Oil Water Separator

What an Oil Water Separator Is and What Is Being Decided

The gravity oil water separator, also written oily water separator when the stream is mostly oil-contaminated water, is a passive vessel: no moving separation mechanism, no chemicals, and no membranes in its basic form. The trade names describe shape history more than physics. API separator refers to the rectangular through-flow basin descended from the American Petroleum Institute design basis now published as API 421; coalescing plate separator and CPI, corrugated plate interceptor, refer to the same vessel family with inclined plate packs installed in it; TPI, tilted plate interceptor, is the parallel-plate ancestor. All of them do one thing: hold the water still enough, and give it enough projected settling area, that free oil droplets rise out of the flowing layer into a skimmed surface zone, while settleable solids drop to a sludge floor.

Two boundaries must be drawn in the first paragraphs because search traffic and requisition systems blur them. First, this is not the air-side product: a coalescing filter removes submicron oil aerosol from compressed air and gas streams inside a pressure vessel, and an industrial filter houses media for process liquids at pressure; neither separates free oil from a wastewater stream by gravity, and neither satisfies a discharge permit. Second, this page is not an instrument page: the oil in water analyzer measures the oil the separator leaves behind, at the outlet or in the effluent line, and that instrument has its own encyclopedia entry. The separator is the tank between them, bought from tank fabricators and water treatment suppliers, specified by flow and oil gravity and effluent target rather than by detection limits.

What is actually being decided in this purchase is a set of coupled commitments made in one order. The hydraulics commitment fixes peak flow and how it enters: a stormwater unit sized for a cloudburst is a different tank from a continuous process unit sized for a steady 20 m3/h stream. The separability commitment fixes the oil itself: free oil with a specific gravity of 0.85 to 0.99 separates by gravity, and a detergent-stabilized emulsion does not separate at any tank size, so the influent characterization decides whether the whole project is viable before any vendor is contacted. The construction commitment fixes API basin or plate pack, aboveground steel or underground double wall, coatings, heaters, and covers. The operations commitment fixes skimming method, sludge handling, and instrumentation. A quotation that answers only liters per minute has answered almost none of this.

The station around the tank matters as much as the tank. Upstream, drain lines bring solids and trash with the oil, so a forebay with a trash rack and a grit removal step, often a small cyclone separator or a settlement sump, protects the separation zone. Downstream, the effluent may flow by gravity to a municipal sewer, be pumped onward with a transfer pump, or pass a monitoring point where discharge permits are demonstrated with a flow meter and an analyzer. Around it, isolation valves, sample connections, and level hardware make the unit operable. The separator purchase is therefore a package decision, and the RFQ chapter treats it as one.

Five reference frames organize the datasheet. API 421 is the design basis for gravity separators in refinery and general oily water service and anchors the rectangular basin geometry most fabricators build to. In the United States, 40 CFR 112, the SPCC rule, decides when a facility must provide secondary containment and controlled drainage for oil, and the separator is often the control point that rule creates. Tank construction standards anchor the vessel itself: UL 2215 for aboveground oil water separators, UL 58 for underground steel tanks, and ULC-S656 as the Canadian secondary containment basis, with STI ACT-100 and UCL class coating systems for buried steel. On ships and offshore units, IMO resolution MEPC.107(49) defines the 15 ppm bilge separator with alarm and automatic stopping arrangement. Coating systems for the interior are qualified under ASTM practice. A datasheet that quotes none of these frames is a price tag, not a specification.

How Gravity Separation Works, and Where It Stops Working

The sizing physics is a single equation a buyer should be able to recite: Stokes law for the terminal rise velocity of a small sphere, in words, rise velocity equals gravitational acceleration times droplet diameter squared times the density difference between water and oil, divided by eighteen times the fluid viscosity. Worked at midrange numbers: a 100 micron droplet of 0.90 specific gravity oil in 20 deg C water, density difference about 100 kg/m3 and viscosity about 0.001 Pa s, rises at roughly 0.5 mm per second, on the order of 2 m per hour. That single number explains the entire product category: separation is slow, so the tank must be large, and every design trick in chapter 3 is a way to buy settling area without buying basin volume.

