What is retention time in a grease interceptor?

Retention time is how long wastewater stays inside a gravity grease interceptor before it flows out to the sewer. During that stay, fats and oils float up and heavier food solids sink. A longer stay means more separation.

### Why does grease need time?

Grease is lighter than water, so it rises. It does not rise instantly. The National Precast Concrete Association (NPCA) puts it this way: if the material passes through the interceptor too fast, the grease does not have enough time to rise to the top. Solids also need time to settle to the bottom.

The NPCA adds that more time means more cooling of the liquids, and the more the liquids cool, the more effective the interceptor. That is one reason big outdoor tanks work well. The NPCA says large outdoor grease interceptors provide a longer retention time.

Think of a pot of chili left on the stove. Stir it and the fat stays mixed in. Leave it alone and a layer forms on top. A grease interceptor is a big, calm pot, and retention time is how long you leave it alone.

### How is retention time counted?

The NPCA describes it as the time it takes one particle of influent to travel through the system and leave the interceptor. It also says flow rates and retention times are inversely proportional. Faster flow, less time.

Here is a simple example of our own, not from a city. If a tank holds 500 gallons and water moves through at 25 gallons per minute, water spends about 20 minutes in the tank. At 50 gallons per minute, it is about 10. Real tanks are messier than that. Some water rushes along the shortest path and some lingers. Still, the direction is right: more flow, less time.

### Do small grease traps have a retention time?

Not in the same way. A small hydromechanical trap, the kind that sits in the kitchen, is rated by flow rate and grease capacity instead. The Plumbing and Drainage Institute (PDI), a trade body of drainage product makers, says these units are tested at their rated flow. A 20 gpm size is tested with 20 gpm of grease-laden water. It also says the vented flow control keeps the designed flow from being exceeded.

Schier, a maker of these units, says a properly designed hydromechanical interceptor does not depend on long retention times, because grease separates quickly when the flow pattern is right. Its blog gives an example of a 200 gpm rated unit that held 277 gallons and ran with a calculated retention time of 83 seconds, and it reports 93.5 percent efficiency in testing. That is a manufacturer's own test, so read it as one company's data.

For the flow-rate side, see how a trap differs from an interceptor and what size grease interceptor a restaurant needs.

It depends on whose code you read. The model code says 30. The Valley cities that publish a method mostly say 12 or 17.

DFU means drainage fixture unit, a code number for how hard a fixture can hit the drain. The sizing worksheet page walks through the math with a Phoenix example.

### Why does Phoenix use 12 minutes when the model code says 30?

Phoenix changed the text on purpose. Its amendment document strikes the words about 30 minutes from two places. In the definition of a gravity interceptor, the words "of not less than 30 minutes" are struck and replaced with a retention time approved by the Authority Having Jurisdiction. In Section 1003.3.7, the peak-flow-times-30-minutes wording is struck and replaced with total DFUs x 3 gpm x 12 minutes, or 17 minutes with food waste disposers.

The struck text does not show up as struck in a plain text copy of the PDF, so it is easy to read the deleted 30 as current. It is not.

Phoenix gives its reason in the same document: the city's water department and surrounding cities use a minimum 12-minute retention time. It adds that adopting 30 minutes would increase the size of required interceptors, which adds expense to the purchase and installation. Phoenix's manual gives the minimum gravity size as 500 gallons.

Scottsdale is a bit different on paper. Its guideline excerpt repeats the model definition that mentions a retention time of not less than 30 minutes, and its formula lists 12 minutes without a garbage disposal and 17 with one. Both are in the same city document. If a plan reviewer asks for one over the other, ask which section they are applying.

Mesa's code has no 17-minute step. Instead it says users shall not install or replace a garbage grinder that sends liquefied waste to the sewer, except for domestic sources. Mesa also sets a 500 gallon minimum and a 2,500 gallon maximum, and at least two chambers.

