Oil/water separator and sand/oil interceptor field guide

The oil/water separator, and the job it does

An oil/water separator is a tank or vault that holds the oily wastewater from a garage, a wash bay, a fueling island, or an industrial floor drain long enough for the petroleum oil to float to the top and the sand and grit to drop to the bottom, so the water leaving for the sewer is clear of free oil. That is the whole job. Slow the flow, let gravity sort the oil from the water, hold the oil and the grit, and pass clarified water on.

It works because petroleum oil is lighter than water and does not mix with it once the flow calms down. The wastewater comes in carrying suspended oil droplets and washed-off grit. Inside the tank the velocity drops, the oil droplets rise and pool against the top, the heavy sediment settles, and an outlet that draws from the middle of the column takes water from below the oil layer and above the sludge.

What the separator protects is the public sewer or the storm system, not your floor drain. Oil that gets past it does not disappear. It coats the inside of the main, fouls the treatment plant, or runs straight to a creek if the drain was a storm drain, and the authority traces the sheen back to your site. The separator is the device that keeps that violation, and that cleanup bill, from being yours. Where the oily drains tie into the sanitary system, the building sewer that carries them off the site is its own piece of work, covered in the building sewer lateral guide.

What is the difference between an oil/water separator and a grease interceptor?

An oil/water separator handles petroleum: motor oil, hydraulic fluid, fuel, and the grit that washes off vehicles and shop floors. A grease interceptor handles FOG, the fats, oils, and grease that come off a kitchen. They look similar and work on the same idea of letting a lighter-than-water layer float, but they are different devices for different waste, and the plumbing code defines them separately. Do not treat one as a substitute for the other.

The distinction is in the waste, not the appearance. Plumbing code language defines grease as material from animal, vegetable, seed, or nut sources, and it specifically excludes petroleum-based products. So a grease interceptor is sized, built, and permitted for kitchen FOG, while an oil/water separator is sized and built for hydrocarbons and sediment. The internals, the venting, and the discharge limits are set for the waste each is meant to catch.

Put car-wash or shop water into a grease interceptor and you have the wrong device in two ways. It is not rated for the grit load that buries a kitchen unit fast, and it does no good on a discharge permit written for oil, because the authority is sampling for petroleum the grease unit was never built to remove. The grease interceptor guide covers the FOG side in full. The line is simple: food grease goes to the grease interceptor, petroleum oil goes to the oil/water separator.

Where are oil/water separators required?

A separator is required wherever a drain can carry petroleum oil, fuel, or flammable liquid into the sewer. The plumbing code names the usual sources directly: repair garages with floor or trench drains, vehicle and equipment washing, factories where oily or flammable liquid wastes are produced, and hydraulic elevator pits. If a floor drain sits where oil can reach it, that drain gets treated before it leaves.

On a real site the list is longer than the code's short version. Fueling stations and the canopy area around the dispensers, parking garages where the floor drains catch crankcase drips and tire grit, fleet maintenance shops, transit and equipment yards, machine shops with cutting-oil mist on the floor, and generator and transformer yards that have to contain a fuel or oil spill. The common thread is a surface that collects petroleum and a drain under it.

Two authorities usually have a say, and they are not the same office. The plumbing AHJ enforces the installation under the adopted plumbing code. The sewer authority or the environmental agency enforces what you are allowed to discharge, through a pretreatment permit. A separator can pass the plumbing inspection and still be out of compliance on the discharge side, so confirm both before you call it done. Whether a given drain even needs the separator is a call the AHJ makes on the use of the space, so verify it rather than assuming.

The sand/oil interceptor: grit and oil together

A sand/oil interceptor is the garage version of the device, built to settle out heavy sand and sediment as well as separate the oil. Repair shops and wash bays do not just shed oil. They shed grit, road sand, brake dust, and the mud that comes off undercarriages, and that solid load would clog and bury a plain oil separator in a hurry. The sand/oil unit handles both in one tank or in a tank with a dedicated grit chamber ahead of the oil section.

