Grease Traps

In the City of Beloit, any facility required by the department to install a grease interceptor, oil interceptor, or sand interceptor shall, at all times, maintain the interceptor in proper working order. The interceptor shall be readily accessible for cleaning and inspection at all times. No interceptor, catch basin, or similar device shall contain greater than 25% combined grease, oil, or solids at any one time based on industry standards. The facility shall keep a log describing the cleaning and repairs performed on the grease interceptor and the dates of such activity. The log shall be kept on site for three years.

Each year, the City of Beloit Water Resources staff inspect all grease traps to help prevent blockages and sanitary sewer overflows.                                                       


Sanitary Sewer Overflow (SSO)

An SSO is a release of raw sewage from a sanitary collection system prior to treatment.  Inadequate flow capacity, power failures, excess inflow and infiltration, and blockages causes SSOs. SSOs are prohibited to protect public health, prevent property damages, protect resource damage and protect waters of the state. 


Grease Traps - Basic Operation 

Most grease traps are "passive" which means they operate without any moving or mechanical devices. The grease traps or interceptors work by separating the grease from the water. The wastewater passes over a series of baffles separating the grease and oil from the water by slowing the flow to allow enough detention time for separation.

 See Grease Trap Flow Chart for schematic)


Frequency of Maintenance

Interior grease traps should be evaluated for cleaning needs on regular basis. If a significant volume of grease is accumulating in the outlet baffle of the trap, it needs to be cleaned more often. Proper and frequency of maintenance is the only sure way to know if your trap is operating properly.

Remove accumulated grease out from the top and place it in a watertight container such as a plastic bag. Bail out any water and place it in a watertight container. Scrape the sides, baffles, and lid and place in a watertight container and place in the garbage. Smaller interior traps can be cleaned by using a shop vac. A 100% of the contents must be removed and may not be introduced into the trap or any drains. DO NOT FLUSH WITH HOT WATER OR USE DRAIN CLEANERS, ENZYMES OR BACTERIA AGENTS. This just removes the problem into the sanitary sewer and is a violation of the Wisconsin Plumbing Code SPS 382.34(5)(e).




DO Not Discharge Hot into the Grease Interceptor

Using hot water, 120 degrees F or higher to rinse the dishes or if a dishwasher is connected, keeps the grease in suspension and does not allow for separation. If the trap does not have sufficient capacity to allow for cooling, grease will again be discharged into the sewer. Many treatment facilities requiring cooling jets that are activated to cool the water as it enters the interceptor. Even if the grease amount isn’t a lot as it cools, it accumulates on the interceptor pipe walls and causes a plug. Hot grease cools quickly in a sanitary sewer.


Best Management Practices 

1. Train kitchen staff and other employees about proper maintenance of your grease trap. 

2. Post “No Grease Down the Drain” signs above sinks and on the front of dishwashers.

3. “Dry wipe” pots, pans, and dishware prior to dishwashing.

4. Dispose of food waste by recycling and/or solid waste removal.

5. Recycle waste cooking oil.

6. Do not pour grease down the sinks or into the toilet. Remember that only the  four P's (Pee, Puke, Poop, and Toilet Paper) down the drain!

7. Avoid or limit the use of garbage disposals.

8. Use a 3-sink compartment dishwashing system, including sinks for washing, rinsing and sanitizing.

9. Use strainers in sinks to catch food scraps and other solids.

10. Keep up with maintenance log on grease interceptor/trap maintenance.



It is difficult to formulate an exact criteria for sizing grease interceptors because of so many variables that exist. Size is often evaluated on the following criteria:

  • Type of restaurant (drive through, walk in)
  • Type of food being served (fried, baked, broiled)
  • Seating capacity or quantity of flow
  • Retention time required for efficient separation of water and grease
  • Frequency and type of maintenance


Why Following the Plumbing Code May Not be Good Enough

The Wisconsin Plumbing Code Comm 82 requires that all nonresidential establishments who prepare food must have a grease interceptor. Comm 82 also provide guidelines for choosing an appropriate grease interceptor for installation. HOWEVER, following the plumbing code will not assure that you will not violate prohibited discharges standards under the sewer use ordinance. The SUO may be more restrictive than the plumbing code when it comes to installing pretreatment equipment. Pretreatment equipment is any piece or pieces of equipment used to treat wastewater to meet pretreatment regulations.


Check out our additional documents:

Grease Trap Best Management Practices (BMPs)

Business Owner's Guide to Grease Traps in Beloit

How often should I have my grease trap cleaned?