrete slabs in the yard of your premises. If you discover one you didn't know about, chances are it needs cleaning out urgently!
The next trick is to find manufacturers specifications for cleaning it. If this is not possible, call a 'sump cleaner' to do this for you. Look under Waste Disposal or Tank Cleaners in the Yellow Pages.
A conventional grease trap, once under control again, should need checking at least monthly. If the FOG layer is thick, it's time to clean it out again. This frequency will vary between traps and premises depending on the volume of the trap and the amount of FOG put down it.