CORE OIL-WATER SEPARATION TERMS
These are the foundational terms most users search, ask, or need clarified.
Absorption
The process by which a fluid or gas is taken up by a solid or liquid. In wastewater, this often refers to contaminants soaked into solids—not a separation mechanism Mercer uses.
Adsorption
The adhesion of oil molecules onto the surface of solids or media. Some separation technologies use adsorption, but Mercer relies on enhanced gravity and coalescence.
API Separator
A shallow, concrete, long-weir oil-water separator defined by API 421 principles. Typically removes droplets down to ~150 microns, making it inadequate for many modern industrial and O&G applications.
Baffle
A plate or wall used to control flow, reduce turbulence, distribute velocity, and prevent short-circuiting inside a separator.
Baffle Weir
A barrier used to control water depth and ensure even flow across the coalescer pack.
Basic Sediment & Water (BS&W)
The solids and water content in recovered or saleable oil. Effective oil-water separation reduces BS&W.
Breakthrough (Oil Breakthrough)
When oil escapes a separator due to turbulence, bypass, or fouled plates. Indicates performance limitations or incorrect hydraulics.
Coalescence
The process of combining small droplets into larger ones so they rise faster in an enhanced gravity separator.
Coalescer
A surface or structure that encourages oil droplets to combine and separate. Mercer uses engineered flat-plate herring-bone coalescers.
Coagulation (not used by Mercer)
A chemical treatment process where chemicals cause contaminants to aggregate. Not a mechanism used in Mercer systems.
Corrugated Plate Interceptor (CPI)
A common plate-based separator using sinusoidal (corrugated) plates. Tends to foul quickly and trap solids; often replaced by Mercer systems.
Dead Zone
An area within a separator where flow stagnates, leading to solids buildup, poor separation, or turbulence. Mercer eliminates dead zones with engineered baffling and Chimney Zones™.
Dispersed Oil
Oil droplets suspended in water that require coalescing surfaces for removal. Mercer systems excel at capturing dispersed oil.
Droplet Size (Micron Rating)
A key factor in separation efficiency. Larger droplets rise faster; Mercer systems routinely capture 20–60 micron droplets depending on loading.
Emulsion (Emulsified Oil)
Oil droplets stabilized by surfactants or chemicals. True emulsions often require chemical or mechanical pretreatment beyond enhanced gravity separation.
Enhanced Gravity Separation
Mercer’s core method: using gravity, coalescence, and laminar flow to separate oils and solids without chemicals or mechanical complexity.
Fouling
Accumulation of solids or hydrocarbons that obstruct flow paths or plate surfaces. Mercer plates minimize fouling through flat geometry and chimney zone design.
Free Oil
Oil that rises quickly due to buoyancy. Easily captured by Mercer systems.
Hydraulic Retention Time (HRT)
Time water remains inside a separator. Critical for droplet rise and separation performance.
Laminar Flow
Smooth, non-turbulent flow essential for predictable oil rise. Mercer plate geometry and baffling maintain laminar flow.
Mesh Pack (Mesh Coalescers)
Fine mesh media used in some separators. Quickly clogs, difficult to clean, and often leads to bypass or failure.
Oil-Hopper
A sloped or coned zone where oil is collected for removal. Mercer automates oil-hopper discharge when needed.
Plastic Media
Lightweight modular blocks used in some low-cost separators. Difficult to clean and prone to severe fouling. A major upgrade opportunity for Mercer.
Rise Rate
The speed at which an oil droplet rises under gravity. Larger droplets rise faster. Coalescence increases rise rate.
Settleable Solids
Particles heavier than water that settle by gravity. Concentrated in Mercer’s hopper bottom for easy removal.
Short-Circuiting
When water bypasses the coalescer pack due to turbulence or poor baffle design. Leads to sheen or permit issues.
Sinusoidal Plates
Corrugated plates used in CPI systems. Tend to trap solids and foul more quickly than flat-plate designs.
Slug Load (Slugging)
A sudden, heavy inflow of oil or solids. Mercer’s baffling and laminar-flow engineering mitigate slug impacts.
Solids Bridging
Accumulation of solids connecting across flow channels, leading to blockage. Mercer’s Chimney Zones™ prevent bridging.
Surfactant Interference
When detergents or surfactants temporarily inhibit separation by dispersing oil droplets.
Turbulence
Chaotic flow that disrupts coalescence and causes re-entrainment or bypass. Mercer engineering eliminates turbulence.
Weir
A plate or wall controlling water depth and overflow. Mercer uses full-width weirs for even flow.