The sequence inside is settle first, then separate. The wastewater enters a sand or sediment chamber where velocity drops far enough for the heavy grit to fall out, then carries on to the oil-separation chamber where the floatables rise and the clarified water leaves. Keeping the grit out of the oil chamber matters, because sediment that piles up under the oil section steals the volume the separation depends on and shortens the time the water sits.

Sizing the sand chamber is a real part of the design, not an afterthought. A busy wash bay can drop a startling volume of sand, and a grit chamber that is too small fills between services and starts passing sediment downstream. The plumbing code gives a minimum interceptor size for garage floor area, but the sand load on a heavy-duty or truck-wash operation can push you well past the minimum, so size for the dirt the bays actually produce and confirm against the AHJ and the manufacturer.

How does an oil/water separator work?

A gravity separator works on the density difference between oil and water and on time. Free oil droplets are lighter than water, so given enough still residence time they rise to the surface, while sand and heavy solids fall. The tank is built to give the water a slow, calm path from inlet to outlet so that rise and that settling can finish before the water leaves. Take away the time, by oversizing the flow or undersizing the tank, and the oil has not finished rising when the water reaches the outlet, so it carries over.

The physics behind it is Stokes' law: the rise rate of an oil droplet depends on how big the droplet is and how far its density sits from water's. Big droplets rise fast. The tiny emulsified droplets, the ones a pressure washer and hot detergent beat into the water, rise so slowly that a plain gravity tank may never separate them in the time available. That limit is exactly what the coalescing types are built to beat.

Three zones do the work. The inlet and a baffle knock down the incoming velocity and turbulence so the flow spreads and slows. The separation zone is the quiet middle where the oil rises and pools at the top and the grit settles to the bottom. The outlet draws clarified water from between those two layers. This is the same family as the classic API gravity separator used in refineries and heavy industry, scaled down for a shop or a wash bay.

The coalescing-plate separator

A coalescing separator packs a stack of closely spaced inclined plates, or a coalescing media, into the separation zone so the small oil droplets have a surface to merge on. As the oily water passes between the plates, droplets stick to the underside of each plate and run together into larger globules, and bigger droplets rise far faster than the small ones did on their own. The result is better oil removal in a smaller tank than a plain gravity unit needs for the same job.

The payoff is the small emulsified droplets that a gravity tank lets slip. By forcing the droplets together, the plate pack pulls the effluent oil down toward the low single-digit milligram-per-liter range that tighter permits now ask for, in a footprint that fits where a big gravity vault would not. That is why coalescing-plate units, often sold as plate-pack or corrugated-plate separators, are the common pick for new vehicle-wash and shop installs working to a strict limit.

The plate pack is also the part that fouls. Grit and sludge cake the plates, biological growth bridges them, and a heavy solids load can blind the pack so the water short-circuits around it and the effluent gets worse than a clean gravity tank would give. So a coalescing unit needs the plate pack pulled and cleaned on a schedule, and it wants grit knocked out ahead of it. The better effluent comes with a maintenance habit attached. Skip it and the advantage disappears.

API or CPI: which separator?

API names the plain gravity separator, the open-tank design that lets free oil rise and solids settle over a long residence time. CPI, the corrugated-plate interceptor, is the coalescing version with the inclined-plate pack. Both rely on the same density separation. The CPI just makes it happen faster and in less space by giving the droplets a surface to merge on, so for the same flow it is smaller and it puts out cleaner water.

Pick the API gravity style when you have the room, when the oil is mostly free and not heavily emulsified, and when a wider effluent limit gives you margin. It is simple, there is little inside to foul, and it tolerates a dirty, variable flow. Its weakness is footprint and the emulsified fines it cannot catch, so a tight permit can push you off it.

Pick the CPI coalescing style when space is short or the permit limit is low enough that you need the small droplets out. It buys performance and a smaller tank and costs you the maintenance the plate pack demands. The honest version is that the choice is set by the effluent limit, the available space, and the solids load, run against what the manufacturer's unit is rated to do. Match the rated performance to the permit number and confirm it with the AHJ, rather than buying on footprint alone.

