Grease trap backing up or smelling: causes, frequency, and cost

A grease trap is a simple device doing an unglamorous job: slowing wastewater down so grease floats and solids sink, keeping both out of the sewer. When it stops working you get slow drains, a smell, and a conversation with a health inspector.

What is actually going wrong

Symptom	Usual cause
Drains slowing across the kitchen	The trap is full and grease is backing into the lines
Smell at the trap or the floor sinks	Overdue, or pumped without scraping the walls and baffles
Grease appearing in a floor drain	The trap is bypassing or the baffle has failed
Trap pumped recently and still smelling	Contents removed, walls and baffles not cleaned
Health inspector note	Usually frequency or records rather than a failure
Trap fills again within days	Undersized for the volume, or a drain line issue upstream

The 25 percent rule

The working standard most jurisdictions apply: pump when grease and solids reach 25 percent of the trap’s liquid depth. It is a useful rule because it is measurable rather than a guess.

In practice that means:

A small indoor trap on a busy line: often every two to four weeks.
A larger indoor trap: monthly to quarterly.
An in-ground interceptor: typically quarterly, depending on volume and size.

The right interval is the one that keeps you under 25 percent, and the only way to find it is to measure for the first few services rather than assume. Ask us to record the measured grease and solids depth at each visit and set the interval from those readings, not from a fixed calendar.

What a proper service includes

Remove the full contents, not just the floating layer. Pumping the top and leaving the solids is how a trap that was serviced last month is full again.

Scrape the walls, the lid area, and the baffles. This is the step that gets skipped, and it is the step that decides whether the smell comes back.

Check the baffles. A trap with a missing or broken baffle is not separating anything; wastewater runs straight through. This is more common than you would think and it is invisible unless somebody looks.

Check the flow restrictor on indoor units, if fitted.

Refill the trap with water before putting it back in service.

Leave a manifest. Where waste goes matters, and the record is part of what a health inspector may ask for.

Enzymes and additives, honestly

Emulsifiers break grease into smaller droplets so it passes through the trap. That makes the trap look better and puts the grease into the sewer, which is why many jurisdictions discourage or prohibit them. Eventually it congeals downstream and becomes a much larger problem.

Bacterial and enzyme products have a legitimate role in maintaining drain lines and reducing odour between services. They are not a substitute for pumping a trap, and anybody selling them as one is selling you a deferral.

What it costs

Service	Range
Indoor trap pump-out, 20 to 100 gallons	$150–$450 per visit
Recurring scheduled service, low end	around $125
In-ground interceptor to ~1,000 gallons	$325–$950 per visit
Single-hood cleaning on the same visit	$300–$700
Multi-hood cleaning	$800–$1,500
Rush service	Commonly a 25 to 50 percent premium

Published on the pricing page, and the cost calculator will price a hood service with a trap added.

Doing the trap on the same visit as the hood is the obvious saving, because it is one mobilisation instead of two, and both are usually done after close anyway. Ask whether one contractor can do both on the same visit, and get both prices on one quote.

The records question

Health inspectors ask about grease traps, and what satisfies them is a record: dates, volumes, and where the waste went. Keep the manifests. A kitchen that can produce a year of them has a very short conversation; one that cannot has a longer one.

The same principle applies on the hood side, which the NFPA 96 page covers.

Areas covered

Rapid City, Box Elder, Sturgis, Spearfish, and Summerset.

Frequently Asked Questions

How often does a grease trap need pumping?

The working rule most jurisdictions apply is the 25 percent rule: pump when accumulated grease and solids reach a quarter of the trap's liquid depth. For a busy kitchen with a small indoor trap that is often every two to four weeks; for a larger in-ground interceptor it is typically quarterly. Waiting until it backs up is both a compliance problem and a plumbing bill.

What does a pump-out cost?

$150–$450 per visit for an indoor trap of 20 to 100 gallons, with recurring scheduled service bottoming out around $125. An in-ground interceptor up to roughly 1,000 gallons runs about $325–$950. If one contractor can do it on the same visit as a hood cleaning, that saves a separate mobilisation; ask when you book.

Why does my trap smell even after pumping?

Usually because it was pumped rather than cleaned. A proper service removes the contents, scrapes the walls and baffles, checks the baffles are present and intact, and refills the trap with water. Skipping the wall and baffle work leaves the material that produces the smell, and a missing baffle means the trap is not really trapping anything.

Can I use enzymes or additives instead?

No, and in many jurisdictions emulsifiers are specifically discouraged, because they do not remove grease, they move it downstream into the sewer where it becomes somebody else's problem and eventually yours again. Additives can have a place in maintaining drain lines; they are not a substitute for pumping a trap.