API 421 & COMPLIANCE TERMS
API 421
The American Petroleum Institute’s guideline titled Monographs on Refinery Environmental Control — Management of Water Discharges. It provides the foundational engineering principles for designing enhanced gravity oil-water separators, including droplet size targets, velocity limits, baffling, and plate design criteria.
API Separator
A traditional, shallow, long-weir concrete oil-water separator sized per API 421 formulas. Typically removes droplets down to ~150 microns. Often retrofitted or replaced due to poor solids handling and large footprint.
API 421 Section 1.2 (Page 2)
States that long-concrete API separators remove droplets only down to ~150 microns, underscoring performance limitations vs. modern coalescer systems.
API 421 Section 2.1.4.1 (Page 7)
Defines the maximum horizontal velocity of <3 ft/min in the separator. Higher velocities create turbulence, bypass, and re-entrainment. Mercer designs maintain this limit through engineered baffling.
API 421 Section 4.1.3 (Page 25)
Establishes 60-micron droplets as the design target for effective enhanced gravity separation. This micron benchmark drives Mercer’s plate geometry and surface area design.
Benzene Emissions
Volatile organic compounds (VOCs) released from petroleum products. Effective oil-water separation and enclosed skimming reduce benzene volatility and operator exposure.
Best Management Practices (BMPs)
Industry-standard practices used to control stormwater pollution. Mercer systems assist facilities in meeting BMP requirements for oily runoff.
Class I, Division 1 / Division 2
Hazardous electrical classifications for explosive atmospheres. Mercer can provide components meeting these standards for refinery and utility applications.
Compliance Master™ (API-Driven System)
Mercer’s flagship separator designed specifically to meet or exceed API 421 performance criteria, including velocity, droplet size, laminar flow, and baffling requirements.
Discharge Monitoring Report (DMR)
Regulatory documentation submitted for NPDES compliance. Mercer provides engineering data that supports effluent monitoring.
Dispersed Phase Oil Limits
Permitted concentrations of dispersed oil in effluent. Mercer’s coalescer systems help achieve predictable compliance with oil and grease limits.
Effluent Quality Standard
Regulatory limit on oil, solids, and contaminants in discharge water. Mercer systems help facilities meet effluent quality requirements through predictable separation.
Environmental Permit (General)
Any permit governing wastewater discharge, stormwater, or spill control. Mercer provides documentation supporting permit applications and renewals.
EPA Multisector General Permit (MSGP)
The EPA’s stormwater permit covering industrial sectors. Mercer separators help reduce sheen and pollutants required under MSGP compliance.
Hazardous Waste (RCRA)
Regulated waste defined under the Resource Conservation and Recovery Act. Mercer systems reduce oil and sludge quantity, decreasing hazardous waste generation.
Industrial Stormwater Permit
Permit governing the quality of stormwater discharged from industrial activities. Mercer separators prevent visible sheen and hydrocarbon discharge.
Laminar Flow (API Requirement)
Defined by API as essential for predictable droplet rise. Mercer’s 55° plates and optimized baffles maintain laminar conditions even under solids load.
Maximum Allowable Velocity
Under API 421, velocity must remain below 3 ft/min through the separator to ensure stable separation. Mercer baffles enforce this limit.
Micron Rating (API 421)
The droplet size that a separator is designed to remove. API uses 60 microns as the design basis, while many CPI systems achieve only 100–150 microns.
NPDES (National Pollutant Discharge Elimination System)
The federal permitting system governing wastewater discharges. Mercer equipment supports compliance through predictable separation.
Oil & Grease Limit
Permit limit on oil concentration in effluent. Mercer systems are engineered to maintain compliance under variable loading.
Opacity / Sheen Standard
Regulatory requirement to prevent visible sheen on discharged water. Mercer separators maintain consistent removal to prevent sheen-causing bypass.
Permit Exceedance
When effluent quality surpasses allowed pollutant thresholds. Mercer systems help prevent exceedances through stable design.
PFAS (Per- and Polyfluoroalkyl Substances)
Man-made chemicals found in some industrial settings. Mercer systems do not remove PFAS but may be part of a larger treatment train.
Primary Treatment (Gravity Separation)
The first stage of wastewater treatment involving physical separation. Mercer provides enhanced gravity primary treatment.
Re-Entrained Oil
Oil that is swept downward due to turbulence, violating API principles. Mercer prevents re-entrainment through laminar-flow engineering.
Refinery Wastewater Standards
Strict industrial guidelines governing oily wastewater discharge. Mercer’s API-driven design helps refineries meet these standards consistently.