The square-law dependence on diameter is the second thing to internalize. Halve the droplet to 50 micron and its rise velocity falls to a quarter; at 20 micron, a common wash-bay detergent droplet, it falls to a twenty-fifth, and the droplet that needed one hour now needs a day. This is why gravity designs state a design droplet size honestly: the API basis removes droplets of 60 to 150 micron and larger, and plate packs push the practical floor to 20 to 60 micron. Below that floor, in the truly emulsified and dissolved range, no realistic residence time or plate area succeeds, and pretending otherwise in a datasheet is the most expensive lie in this product category.

Flow must remain laminar for the arithmetic to mean anything. Design practice keeps the Reynolds number of the horizontal flow below about 500 in the separation zone, which in a basin of practical depth translates to bulk forward velocities of a few centimeters per second. Every disruption matters: an inlet that jets across the basin shortcuts the residence time, a baffle that sheds eddies re-entrains freshly risen oil, and a downstream pump that sprays the mixed layer undoes hours of settling in seconds. The API 421 geometry exists to tame exactly these effects: an entrance forebay with a trash rack to kill the inlet velocity, a long separation section with a depth to width ratio near 0.3 to 0.5 and a length to width ratio above 5, an oil retention baffle ahead of the outlet, and an effluent takeoff that draws from below the floating layer.

Temperature enters through viscosity. Water viscosity falls from about 1.8 mPa s at 0 deg C to about 0.65 mPa s at 40 deg C, so warm effluent separates visibly faster than cold, and a separator sized on winter water needs more volume than the same duty in summer. The oil side matters too: heavy fuel oil and lubricating oils near 0.95 to 0.99 specific gravity rise slowly at any temperature, which is why those services fit heaters that thin the oil and widen the density difference. Where the climate freezes, outdoor units need insulation and heat tracing on the water path and, frequently, an immersed heater in the oil layer so the skimmed product stays pumpable.

Now the limits, stated bluntly. Gravity separates free oil, meaning droplets that exist as discrete oil in water and have not been chemically stabilized. It does not separate dissolved oil, which passes any tank untouched, and it does not separate stable emulsions, whose droplet surfaces carry surfactant charges that keep them dispersed for days. Emulsions are created upstream by detergents, by high-shear pumping, and by process chemicals, so the influent survey must test for emulsion content before the tank is sized: a simple jar test at the drain, standing a sample for 30 to 60 minutes, shows what fraction of the oil rises and what fraction stays cloudy. Where the cloudy fraction dominates, the honest process adds coagulant dosing, dissolved air flotation, or membrane treatment downstream, and the gravity tank becomes the pretreatment stage of a train rather than the whole solution.

Upstream shear is a purchase-relevant failure path even with free oil. A high-speed centrifugal pump ahead of the separator, a static mixer, or a throttling valve across the feed line can shear 100 micron droplets into 20 micron droplets in one pass, silently converting a separable stream into one that partly isn't. Feed arrangements should therefore favor gravity drain to the tank where the site allows it, and where pumping is unavoidable, a low-speed, large-bore transfer arrangement on a straight run. The same logic applies at the outlet: skimmed oil pulled too hard by a positive displacement pump on a fixed timer can emulsify the layer and pump water with the oil, defeating the mass balance the tank was bought to achieve.

Types, Construction, and Duty Boundaries

The API basin is the ancestor and still the answer for large, continuous, high-solids streams. In plan it is a long rectangular steel or concrete channel divided along its length: a forebay with a trash rack and sand-settling pocket, a separation bay sized on the Stokes arithmetic of chapter 2, and an outlet bay behind the oil retention baffle, with the effluent leaving over a weir or through a slot set below the water surface. Free oil rises to the surface layer, where it is removed by a rotatable slotted skim pipe or a belt or drum skimmer; solids settle to floor hoppers or a scraper-pulled trench. Because everything inside is accessible and nothing fouls catastrophically, the API basin tolerates sour water carryover, waxy crudes, and heavy solids loads that would blind a plate pack in weeks, at the cost of the largest footprint in the category.