Inside the separator: the components

Open a separator and the parts map to the job each does. The inlet and an inlet baffle take the incoming flow and knock the velocity and turbulence out of it, because oil will not separate in churning water. A diffusion baffle spreads the flow across the tank so the water moves slowly and evenly through the separation zone instead of cutting a fast channel straight to the outlet.

The separation zone is the quiet middle, and above it sits the oil-storage space where the floated oil collects until it is skimmed or pumped. Many units carry a skimmer, a slotted pipe or a rotating drum, that draws the oil layer off into a holding compartment. Below the separation zone is the sediment or sludge chamber where the settled grit accumulates. On a coalescing unit, the plate pack lives in the separation zone.

The outlet is built to pull clarified water from between the oil cap and the sludge, usually through an outlet baffle or a tee that reaches below the oil layer, so neither the floating oil nor the settled grit can wash over with the effluent. Two more features earn their keep on every install: a vent to carry off flammable vapor, and a downstream sample port where the authority can pull a sample of what you are actually discharging. Both get their own sections below, because both are where installs go wrong.

How do you size an oil/water separator?

You size a separator for the peak flow it has to treat and the residence time the oil needs to rise, then check it against the code minimum and the manufacturer's rating. Too small and the water moves through faster than the oil can separate, so it carries over no matter how clean the install is. The inputs are the flow rate from the fixtures and bays feeding it, the oil and grit load, and the effluent limit you have to hit, because a lower limit demands more time or a coalescing pack.

The plumbing code gives a floor for garage and repair work tied to floor area. A common code minimum sizes the interceptor by the garage floor it serves, with a base volume for the first 100 square feet and an added increment for each additional 100 square feet. Treat that as a minimum, not a design. The exact base and increment vary between the IPC, the UPC, and local amendments, so pull the number from the adopted code rather than from memory.

Flow-based sizing is where the real work is. Add up the contributing fixtures and bays, find the peak flow, and pick a unit whose rated treatment flow meets or beats it at the effluent limit you need. A pressure-washer wash bay and a trench drain catching a hose-down are very different flows, and a unit sized for the quiet case carries over the day the crew really opens the water. Size for the peak, hedge toward the larger unit, and confirm the rating with the manufacturer and the AHJ. The manufacturer's flow rating at a stated oil limit is the number that governs, not a rule of thumb.

What oil limit does the effluent have to meet?

The effluent has to come in under the oil concentration written in your discharge permit, and that number is set by the sewer authority or the environmental agency, not by the separator. Many permits land in the range of 100 mg/L for discharge to a sanitary sewer, while discharges to storm or surface water are far tighter, often in the single digits. There is no single national number to quote, so the permit is the only limit that counts. Read it before you pick the device.

The classic design basis for a gravity (API) separator targets removal of free oil droplets larger than about 150 microns at a specific gravity around 0.85, which a well-built gravity unit holds to roughly the 100 mg/L neighborhood. A coalescing-plate (CPI) unit pushes the effluent far lower, into the rough range of 10 mg/L, by catching the finer droplets down to about 20 to 60 microns that a gravity tank lets pass. That gap is the whole reason to spend on coalescing when the permit is strict. Dissolved or emulsified oil needs chemistry or membranes, not gravity.

Two things wreck a separator's effluent even when it is sized right. Emulsified oil, beaten into the water by hot high-pressure washing and detergents, will not separate by gravity at all, and a soap-heavy wash stream can need treatment beyond a plain separator to meet a low limit. And a separator that is full of accumulated oil and sludge stops working long before it looks full, because the oil layer thickens down into the outlet path and the water carries it over. The number on the permit is met by the device plus the maintenance, never the device alone.

Venting the separator for flammable vapor

An oil/water separator that can receive fuel or flammable liquid has to be vented independently to outside air, because the vapor coming off that waste is flammable and it has to go somewhere safe instead of building up in the tank or backing into the building. The plumbing code treats the separation or vapor compartment as needing its own vent to the outer air, separate from the ordinary sanitary venting, sized and run so the vapor clears the tank.

