Question:

I am a septic system designer in New Hampshire. We are designing a restaurant system and are looking for ideas on cooling the kitchen waste stream either prior to or within the grease trap, which has a 6,500-gallon capacity. Thank you for your ideas.

Answers:

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I read that you should feel the effluent from the grease trap during peak flow, and if it is more than lukewarm, you should add another tank in series.

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What about installing a closed-loop cooler in the trap? What I mean is to run several coils of pipe on the floor of the trap. Then run the pipe up through a fan on the surface to remove the heat.

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I am also a designer in Vermont and New Hampshire, and have had good results from follow-up testing using a 2- to 3-day retention time in the grease trap. I isolate the kitchen flows from other building flows and use that volume in the calculation. I like to use concrete as a tank material when access is good and, if space is available, use two tanks in series.

If you can get a precaster to make it, try a meander tank. The additional surface area and cooling properties will help a lot. I have not had much success using inside grease-removal technology. Grease issues were easy when we all cooked with lard. With the refined oils, high dishwasher temperatures and detergents that clean so easily, the only reliable solution is time and temperature, which means as much exterior tankage as you can fit onsite.

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Here, only the kitchen wastewater (standard safe waste system) goes into the grease trap. The grease trap waste line gets tied into the sanitary waste line. A 6,500-gallon tank should definitely allow sufficient time for the grease to solidify and separate as it does here, and the average fast-food restaurant may have a 1,500-gallon grease trap.

Larger restaurants may have two in sequence such as a 1,000/1,000 or 1,500/1,500, if needed, to allow for cooling and separation. Code-required pumping is performed and manifested every 30 to 120 days, depending on the size of the restaurant and the flow volume. Fryer oil doesn’t go into the grease traps. It gets put into 55-gallon barrels outside to be used for processing or biodiesel.

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We have not worked on any large restaurants, but we have prepared designs for fast-food places and for banquet halls and churches with kitchens. I know it is important to get the kitchen wastes cool enough for the grease to congeal, and it doesn’t cool very well in below-ground tanks. The ground makes a pretty good insulation.

We advise our clients not to put fryer oil down the drain. Newer restaurants have methods of filtering it and reusing it several times. Then they store and haul away the spent oil. But, you still will have a lot of high-strength waste with high FOG, particularly if you have a commercial pass-through dishwasher. It is important first to get the best handle you can on the flow numbers.

We separate the kitchen waste from the cold-water waste stream from the bathrooms. Then we provide several days of residence time in two or more septic tanks in series for just the kitchen wastes, and put a fine-slot effluent filter on the last tank.

The effluent from these tanks is then discharged into the building sewer coming from the bathrooms, which will be predominantly cold water. An additional two or more days of residence time is provided in these tanks for the now-combined flow. We have also installed an effluent filter on the outlet to these tanks, before the effluent goes to a sand filter or soil absorption system.

In another case, we used a recirculating rock filter, along with lots of tankage, to cool and treat the wastewater before a soil absorption system. Both designs seemed to have performed well.