The corrugated plate pack interceptor shrinks that footprint by multiplying settling area. Standard packs are corrugated sheets on 25 to 40 mm spacing, inclined at 45 to 60 degrees from horizontal, installed in the separation zone of a much shorter tank. Oil droplets need only rise the gap between two adjacent plates before corrugations carry them up the incline to coalesce at the crest and break free as large droplets into the surface layer; solids slide down the corrugations to the sludge zone. Effective settling area climbs by a factor of 5 to 10 over an open basin of the same plan, which is the entire commercial proposition. The price is cleanability: the packs must come out for cleaning on heavy oil and biofilm service, so the purchase includes lifting access, davits or rails, and spare pack modules when the stream is dirty.

Vessel construction follows the installation. Aboveground rectangular steel units, shop-fabricated in the Highland Tank class of product, ship with integral sludge hoppers, skim pipe or belt, ladder and platform access, and optionally an insulated and heated enclosure; they list to UL 2215 where the authority having jurisdiction demands a listed separator. Underground units are double-wall fiberglass or coated steel tanks, the steel variants built to UL 58 with ACT-100 or ULC class exterior coating systems, fitted with coalescing packs, access risers, and leak monitoring between the walls; they trade maintenance access for zero surface footprint and are routinely traffic rated under pavement with the appropriate load frame. Rectangular shallow aboveground boxes also serve as retrofit inserts into existing concrete basins on plants that cannot pour new ones.

The marine bilge separator is a distinct type with its own rules and belongs here as a type, not a separate product family. Onboard oily bilge water must be processed through equipment certified to IMO resolution MEPC.107(49): a 15 ppm bilge oil water separator, meaning a gravity and coalescing stage, followed by a filter or coalescer stage, with a 15 ppm oil content monitor, alarm, and an automatic stopping arrangement that prevents discharge overboard when the limit is exceeded. The certification is type-approval of the package, and buying marine means buying the certificate with it; a shop-built gravity tank, however well made, cannot be certified after the fact. Industrial buyers on land sometimes specify the same 15 ppm hardware for engine rooms and compressor stations because the package solves monitoring and shutoff in one skid.

Duty boundaries keep the family honest. A gravity separator is not a grease interceptor, whose animal fats and soaps need their own baffle geometry and maintenance rhythm. It is not a dissolved air flotation cell, which uses chemistry and air to pull emulsions out, and it is not a membrane or chemical treatment system for dissolved hydrocarbons. It is also not a spill containment device: the SPCC plan may reference it, but containment volume for a tank failure is a separate calculation under 40 CFR 112. Within its boundary, free oil and settleable solids from oil-bearing water, it remains the lowest-energy, lowest-maintenance unit operation in the plant, which is exactly why it is the default first stage of nearly every oily water train ever built. The type table below compresses the family into the form a specification meeting argues from; values are common spans of the established families and must be confirmed against the model datasheet before order.

Key Specification Parameters

The table below decodes the datasheet lines that decide whether a quoted separator matches the duty; values are stated as common spans of the established families and must be confirmed against the model datasheet before order.

The flow and droplet rows deserve a paragraph together because they are where quotations become incomparable when left loose. A capacity quoted without the design droplet size means nothing: the same tank passes 10 mg/L when credited with 150 micron droplets and fails badly when the real stream delivers 40 micron droplets after detergent washdown. Demand the sizing basis in writing, flow, design droplet, oil gravity, and temperature, and re-run the Stokes arithmetic of chapter 2 yourself; a fabricator who will not state the basis has sized the tank to a rule of thumb that your permit does not recognize. The interface between design and operation is instrumentation: the oil-water interface is controlled or at least alarmed by a level transmitter reading the surface oil layer, and effluent compliance is demonstrated by a flow meter on the outlet combined with sampling or an online analyzer.

Around the separation stages sit pretreatment and polishing decisions that the RFQ should allocate explicitly, because the gravity tank cannot do them. Heavy grit upstream shortens sludge intervals badly and belongs in a settlement pocket or a small cyclone separator on large installations. Downstream, where the permit runs tighter than 10 mg/L or targets emulsified and dissolved organics, the train continues: cartridge or media filtration for residual suspended oil, a bag filter or a self-cleaning filter on the polishing loop, and chemical or membrane stages beyond this page's scope. Monitoring closes the train: a turbidity meter gives a cheap daily trend, an oil in water analyzer gives the permit-grade number, and a TOC analyzer reads the emulsified and dissolved organic fraction that gravity never touched, which is the honest way to know whether the chemistry stage is needed.

