Why does my grease trap keep overflowing?

Five things cause most overflows. They often stack, so check all five.

1. 1The tank is past its cleaning point. Grease and solids take up the room water needs.
2. 2The outlet, the air relief or the line past the tank is blocked. Water has nowhere to go.
3. 3A baffle, tee or flow control is missing, broken or wrong. Water rushes through or backs up.
4. 4The kitchen sends too much. More fixtures, more flow or the wrong equipment on the line.
5. 5The device is the wrong type or size for the load. A dishwasher on a small trap is the classic case.

If it overflows again soon after a pump-out, the tank being full is probably not the whole story. Read on to the outlet and load sections.

### Is the tank simply full?

Start here, since it is the easiest to check and the most common. A full tank stops separating grease and starts passing it along, or backs up.

How full is too full?

Phoenix's manual says most types of interceptors stop working effectively when fats, oils and solids reach 25 percent of the device's capacity, and above that the business must do a full cleaning. The NPCA's guide says the same for gravity tanks: if solids and grease are greater than 25 percent of the liquid depth, the interceptor needs pumping. See the 25 percent rule page.

Zurn's care sheet explains what happens past that point. If maintenance is not done, excessive grease builds up until water laden with grease passes directly through the unit. So an overfull trap can leak grease downstream long before it spills on the floor.

How often do cities say to clean?

Here is what the Valley documents we read say:

• Phoenix. Hydromechanical interceptors must be fully pumped and cleaned every 30 days unless the Director grants a variance, and none may go more than 90 days without maintenance. Gravity interceptors must be fully pumped at least once every 3 months unless a variance is granted.
• Tempe. Grease traps must be serviced monthly, or more often if needed. Grease interceptors must be serviced quarterly, or more often if needed.
• The NPCA. For gravity tanks, pump at least every three to six months, more often depending on use, and a new installation may need monthly or bimonthly cleaning until the level of FOG buildup is predictable.

Those are floors, not promises. A busy fryer kitchen can fill a tank faster than the calendar says. Our pump-out schedule page for Phoenix and the Tempe schedule page cover the details.

Why does it fill faster than it used to?

• A new menu with more fried or greasy food.
• Skipped scraping. The NPCA and Phoenix both list scraping plates and pans into the trash, using drain screens and keeping grease out of the drain as basic kitchen practices.
• A garbage disposal. The NPCA says disposals can overload an interceptor with solids.
• A pump-out that was not complete. The California Regional Water Quality Control Board slides report that studies show partial pump-outs let food solids build up. Phoenix's manual says the interceptor must be left completely empty when pumping is done.

Yes, and this is the cause people miss when a freshly pumped tank spills again.

### What does a blocked outlet look like?

The NPCA says an unexpectedly high liquid level, or grease above the normal level, could mean a plugged fitting, line or filter. So if the tank is full of water at the top right after service, look past the tank.

Zurn tells owners that once the tank is cleaned, they should check that the inlet, outlet and air relief ports are clear of obstructions. Air relief matters on small traps. Zurn says the flow control device must be properly vented so air can mix with the fluid entering the interceptor and keep the proper operating level in the separation chamber.

### Can grease harden in the line?

It can. Phoenix's manual says FOG from food preparation and dishwashing can and will harden and build up. Its words for the result: "When a blockage occurs, the sewage coming from upstream backs up and finds another way out." Zurn adds that waste pipes leading to a trap may clog if the liquid cools before it gets there, which is why it says to put the interceptor as close as practical to the fixtures it serves.

Phoenix's plumbing code amendment requires a two-way cleanout on the discharge side of all hydromechanical grease interceptors. The city's stated reason is to give an entry point to clean the line downstream of the device and back to the device. If your trap has no cleanout on that side, the line past it is hard to clear. The sewer lateral page covers grease that gets past the tank, and the pumping versus jetting page explains which job clears which part.

### What if the water spills at a floor sink?

A floor sink is a common spill point. Maricopa County's food establishment construction guide says all three compartments of a three-compartment sink should drain indirectly to a floor sink with an air gap. So a floor sink can be the place where a slow drain shows up first.

Zurn's care sheet warns about this for one of its models: indirect piping to the unit could result in standing water in the floor sink, or flooding, if a line blockage occurs. Schier's support page for its GB2 model has a question from an owner whose fixture has an air gap and discharges into an overflowing floor sink. Schier says the low head pressure in that setup is a factor, and that the fix depends on how the unit is certified. So do not pull out or change a flow control part on your own. Check the maker's sheet, and the city's rules, first.

Either one can fill a trap faster than it drains. Parts that steer the water can fail without any warning sign until the tank spills, and a kitchen can outgrow its trap without anyone noticing.