Run the vapor vent on its own, up and out, terminating in open air away from intakes and ignition sources, and do not tie it into the regular sanitary vent stack where flammable vapor could mix with the building's drainage venting. Where a unit has more than one vapor compartment, each is vented to outside air or joined to a header set above the spill line of the lowest floor drain and then carried out, so liquid cannot rise into the vent and vapor still has a clear path.

Confirm the vent size against the adopted code and the manufacturer, since the required diameter for a flammable-waste vapor vent is set there and differs from a standard plumbing vent. The point a rookie misses is that this vent is a safety device, not a plumbing nicety. It is there so vapor cannot accumulate to a level where a spark in a garage full of fuel finds it. Treat it that way.

The fuel and vapor hazard

A separator on a fueling or shop drain is holding flammable liquid and giving off flammable vapor, so it is a fire and explosion hazard, not just a wastewater fitting. Gasoline and solvent vapor are heavier than air and pool low, exactly where floor drains and tank openings are, and it takes very little vapor in the right mix to ignite. A spark from a grinder, a hot work permit nobody pulled, or a dropped trouble light near an open separator is all it takes.

Build and treat the system as flammable-waste handling. The vapor vent has to be independent and carried to safe air, the tank and covers have to seal, and ignition sources have to stay clear of the openings. Confined-space and hot-work rules apply hard at the separator. Opening a vault that has been holding fuel waste and climbing in, or running a torch over an open tank, is how people get killed on this equipment, and it is a written procedure for a reason.

This is the part of the guide to be blunt about. If the drain can carry fuel, the separator and its venting are life-safety, and the flammable-waste provisions of the plumbing code and the fire code both apply. Coordinate with the fire AHJ as well as the plumbing AHJ, because both have jurisdiction here. Skip the vapor vent or seal off the tank wrong and the failure mode is not a backed-up drain. It is a fire.

Keeping the oily drains on their own system

The oily and flammable-waste drains have to run on their own piping to the separator, kept separate from the sanitary drainage until they have been treated, and kept out of the storm system entirely. A shop sink, a toilet, or any ordinary sanitary fixture does not tie into the oily-waste line ahead of the separator, and the oily floor drains do not tie into the clean sanitary or the storm drain. They meet only downstream of the separator, where the code allows it.

There is a real reason for the separation beyond tidiness. Mixing sanitary flow into the oily-waste line floods the separator with water it was not sized for and shortens the residence time the oil needs. Worse, routing flammable waste through the regular sanitary system spreads flammable vapor through piping and venting that was never built to contain it. And an oily drain cross-connected to a storm drain sends raw oil straight to a creek, which is the violation that brings the environmental agency to the door.

Lay it out so the oily-waste branch is its own clearly identified system from the floor drains to the separator, then to the treated discharge point. Where the treated effluent ties into the building sewer that leaves the site, that connection follows the building sewer rules. The two systems stay apart until the oil is out.

What does the pretreatment permit require?

Most sites discharging oily wastewater need a pretreatment permit from the local sewer authority, issued under the broader environmental pretreatment framework, and that permit sets the oil and sediment limits, the sampling, and the records you have to keep. The separator is the equipment that lets you meet the permit. The permit is the obligation. Pull it and read its actual numbers before you size or install anything, because the limit drives the device.

The permit typically spells out the maximum oil concentration in the discharge, sometimes limits on other parameters like total suspended solids or pH, a requirement to install and maintain an approved separator, a sample point the authority can use, and a schedule for monitoring and reporting. Some authorities sample for you. Some require you to sample and report on a cadence. The exact limits, methods, and reporting differ by jurisdiction, so the local sewer-use ordinance and the permit are the only authorities to cite.

Enforcement has teeth. The authority can sample at your port, fine you for being over the oil limit, fine you for missing maintenance or disposal records, and in a serious case suspend the discharge permit, which can stop the operation. Discharging to a storm system instead of the sanitary sewer is a separate and usually harsher regime under stormwater rules. Confirm with the AHJ which system you are even allowed to discharge to, because the answer changes the whole compliance picture.

