How do I control odor from a grease trap or interceptor?

Do four things, in this order.

1. 1Clean it on time. Full cleanings on the schedule your city sets, or sooner.
2. 2Seal it. Lids, gaskets and bolts in good shape, so odor stays in the tank.
3. 3Vent it. The building's plumbing vent is what carries gas up and away.
4. 4Feed it less. Keep grease, food scraps and additives out of the drains.

The National Precast Concrete Association (NPCA) lists poor ventilation, long gaps between service and sporadic use among the causes of interceptor odor. Each of those maps to one of the four steps above.

As often as your city says, and more often if the tank fills faster. The rule that matters most is the 25 percent mark. Phoenix says most types of interceptors stop working effectively when fats, oils and solids reach 25 percent of the device's capacity, and that a full cleaning is required above that.

### What do Phoenix, Scottsdale and Chandler say?

Phoenix. The city's FOG manual sets these minimums:

• Hydromechanical interceptors (the small, indoor kind) must be fully pumped out and cleaned every 30 days unless the city grants a variance. None may go more than 90 days without maintenance.
• Gravity interceptors at food service establishments that capture FOG must be fully pumped out and cleaned at least once every 3 months unless the city grants a variance.
• A new facility should watch its interceptor with its pumping company or a city inspector and adjust to the real rate of buildup. It may need more frequent pumping.

Phoenix also says cleaning must be performed by a licensed pumper, and the interceptor must be left completely empty afterward.

Scottsdale. Its 2021 plumbing code amendments name four triggers for cleaning: captured fats, oil, grease and solids displacing more than 25 percent of the interceptor volume, effluent pH below 5.0, odor becoming a health issue or nuisance, and a city decision that cleaning is needed. The same amendments set a complete cleaning at least every 90 calendar days for interceptors above 100 gpm or 200 pounds, and every 30 calendar days at or below that size, with an exception process for the smallest units. Complete cleaning is by a licensed contractor. Check that Scottsdale has not adopted a newer version before you rely on those numbers.

Chandler. The city's page calls 25 percent a rule of thumb for the combined volume of FOG and solids. It says that if FOG is seen leaving a treatment device, the cleaning frequency must be increased.

Our pump-out schedule page for Phoenix and the general pump-out page go further on setting an interval.

### What does the precast industry add?

The NPCA guide says a new gravity interceptor may need cleaning monthly or every other month until the service company learns the buildup rate. It says at a minimum a gravity interceptor should be pumped every three to six months, and that a facility should follow the local pump-out level. It says that when the combined FOG and solids reach 25 percent of the liquid depth, the interceptor needs pumping. It also suggests thorough vacuuming of settled material, to prevent biological breakdown and the unwanted release of gases.

One more useful line for seasonal or part-time kitchens. The NPCA says that when a precast gravity interceptor will sit dormant for more than 30 days, the preferred practice is to pump it and refill it with clean water before the quiet period. Schools and churches are its examples.

For who is allowed to haul the waste, see who can haul grease trap waste in Maricopa County.

Yes, and the NPCA puts it strongly. It says odor problems with outdoor interceptors can be reduced when a properly designed interceptor is part of the building's plumbing and venting system. It says that in almost all cases odor problems are caused by improper venting of the building's plumbing system, which lets gas build up in the interceptor and escape. Proper ventilation and design, along with gastight manhole covers and seals, keep odors in the interceptor so they can leave through the roof vents.

### What should you check on the lids?

• Gaskets. The NPCA says to check that the gasket is in place and not cracked or broken, and to replace it if needed.
• Bolts. It says to secure access covers with bolts where applicable.
• A spare. It says to keep an extra gasket on site so a failed one does not turn into leaks and odors.
• Rating. Mesa's code says bolted covers, where required, must be gastight and watertight.
• The lid itself. A cracked or sunken cover breaks the seal. See the riser and extension collar page and the traffic rated lid page.

### What should you know about the vent?

Interceptor venting is a design matter. Scottsdale's amendments say interceptors must be designed so they do not become air bound where tight covers are used. Our venting page covers what the model code and Valley cities say, and what they leave open. A vent problem is a plumbing repair, so it is not something to fix with air fresheners or cleaners.