Regulatory Compliance Audit
A review of systems and practices against environmental regulations. Mercer provides data and engineering support for compliance evaluations.
Secondary Containment
Structures used to capture spills from tanks and transformers. Mercer separators prevent oily water from entering storm drains or waterways.
SPCC (Spill Prevention, Control, and Countermeasure)
EPA regulation requiring prevention of oil releases to navigable waters. Mercer systems support SPCC requirements for containment areas.
Stormwater Pollution Prevention Plan (SWPPP)
A plan required under MSGP permits. Mercer separators help meet hydrocarbon control requirements in the SWPPP.
TSS (Total Suspended Solids)
A common permit parameter measured in effluent. Mercer systems remove solids via hopper bottoms and settleable solids control.
VOC (Volatile Organic Compound)
Emissions released from oils and hydrocarbons. Proper oil handling and enclosed skimming reduce VOC emissions.
Water Quality Standard (WQS)
Any limit placed on discharge water contaminants. Mercer systems help maintain compliance with hydrocarbon and solids standards.
INDUSTRIAL WASTEWATER TERMS
Activated Sludge (Not Part of Mercer’s Process)
A biological treatment method using aerobic microorganisms to break down organics. Mercer systems are typically installed upstream of activated sludge systems to reduce oil load.
Aeration Basin
A tank used in biological wastewater treatment where oxygen is added to support microbial breakdown. Cleaner influent from Mercer separators reduces basin foaming and shock load.
Agglomeration
The clustering of particles or droplets into larger masses. Mercer’s coalescing plates promote agglomeration of oil droplets for easier separation.
Anoxic
An environment with little to no oxygen. Typically applies to biological wastewater treatment, not to gravity separation equipment.
Batch Wastewater
Water discharged intermittently from a specific operation. Mercer evaluates batch profiles during audits to ensure the separator handles surge loads.
Break Tank
A tank used to equalize flow before treatment. Often used upstream of Mercer systems in highly variable or batch operations.
Chemical Oxygen Demand (COD)
A measure of organic pollutants in wastewater. While Mercer systems do not remove COD, they reduce oil/solids loading on downstream COD-removal processes.
Clarifier
A settling tank used to remove solids after biological treatment. Reducing oil/solids at the front end improves clarifier efficiency.
Cooling Water Contamination
Oil ingress into cooling water loops due to leaks or seal failures. Mercer separators remove lube and hydraulic oils from contaminated cooling water.
Design Flow
The intended operational flow rate for a separator. Exceeding design flow causes bypass and turbulence.
Detergent Load
The amount of surfactant or cleaning chemicals in wastewater. Can temporarily inhibit separation by dispersing oil droplets.
Effluent
The treated wastewater leaving a system. Mercer systems improve effluent clarity and reduce oil/solids.
Equalization (EQ) Tank
A tank used to moderate flow and load variation. Mercer sometimes specifies EQ tanks for highly surging industrial streams.
Feed Pump
The pump delivering wastewater to a separator. Its behavior (surges, cavitation, cycling) greatly influences separator performance.
Fines (Solids)
Very small particles (clays, metal fines, grit) often present in industrial wastewater. Mercer’s hopper bottoms concentrate fines for removal.
Floatables
Oil, grease, and other low-density materials that rise to the surface. Mercer separators are engineered to collect floatables efficiently.
Flow Change-Out / Flow Reversal
Sudden changes in direction or rate of flow. Good baffling minimizes their negative impact on separation.
Hydraulic Loading
The volume of water entering the system relative to time. High hydraulic loading creates turbulence; Mercer designs maintain control at higher loadings than CPI systems.
Hydrocarbon
Petroleum-based compounds (fuels, oils, lubricants) commonly found in industrial wastewater. Mercer specializes in hydrocarbon removal.
Industrial Washdown Water
Cleaning water from machinery, floors, or equipment. Typically contains grit, oils, and grease. Ideal candidate for Mercer separation.
Influent
The raw wastewater entering a treatment system. Influent characteristics determine separator design.
Interceptor
A device designed to capture oils/solids before they reach the sewer or environment. Mercer separators are high-efficiency interceptors.
Lube Oil
Lubrication oil used in bearings, gearboxes, turbines, etc. Frequently present in power generation and manufacturing wastewater.
Mechanical Seal Leakage
Oil leakage from pump seals or compressors that enters washdown or sump systems. A common source of oily wastewater.
Oil & Grease (O&G)
A standard regulatory measurement representing hydrocarbon concentration in water. Mercer separators reduce O&G significantly.
