Which grease trap?

Choosing a grease trap

This information is a non-technical guide for anyone unfamiliar with how grease traps operate. It is for guidance only. For more information please contact Mactrap.
In New Zealand there are 3 main types of grease traps: passive, grease removal units and grease converters. Choosing which one is best suited to your premises will depend on local council requirements, the space you have and your budget.

Passive grease traps
These are large tanks, designed to hold and slow the flow of waste water containing fat, oil and grease (FOG) before it reaches the sewer. The passive trap has baffles which slow the flow of waste water FOG from kitchen sinks or other kitchen waste water outlets. This slowing process allows the FOG to separate from the water - the FOG rises to the top and the water below flows out of the trap into the sewage system. Over time the FOG builds up (along with food waste allowed to wash into the trap) and the trap has to be cleaned out (usually by a tank cleaning service).
Passive traps are usually installed outside your premises and generally speaking, councils in New Zealand prefer external passive grease traps to internal traps. Most councils will insist that if you have space outside for a grease trap, this is where it should be installed. Mactrap passive traps are made from a high density polyethylene and can be installed above or below ground as their structure does not need the support of surrounding earthworks to keep them in shape.

Passive trap

Grease Removal Units
These are usually installed internally, close to the sink where the waste water containing the FOG comes from. They are usually made from stainless steel. Grease Removal Units (the Mactrap Grease Boss) work differently to the passive trap. Waste water with FOG flows into the trap, through a filter to remove food particles, then through a baffle system where the FOG separates and rises to the top. However instead of the FOG building up, the Grease Removal Unit has a mechanical means of continuously removing the FOG into a container that can be discarded (or re-used). If the Grease Removal Unit is kept free of waste food (if food particles get into the trap there is no means for them to escape), the Grease Removal Unit should be able to remove most FOG allowing the cleaned water to flow into the sewage system.

It is important to keep the Grease Removal Units clean - the filter systems must be cleaned frequently (the Mactrap Grease Boss has a fine mesh filter basket which is removeable to empty the waste that has gathered. This waste is then discarded). The containers which catch the discarded FOG must also be emptied. The units generally require a professional clean out every 6 months.

Choosing.grease boss

Grease Converters
Many councils do not allow these units so you need to check if they are allowed in your area.  Grease Converters are usually installed internally, close to the sink where the waste water containing the FOG flows out and they are usually made from stainless steel. They are fitted with an automatic dosing pump which pumps a set amount of enzyme or equivalent into the tank. Waste water with FOG flows into the trap and this is broken down by the enzyme so the FOG flows in solution with the waste water into the sewage system.

The reason many councils do not allow these units is because any waste food allowed into the grease converter reacts with the enzyme (or bio-material) and stops it working effectively. This means the FOG is not properly broken down before it flows into the sewer. However if you are scrupulous about keeping food out of the unit and ensure the enzyme dosage is set correctly, they should work.

Although grease converters are cheaper to buy than the grease removal unit, you should also budget for the higher cost of maintenance (they usually require cleanouts ever 2/3 months and enzyme over one year will cost between $600 - $2000, depending on whether your restaurant has high or low fat/oil content.

Mactrap grease converter

What about plumbing in a dishwasher?
Dishwashers: Dishwashers are usually plumbed straight to the drain, although some councils may prefer they are connected to the grease trap. Dishwashers and degreaser units will severely affect the efficiency of the grease trap because of the cleaning agents used with them. The cleaning agents react with the enzyme/bacteria in the grease converter which prevents the grease converter performing as it should.

With the Grease Removal Units, the dishwasher cleaning will also reduce the effectiveness of the FOG removal although they will still work to a certain extent. The cleaning agents can often create a sludge with the FOG which has to be manually removed.
In passive traps, the cleaning agents will also affect the FOG separation within the tank.
Need help?
Contact us if you need more information - we hare happy to help you work out the best grease trap for your situation.
If you have a grease trap or interceptor and believe that it may be ineffective at keeping FOG out of the sanitary sewer (i.e. needs frequent cleaning, backups occurring in kitchen, etc.), you may need to upgrade or replace your existing grease trap/interceptor.