Decaying grease and food solids release hydrogen sulfide gas. Above the waterline, bacteria turn it into sulfuric acid, which eats the concrete until aggregate and rebar show. Light damage and broken baffles can often be repaired or coated, but an interceptor that has rotted through usually gets replaced, and in Phoenix that means plan review.
How does grease make concrete corrode?
Through a chain of gas, bacteria and acid. The EPA's technical report on hydrogen sulfide corrosion, written about sewers and concrete wastewater structures, describes it: "The released H2S combines with moisture on the non-submerged surfaces of the pipe and is oxidized to sulfuric acid by aerobic bacteria of the genus Thiobacillus." Those bacteria live on the surfaces above the water level.
The acid reacts with the cement and forms new compounds. The same report says "Both gypsum and ettringite occupy considerably greater volume than the compounds they replace. This leads to expansion and disruption of the concrete, and loss of aggregate." In other words, the concrete swells, softens and flakes away from the inside.
Grease interceptors are set up for this. Pima County's pretreatment code notes that "solids accumulation and low flows in an interceptor may, over an extended period of time, produce a corrosive environment which can damage the structural integrity of the interceptor." Grease and food solids sitting in a warm tank are exactly the kind of waste that makes hydrogen sulfide.
### Why does pumping frequency matter?
Because solids left in the tank keep producing gas. Scottsdale's plumbing code amendments require cleaning when trapped grease and solids fill more than a quarter of the interceptor's volume, when the "pH of a sample taken from the interceptor effluent is below 5.0," or when odor becomes a problem. Low pH and strong odor are both signs of the conditions that feed corrosion. See how the fill-level pumping rule works and how often to pump a grease trap.
Hydrogen sulfide is also a safety hazard on its own. Scottsdale's code says that when levels reach 100 ppm, the OSHA level for "Immediately Dangerous to Life and Health," immediate action is required, and "If the hydrogen sulfide level cannot be immediately resolved, all use of the interceptor shall cease until the level becomes safe." Never lean into or enter an open interceptor. See whether a hydrogen sulfide detector makes sense.
What does a corroded interceptor look like?
The EPA report's definition of severe corrosion in concrete is a useful yardstick: "loss of one inch or more of concrete, loose or missing aggregate, exposed reinforcing steel." In an interceptor, you'd see it when the tank is pumped out and empty:
| What you see | What it usually means |
|---|---|
| Rough, sandy concrete above the waterline | Early acid attack on the surface |
| Loose or missing gravel in the walls | Advanced corrosion |
| Exposed or rusted rebar | Severe corrosion, structure at risk |
| Crumbling, broken or missing baffles or tees | Internal parts failing; grease can pass through |
| Cracked or sagging lid or riser | Corrosion at the top where gas collects |
| Groundwater or soil entering the tank | Walls or joints no longer watertight |
Mesa's guidance for restaurants says that while the device is empty, someone should check "that there are down-turned elbows/tees on the inlet and outlet piping, and that they are in good condition." Pumping day is the best chance to look. See what happens during a grease trap cleaning.
Cities look for the same things. Phoenix's FOG manual says "The inspector will check the grease Interceptor to see if any parts are missing or if the interceptor itself has rotted out and needs to be replaced." See what city inspectors check on a grease trap.
Can a corroded interceptor be repaired or lined?
Sometimes, when the damage is limited to internal parts or the surface of the concrete. It depends on how far the corrosion has gone and what your city accepts.
Broken internal parts. A failed baffle, tee or outlet pipe is a common, fixable problem. Phoenix's city code lists "necessary repairs to internal structures" as part of required interceptor maintenance. See what to do about a broken baffle or tee.
Surface damage. Coatings and specialty repair concretes exist, but results vary. The EPA report says specialty concretes "are used mostly for spot repairs," and that coatings include "a myriad of proprietary epoxies, resins, sealers, silicones, urethanes, and coal tars." It also found that only about a quarter of the coatings tested by the Los Angeles County sanitation districts showed good acid resistance, with most failures traced to application problems. The EPA report also warned that "Hydrogen sulfide gases have been documented to pass through poor joints and cause failure by attacking the concrete substrate behind the liner." Preparation and application matter as much as the product.
Some cities set a standard. Flagstaff's FOG manual says gravity interceptors "made from materials that are subject to corrosion such as concrete or steel, shall be lined or coated with a durable material approved by the City that is compatible with a pH of 3," and that acid-resistant enamel coatings "are not acceptable." That's Flagstaff's rule, not the Valley's, but it shows what a durable coating is expected to handle.
Structural failure. Exposed rebar, walls that have lost inches of concrete, or a tank that lets groundwater in usually means replacement. Scottsdale's inspection form gives inspectors two options to check, "Repair Interceptor" with a deadline, or "Replace Device," with 30 days for plan review and permits and 60 days to install.
### What does replacement involve in Phoenix?
A permit and plan review. Phoenix's FOG manual says "Per plumbing code all grease interceptors installed and replaced in the City of Phoenix must go through plan review," so the city can confirm the new unit is sized correctly, approved, and connected to every grease-bearing fixture. Like-for-like replacement of hydromechanical interceptors can go through over the counter. The same manual requires hydromechanical units to be built of "corrosion resistant material (stainless steel or polymer)." For the permit side in other cities, see grease interceptor permits for a replacement in the Valley and the Valley grease trap and interceptor rules by city.
How do you slow the damage down?
Keep solids and grease from sitting in the tank:
- 1Pump on schedule. Don't let grease and solids build past your city's pump-out point. See Phoenix's pump-out schedule.
- 2Scrape plates and use strainers. Fewer food solids means less decay and less gas. See what can go down a restaurant grease trap.
- 3Inspect when it's empty. Look at walls, the lid underside, baffles and tees every time it's pumped, and note them in your log, along with any repairs such as a replaced baffle. See what to record in a grease trap maintenance log.
- 4Fix small problems early. A broken tee or a patch of bare aggregate is cheaper to fix than a failed wall.
HQ Plumbing & Air repairs and replaces grease interceptors for restaurants across the Phoenix area, including baffles, tees and the plan review for a new unit. We're licensed and insured, with 24/7 emergency service.