The comparison table below compresses the two core constructions into the form a project engineer can argue from in a specification review.

What the datasheet must state, in plain terms: design flow on a stated basis, peak and continuous; influent oil specific gravity and an emulsion statement; design droplet size; predicted effluent in mg/L at those conditions; geometry ratios or pack specification with material; vessel standard and coating system inside and out; sludge volume and cleanout access; skimmer type; heater and freeze protection scope where the climate requires it; and the instrument list with alarm logic. A quotation without the sizing basis, droplet, gravity, and temperature, is incomplete at any price, and that omission is the most common one on the market.

Selection Decision Factors and RFQ Language

The selection sequence runs as a checklist, and each line eliminates a class of quotations before price comparison begins.

1. Characterize the influent before contacting anyone: flow profile, oil specific gravity, emulsion fraction from a jar test, solids load, temperature range, and the detergent and chemical inventory upstream that could stabilize emulsions.
2. Fix the effluent target from the actual permit or sewer ordinance, in mg/L and measurement method, because the target decides whether gravity alone can close the case or a polishing train is part of the project.
3. Choose the hydraulics honestly: stormwater units size on a stated storm event and first-flush capture, process units on continuous flow plus upset margin, and mixing the two bases is the classic sizing error.
4. Choose the construction: API basin for large continuous flows, heavy solids, and full access; plate pack where footprint rules; listed aboveground or double-wall underground by the site and the authority having jurisdiction.
5. Specify the skimming and heating to the oil: light oils skim with a pipe, waxy and heavy oils want a belt or drum and a heated layer to stay pumpable.
6. Allocate pretreatment and polishing explicitly: grit removal upstream, filtration or chemistry downstream, and the instruments that prove the numbers, so no stage is silently assumed into someone else's scope.
7. Fix the compliance package: SPCC reference where applicable, vessel listing required, coating qualification, and freeze protection for the climate.
8. Fix the evidence: sizing basis in writing, effluent prediction at stated conditions, fabrication and coating standards, and the cleanout and sludge disposal procedure the plant can actually operate.

Against neighboring equipment the gravity separator holds specific ground, and the comparison belongs here in the selection chapter. On the gas side, the coalescing filter and the pressure-rated industrial filter remove oil aerosol from compressed air, a different phase and a different purchase that no wastewater tank can substitute for. Mechanically, a cyclone separator removes settleable grit from liquid streams by spin, protecting the gravity tank rather than replacing it, since it does not capture rising oil. On the liquid polishing side, a bag filter or a self-cleaning filter removes the residual suspended droplets and solids after the tank, at media cost and maintenance that gravity avoids. Chemically assisted stages, coagulation and dissolved air flotation, handle the emulsions gravity cannot, at operating cost and complexity gravity never charges. The gravity tank wins first position in every oily water train because it removes the bulk of free oil with no energy, no consumables, and almost no operator attention; it loses whenever asked to do the chemistry stage's job.

The application map below compresses the practice into the form a project file carries.

Downstream monitoring deserves its own sentence before the RFQ block, because permits are demonstrated at the outlet, not in the tank. Where effluent is discharged to a sewer or receiving water, the proving set is an oil in water analyzer or scheduled sampling, a flow meter for load calculations, and where the treatment continues biologically, a dissolved oxygen meter at the aeration stage and a residual chlorine analyzer where the outfall is disinfected. Buying the tank without the proving set simply postpones the cost to the first inspection.

The RFQ block should be written once and reused. A complete block reads: service description, stormwater or process, with the SPCC or permit reference; peak and continuous design flow with the basis; influent oil specific gravity, expected oil loading, and emulsion statement from a jar test; design droplet size and predicted effluent at stated temperature; solids load and sludge volume expectation; construction, API basin or plate pack with pack specification, aboveground or underground, listed to the standard the authority requires, with coating systems stated inside and out; skimmer type and heater scope; freeze protection for the climate; pretreatment and polishing scope boundaries; instrumentation, interface level transmitter, outlet flow meter, sampling ports, and analyzer scope; and the documentation set, sizing basis, fabrication and coating standards, and cleanout procedure. A quotation returned without the sizing basis is incomplete at any price.