### Where did the 30-minute figure come from?

The NPCA's guidelines slides say 30 minutes minimum is referenced in the Plumbing Code, and tell owners to check state or local guidelines for possibly different retention times. Older methods were different. The NPCA's design article shows that the 2003 Uniform Plumbing Code method used hours of retention time, 2.5 hours with a dishwasher. When the NPCA ran one example restaurant through four common formulas, the tank sizes came out at 900, 1,000, 1,224 and 2,132 gallons, and it concluded that there is no right or wrong formula until more research is done.

Schier tells a sharper story. It says the 30-minute idea traces to a 1998 textbook that suggested a septic tank used as a grease interceptor should hold liquid for at least 30 minutes, and that the textbook offered no research behind the number. It also says a 2008 report from the Water Environment Research Foundation (WERF) found that gravity interceptors were generally improperly sized and often failed due to short-circuiting. Schier sells hydromechanical units, so weigh that as one side of the debate. PDI says most of the disagreement is over which flow rate to size for, not over the method.

### Is a bigger tank always better?

No. PDI says field experience has shown that oversizing can lead to hydrogen sulfide gas and sulfuric acid that destroy the interceptor and the drainage system. Schier says oversized tanks often generated hydrogen sulfide gas. So a long retention time is not a goal on its own. The target is enough time, in a calm tank, at the flow the kitchen actually produces.

Here the sizing math meets the real kitchen. A tank sized for 12 or 30 minutes only gives that time when things are working.

### Do flow surges matter?

Yes. PDI says a gravity interceptor sized with DFUs could see peak flow above its sizing, which shortens the detention time, and that at some point the shorter time will reduce grease separation. A dishwasher dumping a load, or a full pot sink draining fast, is a surge.

### Does a full tank lose time?

Yes. The NPCA says the FOG and solids layers that build up reduce the clear zone and slightly speed up the flow of water through the tank. That is why it stresses regular maintenance. The California Regional Water Quality Control Board slides make a similar point about partial pump-outs: they let food solids build up, which can cause short-circuiting. That is one reason the 25 percent rule exists. See the 25 percent rule page.

### Do hot water and soaps matter?

They can. The NPCA says emulsifying detergents reduce the size of grease globules, and smaller globules separate less easily. It also says hotter water may produce smaller globules, and that water hotter than needed will cause longer separation times of FOG. Its design article notes the effect is mixed, since hot water also thins out and lets globules rise faster, and a big tank cools the flow. It notes that several agencies limit water temperature to 140 degrees Fahrenheit. It also warns owners not to use enzymes, grease solvents or emulsifiers, since they can let grease pass through and set up later in the sewer pipes. Phoenix prohibits those products in its manual; see whether enzyme treatments are allowed in Phoenix.

### Can the tank design shorten it?

Yes. The NPCA says small holes or narrow baffles between compartments speed up the water, which can cause short-circuiting, and that in some WERF cases a single-compartment tank did better than a two-compartment tank with a narrow transfer baffle. Its guidelines slides say baffles increase retention time and prevent short-circuiting. A broken or missing baffle or tee undoes that. See the broken baffle page.

You cannot change the tank's size, but you can keep it doing its job.

1. 1Pump it on the city's schedule, and before it reaches 25 percent full of grease and solids where your city uses that rule.
2. 2Watch the pump-out. Confirm the tees and baffles are in place afterward.
3. 3Keep grease, solvents and enzymes out of the drain.
4. 4Avoid hotter water than the job needs.
5. 5Do not add fixtures without checking the size. Scottsdale, for one, says an interceptor must be sized for the new volume when additions or remodeling will increase the volume to it.
6. 6Keep records. Phoenix requires three years on site.

For schedules, see how often to pump a grease trap and the Phoenix pump-out schedule. If a tank is overflowing, see why a grease trap keeps overflowing.

Call a licensed plumber when you are adding fixtures or a dishwasher, replacing a tank, or seeing signs that water is rushing through, like grease downstream or a smell that returns fast. A plumber can check the fixture count, the plumbing plan and the parts, and can install or replace them under permit. Our plumbers are licensed and insured, we give free estimates with upfront pricing, and we offer 24/7 emergency service when a grease line backs up. Start at our commercial plumbing page.