### What about gravity interceptors?

The California Regional Water Quality Control Board slides say to leave the interceptor with all proper components, meaning the outlet tee, inlet tee and mid-wall baffle sweep. A missing or broken tee can change how water moves through the tank. We cover this on the broken baffle and tee page. The NPCA also says to visually inspect all fittings, baffles and fixtures after a pump-out.

### What about small hydromechanical traps?

These have one more part: the flow control. The International Plumbing Code says grease interceptors shall have devices to control the rate of water flow so flow does not exceed the rated flow, and Phoenix carries that wording through in its amendments. Mesa and Scottsdale both say a flow restriction valve goes upstream of the trap and must be vented properly.

Zurn says these units are tested and rated with a vented flow control with a properly sized orifice, and that failing to use it will cause improper function and could cause grease to bypass the unit. It also says if the maximum recommended flow is exceeded, the efficiency will decrease considerably.

The Plumbing and Drainage Institute (PDI) says a trap with a vented flow control will not exceed its designed flow, but that drain-down times get longer. In a busy kitchen, that can look like a sink that will not empty, or a floor sink that fills and spills. For flow-control venting, see the venting page.

### Can a dishwasher be the cause?

Very often. Here is what the Valley documents say about dishwashers and small traps:

• Phoenix's manual says no food waste disposals or dishwashers can discharge to a hydromechanical interceptor, and its code amendment says commercial dishwashers and food waste disposal units shall discharge to a gravity grease interceptor.
• Scottsdale's guidelines say dishwashers shall not be connected to a grease trap.
• Mesa's code says any facility installing a dishwasher shall install a grease interceptor.
• Tempe says a facility with a commercial grade dishwasher must install a grease interceptor to accept its wastewater.

If a dishwasher was added to a kitchen after the trap went in, that alone can explain repeat overflows. The dishwasher and grease trap page covers it in detail.

### What about garbage disposals?

Zurn says not to install its interceptor in a waste line from a garbage grinder, because the rapid buildup of solids greatly reduces efficiency. Tempe says garbage disposals and food grinders are not permitted. Maricopa County's guide says grinders are not permitted on three-compartment sinks or food prep sinks and are approved only case by case.

### Was the device sized for the fixtures?

Phoenix sizes small traps by the number of fixtures connected, up to four, and its amendment table steps up the flow rating as the count goes up. Scottsdale and Mesa allow traps only with four or fewer fixtures used for food preparation. Once a kitchen adds fixtures, the old device may be too small. The sizing worksheet page explains gravity tank sizing, and the under-sink trap page covers how a three-compartment sink fits a trap. When a change of use or a remodel adds load, see the former retail space page.

### Do soaps and hot water play a part?

They can. Zurn lists grease-cutting detergents as a factor, because they break liquid grease into tiny particles that can pass through. The NPCA says to use detergents that promote rapid oil and water separation and to avoid enzymes, solvents and emulsifiers, which it says let grease pass through and later coagulate on the inside walls of sewer pipes.

Work through these steps in order.

1. 1Stop sending water to the trap. Pause dishwashing and pot sink use until the level drops.
2. 2Contain and clean the spill dry. Phoenix's manual recommends spill kits with absorbent material, like pads or kitty litter, before mopping. Phoenix's manual notes that oil left on the ground washes into the storm drain system when it rains, so do not hose a spill outside.
3. 3Keep people away from an open tank. The NPCA warns never to enter the interceptor. Its guide also notes that lids at ground level are a fall risk when damaged or left unsecured.
4. 4Skip chemicals. Phoenix's manual prohibits chemicals, enzymes, bacteria, solvents and emulsifiers meant to dissolve grease. See enzyme treatments in Phoenix.
5. 5Book a full pump-out through a permitted hauler, and watch it. Phoenix says to witness the cleaning to make sure the pumper cleans the interceptor completely. Ask the hauler to confirm the inlet, outlet, air relief ports, tees and baffles are clear and in place. Our hauler page covers who is allowed to haul in Maricopa County.
6. 6If it spills again, look past the tank. That points to the outlet, the line, the flow control or the load.

Write down what happened and when. The maintenance log page shows what to record, and Phoenix requires records on site for three years.

Call a licensed plumber when the tank spills again soon after a pump-out, when a baffle, tee or flow control needs repair or replacement, when a line past the tank needs to be cleared or a cleanout added, or when a dishwasher, disposal or new fixture means the device is the wrong type. In Phoenix, every interceptor installed or replaced goes through plan review, and a like-for-like swap of a small hydromechanical unit can go through over the counter. Our plumbers are licensed and insured, we give free estimates with upfront pricing, and we run 24/7 emergency service when a grease line is overflowing during service. Start at our commercial plumbing page.