If you work around fuel storage, refineries, or vehicle maintenance facilities, you’ve probably heard the term “API tank” thrown around — usually in the context of keeping oily wastewater out of the sewer system.
It’s a specific piece of equipment with a specific job, and understanding what it does (and doesn’t do) makes a big difference when you’re specifying one for a project.
What Is an API Tank?
An API tank — more precisely called an API oil-water separator — is a tank designed to remove free oil and suspended solids from industrial wastewater before that water is discharged or sent on for further treatment. It works by gravity separation: oil rises to the top, solids settle to the bottom, and the cleaner water sits in the middle layer between them.
The “API” in the name comes from the American Petroleum Institute, which publishes the design standards these separators are built around. That’s why you’ll see them at refineries, petrochemical plants, natural gas facilities, fueling terminals, and vehicle maintenance shops — anywhere oily water needs to be treated before it goes anywhere else.
Quick note on terminology: “API tank” is sometimes used loosely for large aboveground storage tanks built to a different API standard, API 650, which governs welded tanks for storing crude oil and other liquids, not treating wastewater. An API 650 tank is a welded steel storage vessel built to the American Petroleum Institute’s specification for oil, chemical, and water storage. This post focuses on the separator — the version most people searching this term actually mean, and the type Tex Tanks builds. You can see our API tank fabrication work here if that’s what you’re after.
Types of API Tanks
Not every API separator is built the same way. The right type depends on flow rate, oil droplet size, and how much space you have.
Conventional API Separators
These are gravity separators built to American Petroleum Institute guidelines, and they work by giving wastewater enough retention time in a quiet zone for oil to float and solids to settle. They’re the standard choice for higher-flow applications.
Parallel-Plate (CPI) Separators
Parallel-plate separators use closely spaced plates to shorten the distance oil droplets need to travel to separate out, which lets them treat more flow in a smaller footprint than a conventional separator. These are a good fit when space is tight.
Material Options
The shell material matters as much as the design. Depending on what’s in the wastewater, an API tank might be built from carbon steel or from stainless steel grades like 304 or 316 for better corrosion resistance. Tex Tanks fabricates API tanks in a full range of steel and alloy options to match the chemistry of the specific waste stream.
What Does an API Tank Include?
A basic API separator has a handful of core parts working together:
- Inlet baffle – slows and evens out incoming flow so it doesn’t disturb the separation process
- Separation (quiescent) chamber – the main body where oil rises and solids settle
- Oil skimmer – removes the floating oil layer
- Sludge hopper or sump – collects settled solids at the bottom
- Outlet weir – lets the treated water exit near the bottom, below the oil layer
Some units also add coalescing or parallel plates inside the separation chamber to increase surface area and speed up oil removal without increasing the tank’s footprint.
Where Are API Tanks Used?
API tanks show up anywhere petroleum products meet water. Common settings include:
- Petroleum refining and terminal/pipeline operations
- Fueling stations and bulk fuel storage sites
- Vehicle and equipment maintenance washing
- Chemical processing plants
- Power generation facilities
- Site remediation projects
If your operation falls into any of these categories, there’s a good chance local or federal regulations require some form of oil-water pretreatment before discharge — which is exactly what these tanks are built for.
How Often Should an API Tank Be Inspected and Cleaned?
This depends on your local discharge permit and how much flow the tank handles, but a few benchmarks show up consistently in municipal and federal guidance:
- Visual inspections are typically expected on a routine basis, and some federal air-quality rules specifically require quarterly checks of separator covers and seals to catch cracks or gaps.
- Cleaning thresholds: one municipal maintenance standard recommends pumping the tank out once the sludge layer reaches about 25% of the wetted height, the oil layer reaches about 5% of the wetted height, or the tank reaches 75% of its retention capacity — whichever comes first.
- Minimum cleaning frequency: a full cleanout at least once a year is a common baseline requirement, even if none of those thresholds are hit sooner.
- Keep records. Facilities covered under federal spill-prevention rules are expected to document inspection and maintenance activity for their oil-water separators, which matters if you’re ever audited.
The actual schedule for your site should come from your discharge permit, your manufacturer’s recommendations, and how much oily water you’re actually pushing through the tank day to day.
What Does an API Tank Cost?
There’s no single number here — cost depends on several factors that shift the price up or down:
- Size and flow capacity – bigger tanks built for higher flow rates cost more
- Material – carbon steel is generally more affordable than stainless steel or specialty alloys, but the latter last longer in corrosive conditions
- Aboveground vs. underground installation – underground units typically involve more site prep and excavation
- Internal components – coalescing plates, skimmers, and automated monitoring add to the price
- Permitting and site work – local requirements can add engineering or installation costs
Because these variables shift so much from site to site, the most reliable way to get an accurate number is to request a quote through our tanks page based on your specific flow rate and site conditions.
How Long Does an API Tank Last?
Lifespan comes down to a few practical factors:
- Material – stainless steel and specialty alloys resist corrosion far better than carbon steel, especially with aggressive or acidic waste streams
- Coatings and corrosion protection – properly coated carbon steel tanks last significantly longer than uncoated ones
- Maintenance consistency – tanks that get cleaned and inspected on schedule avoid the internal buildup and corrosion that shorten service life
- Waste stream characteristics – wastewater pH, temperature, and chemical makeup all affect how hard the tank has to work and how fast it wears
A well-built, well-maintained API tank in the right material for its application can run for decades. A neglected one, regardless of material, will need repairs or replacement much sooner.
Is It Worth Installing an API Tank?
For most facilities that generate oily wastewater, yes — it’s less a question of “is it worth it” and more “is it required.” Here’s what an API tank does for you:
- Keeps you compliant. Many facilities with oil-water separators are subject to federal spill-prevention regulations that require proper design, capacity, and maintenance documentation.
- Avoids fines and shutdowns. Skipping inspections or letting sludge and oil build up is one of the most common causes of permit violations at industrial sites.
- Protects downstream systems. Treating oily water before it reaches the sewer or further treatment stages prevents damage and clogging further down the line.
- Recovers usable oil. Skimmed oil can often be reclaimed rather than lost entirely.
One caveat worth knowing: API separators are designed to remove free oil, not oil that’s already dissolved or emulsified into the water. If your waste stream includes detergents, solvents, or heavy emulsification, you may need a secondary treatment step alongside the separator — worth discussing with your tank provider before you finalize a design.
If your site handles fueling, vehicle maintenance, refining, or any process that puts oil into your wastewater, an API tank isn’t optional so much as it is the baseline. Take a look at our full lineup of tank types to see how an API separator fits alongside your other storage and treatment needs, or reach out for a quote specific to your site.