The sample port

A sample port is an accessible point downstream of the separator where the authority, or you, can pull a sample of the treated effluent to check it against the permit limit. Many permits require it, and it has to be where someone can actually reach it and get a representative sample of what is leaving, not a spot buried under a parking slab or upstream of the separator where the reading means nothing.

Put it on the discharge side of the separator, before the effluent mixes with any other flow, in a structure or a cleanout-style access a person can open and sample without heavy equipment. A port placed where the sample is diluted by other drainage, or placed ahead of the treatment, gives a number that does not represent the discharge and can read either falsely clean or falsely dirty. The inspector and the sampling crew both check that the port is genuinely representative and reachable.

How often do you pump an oil/water separator?

You pump a separator on a schedule and on a trigger, before the accumulated oil and sludge reach the point where the unit carries over. The schedule is whatever your discharge volume and oil load demand, ranging from monthly on a heavy wash operation to a minimum of about once a year on a light one. The trigger overrides the schedule: pump when the oil layer or the sludge gets deep enough to threaten the separation, even if the calendar says you have time.

Common operating triggers used in the field are sludge built up to roughly 25 percent of the separator's wetted depth, an oil layer near 5 percent of that depth, or the unit at about 75 percent of its total holding capacity. Whichever comes first is the call to service it. The reason for pumping early is mechanical, not cosmetic. As the oil layer thickens it reaches down toward the outlet, and as the sludge piles up it steals the volume the separation needs, so a unit that is not yet visibly full is already letting oil over.

This is the single biggest ongoing requirement, and the most common failure, because a separator is out of sight and easy to forget until it shows up as oil in the sewer. Pull the oil and the sludge with a vacuum truck, clean the plate pack on a coalescing unit, and write down the date, the volumes removed, and where it went. The records are not paperwork for its own sake. They are what you hand the authority to prove the separator is doing its job, and a missing record is its own violation. Do not let it carry over.

Disposing of the recovered oil

The oil and sludge pulled out of a separator are a regulated waste, often a hazardous waste depending on what is in it, and they have to go to a licensed handler with the manifest that proves the chain of custody. Recovered used oil from a clean shop stream may go through a used-oil recycler, while contaminated sludge or solvent-laden waste can be a hazardous waste with stricter handling. The classification depends on the waste, so it is a determination the disposal contractor and the regulations make, not an assumption.

Keep the manifest and the disposal records with the maintenance log. The authority that polices your discharge also wants proof that what came out of the separator was disposed of legally and did not just get poured down another drain or dumped. Use a licensed hauler, get the signed manifest for each pump-out, and file it. A separator with clean effluent and no disposal paper trail is still a compliance problem waiting to surface.

Indoor point-of-use versus in-ground vault

Separators come as indoor point-of-use units and as outdoor in-ground vaults, and the pick follows the flow, the space, and where a pump truck can reach. A point-of-use unit sits indoors near or under the floor drain it serves, often a compact coalescing box for a single bay or a small shop, and it is serviced from inside. It fits where there is no yard to bury a tank and it treats close to the source.

An in-ground vault, usually precast concrete or a large fiberglass tank, sits outdoors and handles the higher flows from multiple bays, a fueling island, or a wash line, with the volume for longer residence time and more oil and sludge storage between services. It needs a spot a vacuum truck can reach for pumping, and it has to be rated for the traffic load if it sits under a drive or a lot.

The trade-offs are straightforward. A small point-of-use unit holds less, so it is serviced more often and carries over sooner if it is missed. A big buried vault holds more and rides longer between pump-outs, but it costs more, needs the outdoor room and truck access, and is the easier one to forget precisely because it is out of sight. Size and place it for the flow and the service access, not just for what fits on the plan.

