How often should a grease trap be pumped?
"Every 30 to 90 days" is the answer most pumping companies give. City sewer codes are both stricter and looser than that, depending on what kind of trap you have — from every two weeks for a small indoor trap in one city to once a year for a large outdoor interceptor in another.
The rule that applies almost everywhere: 25%
Cities that regulate grease traps nearly all use the same trigger. In Phoenix's city code, the accumulated grease and solids must never exceed 25 percent of the total liquid volume of the interceptor. San Jose's version: pump "when the final chamber is twenty-five percent (25%) full of FOG and solid food waste." FOG is the regulators' shorthand for fats, oils and grease.
The reason is how a trap works. Grease floats, solids sink, and the clear water in between flows out to the sewer. Once grease and solids take up about a quarter of the depth, there isn't enough clear water left for the grease to separate, and it starts to pass straight through into the sewer line — which is what the city is trying to prevent.
The 25% rule applies on top of any fixed schedule. If your trap reaches 25% before its scheduled date, it must be pumped then.
First: which kind do you have?
| Grease trap (hydromechanical interceptor) | Grease interceptor (gravity) | |
|---|---|---|
| Where | Indoors, usually under or near a sink | Outdoors, in the ground |
| Size | Small — often under 100 gallons | Large — often 1,000 gallons or more |
| Fills | Fast | Slowly |
| Typical schedule | Weekly to monthly | Quarterly or longer |
Most of the confusion online comes from mixing these two up. A small indoor trap and a large outdoor interceptor are on completely different schedules.
What cities actually require
| City | Indoor trap (hydromechanical) | Outdoor interceptor (gravity) | Records |
|---|---|---|---|
| Phoenix, AZ | At least every 30 days; never more than 90 days without maintenance | At least every 12 months; food service establishments at least every 3 months | 3 years |
| San Jose, CA | At least every 90 days, or at 25% if sooner | 3 years | |
| Lawrence, KS | At least monthly; small units under 100 gallons may need weekly | At least quarterly | Keep a log |
| Pleasanton, CA | Inspected weekly, cleaned at least every 2 weeks | Every 60 days unless the city sets otherwise | 2 years |
Phoenix and the others allow a variance from the fixed schedule, typically where you can show the trap stays under 25% for longer. Your city's sewer or FOG program office sets your schedule; many issue it in writing when you open.
How to tell where you are
The 25% point is measured, not guessed. Pleasanton's inspectors use a sampling stick — a "Dip Stick Pro" or similar — to measure the grease and solids layers against the water depth. Lawrence gives a rule of thumb for interceptors: cleaning is typically due when there are 3 inches or less of floating grease.
Signs you are already past due:
- Grease visible downstream of the trap. Lawrence says to increase frequency if you see it.
- Repeated blockages in the line from your building. Pleasanton notes that clogged lateral sewer lines needing a plumber are often a sign the interceptor needs cleaning more often.
- Smells from floor drains and slow-draining sinks.
A pump-out means all of it
San Jose requires the pumping service to completely pump out all contents, and forbids it from decanting — pumping the tank and pouring the separated water back in. Decanting makes a pump-out quicker and cheaper for the pumper, but it leaves the trap partly full of greasy water. Phoenix similarly prohibits returning removed contents to the interceptor.
San Jose also requires the pumping invoice to show where the waste was taken. Your business can be liable for illegal dumping of its own waste, so a pumper who can't tell you where it went is a risk to you, not just to them.
Small indoor traps you clean yourself
Many small under-sink traps are cleaned in-house. Lawrence's guidance:
- Scoop out the floating grease weekly or more often, and leave the water behind.
- Empty and clean the entire trap at least once a month.
- Check the outlet, clear deposits from the piping, and refit the flow baffles after each cleaning.
- Log every cleaning.
What makes a trap fill faster
- Hot water. Lawrence says to keep it below 140°F. Hotter water keeps grease liquid so it passes through instead of separating.
- Garbage disposals. Pleasanton bans food grinders upstream of a grease control device, and Lawrence says not to use them. Ground food goes straight into the trap as solids.
- Grease down the drain. Fryer oil and rotisserie fat belong in a used-oil container, not the sink. San Jose says pouring them into drains connected to the sewer violates local ordinances.
- Additives. Pleasanton bans bacteria, enzymes and emulsifiers in grease control devices. They loosen grease so it flows on, only for it to solidify again further down the sewer.
Fit drain strainers and scrape plates into the bin before washing. Every bit of food that doesn't reach the trap is pumping you don't pay for.
Grease trap maintenance checklist
Pulling the city rules above into one schedule. Your city's intervals override these where they are stricter.
| When | Task | Source |
|---|---|---|
| Every shift | Scrape plates into the bin; keep drain strainers in place; fryer oil and fats into the used-oil container, never the sink | San Jose, Lawrence |
| Daily | Keep dishwashing water below 140°F where you control it; no garbage disposal upstream of the trap | Lawrence, Pleasanton |
| Weekly | Small indoor traps: inspect, and skim off floating grease with a scoop, leaving the water | Lawrence, Pleasanton |
| Every 2 weeks to monthly | Small indoor traps: empty and clean the whole trap, refit baffles, log it | Pleasanton (2 weeks), Phoenix and Lawrence (monthly) |
| Every 60 to 90 days | Outdoor interceptors: complete pump-out by a pumping service, with an invoice showing where the waste went | Pleasanton (60 days), San Jose (90 days), Lawrence (quarterly) |
| Whenever grease and solids reach 25% | Pump, whatever the schedule says | All four cities |
| Always | Keep every pumping and cleaning record on site, 2–3 years | Phoenix, San Jose, Pleasanton |
Not on the list: enzyme or bacterial additives. Several cities ban them, and they don't replace a pump-out.
Keep the records
Phoenix and San Jose require pumping records on site for three years, and Pleasanton for two. San Jose says failing to keep them can bring fines. Keep every invoice, in date order, where an inspector can see it. They are your evidence that the trap was maintained if a sewer line blocks downstream.
What a pump-out costs, size by size, is in what does grease trap pumping cost.