Installation, Standards, Failure Modes, and Maintenance

Installation quality decides whether the tank ever meets its paper effluent. Bring the influent in low and slow: drain by gravity where the site allows, size inlet piping for velocities that do not shear droplets, and where a feed pump is unavoidable, use a low-speed centrifugal pump on a straight, valve-free run. Set the unit level and at the design waterline, because the API geometry and the plate pack both depend on the operating depth, and an outfall weir set wrong by centimeters moves the oil retention baffle's working margin with it. Isolation belongs around the unit: a butterfly valve on large inlet lines and a gate pattern isolator on small ones, and a check valve on any pumped discharge to stop backflooding the basin from the sewer in a storm. Sample connections at inlet and outlet, a ball valve at each tap, make the permit routine possible from grade instead of from a manhole lid.

Instrument the tank so it fails loudly. A level transmitter reading the oil-water interface gives the skim schedule and alarms a rising layer before oil reaches the effluent weir; many packages use it to automate the skimmer or the decant. At the outlet, a flow meter turns grab samples into loads and supports the permit record, and an oil in water analyzer closes the loop continuously where the discharge justifies it. Panels that consolidate an online water analyzer, a turbidity meter, and a TOC analyzer on the effluent line are standard at inspected sites, and the same shelter can carry a dissolved oxygen meter when the flow continues to aeration and a residual chlorine analyzer where the outfall is disinfected. Alarms belong on high interface, high effluent oil, and high sludge, and the alarm list should be tested at commissioning, not read about in the manual.

The standards frame is short but binding. API 421 anchors the gravity design basis most fabricators quote. The SPCC rule, 40 CFR 112, decides the containment and drainage context around oil handling in the United States and frequently creates the requirement the separator satisfies. Vessel standards anchor construction: UL 2215 for aboveground separators, UL 58 for underground steel, ULC-S656 as the Canadian secondary containment basis, and STI ACT-100 or ULC class coatings for buried steel, with interior linings qualified under ASTM practice. Marine installations require the MEPC.107(49) type approval of the whole 15 ppm package, monitor and shutoff included. Name the standards in the RFQ; the burden of proof then sits with the fabricator, where it belongs.

Failure modes follow a short and predictable list. Emulsion breakthrough is the most common: a detergent change upstream, a new parts washer, or a high-shear pump swap quietly converts separable oil into a stable haze, effluent climbs, and the tank gets blamed for an upstream chemistry problem; the jar test finds it in an hour. Short-circuiting follows poor inlet condition or missing baffles, where the stream jets across the surface and residence time collapses; dye tracing at commissioning exposes it. Sludge fill reduces effective depth and volume until solids re-entrain; the countermeasure is a measured cleanout interval, not a visual one, because a tank can look acceptable from the riser while holding half its volume in grit. Over-pumping and over-skimming emulsify recovered oil and pump water into the oil tank, wrecking the mass balance the permit reports; timers belong on interface evidence, not on a calendar. A failed check valve on a pumped discharge lets a storm sewer backflood the basin and lift the oil layer over the weir, so it belongs on the quarterly valve exercise list with everything else. Corrosion under insulation attacks heated outdoor units, and a leaking cover seal on a VOC-controlled unit shows up as an air compliance finding rather than a water one, an expensive way to learn that the cover was part of the permit.

Maintenance is deliberately modest and budgeted in pump-outs and gaskets. Weekly: interface level against the commissioning baseline and a glance at the effluent clarity sample point. Monthly: skim operation, sludge depth sounding at the marked points, and a walk of the inlet trash rack. Quarterly to annually by solids load: sludge pump-out by licensed hauler, plate pack pull and wash where fitted, coating inspection at the waterline and under the oil layer, exercise of the inlet butterfly valve and the sample point ball valve set, and function tests of the analyzer set, the oil in water analyzer zeroed and span-checked, the online water analyzer and TOC analyzer per their own schedules. Underground double-wall units add annual tightness testing of the interstice per the tank standard. Hold spares against the package: cover gaskets, skimmer wear parts, heater elements where fitted, and analyzer reagents and cells. Held to that rhythm, a gravity separator runs for decades, and the effluent record it produces turns the next permit negotiation and the next RFQ into paperwork instead of archaeology.