### Which kitchen habits cut the smell?

Less grease and less food in the tank means less to rot. The NPCA guide lists practices that also reduce interceptor upkeep:

• Dry scrape or wipe pots, pans, plates and utensils into the trash or compost before washing.
• Use in-sink strainers to catch food scraps.
• Avoid garbage disposals. The guide says food particles can lead to odor problems, because they break down biologically and release gases in the interceptor.
• Never pour grease or oil down sinks or floor drains. Post no grease signs.
• Do not use water hotter than needed. The guide notes that several agencies limit wash water to 140 degrees Fahrenheit, and that hotter water lengthens the time grease takes to separate.
• Use detergents made to help oil and water separate.
• Keep dry absorbent on hand for spills.

The NPCA's general guidelines also say sanitary waste should not be routed into a grease interceptor. That is a design issue, so it belongs on the plans. See what can go down a restaurant grease trap.

### Are enzymes or bacteria a good fix for odor?

No, and in Phoenix they are prohibited for this purpose. The FOG manual says using chemicals, enzymes, proteins, bacteria, grease solvents or emulsifiers to temporarily dissolve fats, oils and grease is prohibited. Chandler's page says its city code prohibits adding enzymes and bacterial products directly to a FOG treatment device to reduce or eliminate cleaning or pumping, because they emulsify the FOG only until it re-congeals farther down the pipe. The NPCA says the same and adds that most regulators prohibit additives. See are enzyme grease treatments allowed in Phoenix.

### What should you not do?

• Do not skim or pour it back. Phoenix says a pumper may not reintroduce removed water or materials into the interceptor or the sewer. Scottsdale prohibits skimming, decanting or discharging removed waste back into any interceptor or into the collection system.
• Do not add enzymes, bacteria or solvents. See above.
• Do not stretch the schedule. Odor is a cleaning trigger in Scottsdale's text, and Phoenix sets minimum intervals.
• Do not leave a torn gasket or cracked lid. It lets gas out and water in.
• Do not assume it is the tank. A dry floor drain trap or a vent problem can smell like sewer with a clean interceptor. See why a restaurant smells like sewer and sewer gas smell in a commercial building.
• Do not skip the log. Phoenix and Scottsdale both require written records kept at least three years. See what to record in a maintenance log.

Yes, it can be. A rotten-egg smell can be hydrogen sulfide, a toxic and flammable gas that can collect in tanks, manholes and sewers.

OSHA says hydrogen sulfide occurs naturally in sewers and is heavier than air, so it can collect in low-lying and enclosed spaces such as manholes and underground vaults. Its rotten-egg odor is noticeable at very low levels. OSHA also says that after a while at low levels, or more quickly at high levels, you can no longer smell it. Its table lists loss of smell at 100 to 150 ppm. So a smell that seems to fade is not proof the gas is gone.

OSHA's hazards page lists 100 ppm as the level NIOSH calls immediately dangerous to life and health. OSHA calls the gas highly flammable, so keep sparks and open flame away from the area.

### What does that mean for you?

• Do not enter the tank. The NPCA warns not to enter the interceptor to perform maintenance. OSHA defines a permit-required confined space as one that contains or could contain a hazardous atmosphere, among other traits.
• Do not lean into an open tank. That is our advice, not a code line. Gas collects where you would put your head.
• Know the city rule. Scottsdale's amendments say that when hydrogen sulfide reaches 100 ppm or above, immediate action is required, and if the level cannot be fixed at once, all use of the interceptor must stop until it is safe.
• Act on symptoms. OSHA lists headaches, dizziness and eye irritation at lower levels. If anyone feels unwell near a tank, move away from it. If someone is dizzy, confused or collapses, call 911.

Call one when the smell comes back right after a cleaning, when a vent or lid seems to be the problem, or when a floor drain or line near the interceptor smells or backs up. A licensed plumber can trace the vent, check the drain traps and repair or replace parts. HQ is licensed and insured, and estimates are free with upfront pricing. A backup during service is an emergency, and we offer 24/7 emergency service. Start on our commercial plumbing page.

If a very strong smell comes with dizziness, headaches or eye irritation, leave the area first, and call 911 if anyone is dizzy, confused or collapses.