The 25 percent rule says a grease trap or interceptor stops working well once fats, oils and solids fill a quarter of its capacity. Phoenix requires a full cleaning at that point. Chandler calls it a rule of thumb and says to clean more often if grease is seen leaving the device.
What the cities say
Phoenix's FOG manual states that most types of interceptors stop working effectively when fats, oils and solids reach 25 percent of the device's capacity. If they occupy more than that, the restaurant is required to do a full cleaning, and a licensed pumper has to do it.
Chandler words it as a rule of thumb: the combined volume of FOG and solids inside the device should not exceed 25 percent of its total capacity. Chandler adds a second test. If FOG is seen leaving the treatment device, the cleaning frequency must go up.
How it fits with the calendar schedule
The rule works alongside a calendar, not in place of one. Phoenix sets a floor: traps pumped and cleaned every 30 days and never left past 90, interceptors at least every three months. Tempe sets the same monthly and quarterly cycle "or more frequently if necessary." A busy fryer line can cross 25 percent well before the date, so the earlier of the two wins. Outside food service, Phoenix applies the same 25 percent limit to gravity interceptors with a once-a-year minimum, which matters for car wash sand and oil interceptors.
Phoenix also tells new restaurants to watch the device with their pumper or an inspector after opening and adjust the schedule to how much grease actually builds up.
What to do
- 1Ask the pumper to note how full the device was at each visit, so you can see the trend.
- 2If it arrives near or past a quarter full, shorten your interval.
- 3Watch the outlet side. Grease leaving the device means you are already late.
- 4Keep the invoices. Phoenix wants records on site for three years.
Not every city writes the 25 percent figure into its code. Where it is missing, your city's fixed schedule still applies. See how often to pump a grease trap, the Tempe cleaning schedule, and the Valley rules by city.
Some kitchens add an automatic grease removal device next to the interceptor. Whether that is allowed depends on the city.
Retention time is the other half of the picture; see what it is and why it matters. Letting solids sit past that point also feeds the gas that eats concrete tanks; see why concrete grease interceptors corrode.