Fueling islands, parking garages, and generator yards

Fueling stations are the sharpest case, because the drainage under the dispensers and the canopy catches fuel spills and the device has to handle flammable liquid as its normal duty. The fuel-island and canopy drainage runs to a separator built and vented for flammable waste, and a fast fuel spill is a slug load the system has to take without passing fuel through. This is where the vapor venting and the fire-code coordination are not optional.

Parking garages drain crankcase drips, washdown, and a heavy load of tire and brake grit, so the floor and trench drains run to a sand/oil interceptor that settles the dirt and skims the oil. The grit load is what drives service here, since a garage drain produces sediment faster than oil. Maintenance shops and equipment-wash bays are the same pattern with more oil in the mix.

Generator and transformer yards, including the fuel storage at data centers, use a separator as spill containment. The yard drainage is normally clean rainwater, but it has to be ready to catch a diesel or transformer-oil spill and keep it out of the storm system, often with a valve or an arrangement that can hold a spill rather than discharge it. The design intent there is containment of an accidental release, so coordinate it with the spill-prevention plan for the site as well as the plumbing and fire AHJ.

Stormwater oil-grit separators

A stormwater oil-grit separator is a structure on a storm drain, common in parking lots and at vehicle areas, built to catch the sheen of oil and the road grit that washes off pavement in a rain event before it reaches a creek or a storm outfall. It is a water-quality device, sized for a storm flow rather than a steady process flow, and it lives under a different set of rules than a sanitary-sewer separator.

Treat it as its own regime. Stormwater discharges to surface water are governed by stormwater permitting and the local water-quality authority, the limits are tight, and the maintenance still comes down to pumping out the captured oil and sediment before a storm resuspends and flushes them through. A stormwater oil-grit unit that is never cleaned does worse than nothing, because the next big rain washes the accumulated load straight out the outfall.

Common failures in the field

The failures cluster, and they are almost all avoidable. The most common by far is no maintenance: the unit fills with oil and sludge, the oil layer reaches the outlet, and it carries petroleum straight to the sewer while looking fine from the lid. Undersizing is next, where a unit sized for a quiet flow carries over the day the bays really run, because the residence time collapsed under the peak.

The wrong device shows up often too, usually a grease interceptor put on a petroleum drain because someone treated the two as interchangeable. It does not meet an oil permit and it loads up with grit it was never built for. Emulsified oil from hot, soap-heavy washing defeats a plain gravity unit that has no coalescing or no upstream treatment, so the effluent fails even when the tank is clean and correctly sized.

Then there are the dangerous ones. No vapor vent, or a vapor vent tied into the sanitary stack, lets flammable vapor build, which is a fire risk, not just a code miss. Oily drains cross-connected to the sanitary ahead of treatment, or worse to the storm drain, send untreated oil where it does not belong. And a missing sample port or no permit means there is no way to prove compliance even when the water is clean. None of these are exotic. They are the same handful, over and over.

What the inspector checks

The plumbing inspector starts with whether the right device is on the right drain, sized to at least the code minimum for the use, and whether the oily-waste system is kept separate from the sanitary and the storm until the waste is treated. They check that the separator is the type and rating the application calls for, not a grease unit standing in for an oil separator.

Then the venting, because that is the safety item. The inspector confirms the vapor compartment is vented independently to outside air, sized per the code and the manufacturer, and not tied into the ordinary sanitary venting where flammable vapor could spread. On a fueling or flammable-waste install the fire AHJ may inspect alongside, looking at the flammable-waste handling as a fire-code matter.

Last comes the compliance side: a sample port that is downstream, reachable, and representative; the discharge permit in hand; and access for service so the unit can actually be pumped. An inspector who has seen a few of these also looks for the maintenance and disposal records, because a separator with no service history is one that will fail quietly, and the records are the proof that it will not.

What to document

Clean discharge proves nothing on its own, and without records a separator running clear is still a unit nobody can show was sized for the duty or serviced on schedule. The records are what you hand the authority to prove the device is the right one, that it is being serviced, and that the oil is being disposed of legally. Capture the install details once and the service details every time.

Record the device type and rated flow, the permit and its oil limit, the sizing basis, the sample-port location and any effluent results, the pump-out dates with the oil and sludge volumes removed, where the waste went, and the signed disposal manifest for each haul. If the unit is coalescing, note the plate-pack cleaning. Tie each entry to a date and a name, because a record only holds up later if someone can point to who did the work and when.

Common mistakes

• Putting a grease interceptor on a petroleum drain, or routing kitchen FOG to an oil separator: different devices for different waste.
• Undersizing the unit so the peak flow outruns the residence time and the oil carries over.
• Letting it go without maintenance until oil and sludge reach the outlet and the unit passes petroleum to the sewer.
• Omitting the flammable-vapor vent, or tying it into the sanitary stack instead of running it independently to outside air.
• Cross-connecting the oily drains to the sanitary ahead of treatment, or to the storm drain at all.
• Skipping the coalescing pack or upstream treatment on a hot, soap-heavy wash stream that emulsifies the oil.
• Installing with no sample port or no discharge permit, so compliance cannot be proven even when the water is clean.
• Hauling the recovered oil with no licensed hauler and no signed disposal manifest.

Field checklist

Standards and references

The plumbing code is where the installation rules live. Both the International Plumbing Code and the Uniform Plumbing Code cover traps, interceptors, and separators in their Chapter 10, naming where oil and flammable-waste separators are required, the minimum interceptor sizing for garage floor area, and the requirement to vent the separation or vapor compartment independently to outside air. The exact section numbers, the sizing base and increment, and the vent diameter differ between the two codes and shift between editions, so confirm them against the edition the jurisdiction has actually adopted and any local amendments.

The discharge side runs under a different authority. The local sewer authority enforces a pretreatment permit under the broader environmental pretreatment framework and the local sewer-use ordinance, and that is what sets the effluent oil limit, the sampling, and the reporting. Discharges to a storm system instead fall under stormwater rules and the water-quality authority. For separator design and performance, the API gravity-separator design basis and the corrugated-plate (CPI) coalescing approach are the recognized references, with effluent expectations stated against droplet size and specific gravity.

Two more bodies touch the work. The flammable-waste and fire provisions of the fire code apply where fuel or flammable liquid reaches the separator, so the fire AHJ shares jurisdiction. And the manufacturer's listing and instructions set the rated treatment flow, the effluent at that flow, and the service intervals, which govern alongside the code. Cite the standard that controls the point, hedge the sizing and the limits to the AHJ and the manufacturer, and let the permit number override any rule of thumb. The grease interceptor guide covers the FOG side, and the building sewer lateral guide covers the pipe that carries the treated discharge off the site.

Units, terms, and conversions

The same device and the same limits go by several names across a drawing set, a permit, and a manufacturer sheet, so the vocabulary is worth pinning down.

An oil/water separator is also called an oil separator, an oil interceptor, or by its design family as an API separator or a CPI plate-pack separator. A sand/oil interceptor is the grit-plus-oil garage version, sometimes called a gas and oil interceptor in older code language. Effluent oil is given in milligrams per liter (mg/L), which is parts per million by mass in water. Tank size is in gallons in US sources and may appear in cubic feet in code sizing tables, where 1 cubic foot is about 7.5 gallons. Flow is in gallons per minute (gpm). Oil droplet size is in microns, and specific gravity compares the oil's density to water at 1.0.

Oil/water separator

Tank that floats petroleum oil and settles grit out of wastewater before discharge; also oil separator or oil interceptor

Sand/oil interceptor

Garage device that settles sand and sediment and separates petroleum oil from floor-drain wastewater

API separator

Plain gravity oil/water separator using residence time to float free oil

CPI / coalescing plate

Corrugated-plate separator whose inclined plates merge small oil droplets for better effluent in a smaller tank

Effluent oil limit

Maximum oil concentration in the discharge, in mg/L, set by the pretreatment permit

Pretreatment permit

Authorization from the sewer authority setting the oil and sediment limits, sampling, and records for the discharge

Vapor vent

Independent vent carrying flammable vapor from the separator to outside air, separate from sanitary venting

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