Oily Bilge Water
Water collected in ship bilges containing fuel, oil, and contaminants. Mercer systems are frequently used at ports and marine service areas.
Operator Loading
The amount of labor required to operate a system. Mercer’s automation reduces operator loading dramatically compared to CPI or mesh-based systems.
Organic Load
The amount of biodegradable matter in wastewater. While Mercer does not remove organics, removing oil/solids upstream protects biological systems.
Overloaded Separator
A separator receiving more flow, oil, or solids than it was designed for. Common with CPI, plastic media, or aged systems.
Pit Water / Sump Water
Runoff that accumulates in maintenance shop pits, lift stations, and sumps. Often contains oils, grit, and detergents.
Primary Treatment
The first stage in wastewater treatment focused on removing solids and oils. Mercer provides enhanced primary treatment.
Process Water
Water used in production processes that may collect oils, lubricants, or solids. Mercer separators stabilize process water for reuse or discharge.
Recycle Flow
Water that is reused within an industrial plant, such as cooling towers or wash systems. Cleaner water improves system reliability.
Re-entrainment
Oil that is pulled downward or sideways due to turbulence, defeating separation. Mercer eliminates re-entrainment through laminar-flow design.
Retention Time
The amount of time water remains in the separator. Shortened retention time reduces separation efficiency.
Secondary Oil Recovery
Oil recovered from wastewater for potential reuse or sale. Mercer systems improve oil recovery through finer droplet capture.
Separator Loading
The amount of oil/solids entering the separator. Consistent loading improves performance; erratic loading requires good baffling.
Sheen
A reflective, rainbow-like film of oil on water. Visible sheen at the outfall is a regulatory violation. Mercer helps prevent sheen discharge.
Solids Accumulation
Build-up of settleable solids inside a tank or separator. Mercer’s hopper bottom design manages solids accumulation predictably.
Split-Flow / Parallel Flow
Flow routing that divides influent into separate treatment paths. Often used in large plants; requires careful engineering.
Storm Surge Loading
Sudden inflow of runoff during rainfall. Mercer separators—especially baffled systems—handle surge loads better than CPI units.
Strainer / Screen (Upstream Equipment)
Mechanical devices that remove large debris before water enters a separator. Recommended upstream of any coalescer.
Sump Pump
A pump used to move sump or pit water to a separator. Pump surges can cause turbulence that impacts separator performance.
Suspended Solids
Non-settleable particles suspended in water. Excessive suspended solids may require pretreatment.
Thermal Shock
Sudden temperature change in wastewater that can affect oil viscosity and rise rate. Mercer’s plate area compensates for moderate thermal variation.
Turbidity
Cloudiness caused by suspended particles. Lower turbidity results from effective solids removal.
Viscosity
The thickness of oils or fluids. Higher viscosity slows oil rise rate; temperature impacts viscosity significantly.
Washdown Detergents
Chemicals used in cleaning that may temporarily suspend oil droplets, impacting separation. Mercer identifies detergent issues during audits.
Wastewater Characterization
The analysis of wastewater’s physical and chemical properties. A key step in Mercer’s free site audit.
OIL & GAS INDUSTRY TERMS
API Gravity
A measurement of petroleum fluid density. Influences oil/water separation because lower-density oils rise faster in water.
AST (Aboveground Storage Tank)
Large tanks used to store produced water, slop oil, or flowback. Mercer systems reduce the volume and solids load entering ASTs.
Beam Pumping / Pumpjack
A common method of extracting oil in upstream production. Produced water from pumpjacks typically enters a separation system requiring treatment.
Basic Sediment & Water (BS&W)
A measure of solids and water in crude oil. Mercer systems improve oil recovery quality by reducing BS&W in skimmed oil.
BOP (Blowout Preventer)
A critical drilling safety device. Hydraulic fluid and washdown from BOP maintenance may contain oils requiring separation.
Casinghead Water
Produced water collected from the well casing annulus. Frequently contains emulsions, solids, and hydrocarbons.
Chemical Injection
Addition of chemicals (corrosion inhibitors, demulsifiers, biocides) that may impact droplet size and separation behavior.
Completion Fluids
Fluids used during well completion that may contain hydrocarbons when returned. Mercer systems help remove free and dispersed oil.
Condensate
Light hydrocarbon liquid from natural gas production. Often appears in produced water and must be separated.
Crude Oil
Unrefined petroleum containing hydrocarbons and solids. Crude-contaminated wastewater requires enhanced gravity separation.
Downstream Operations
Refinery and petrochemical processing of crude oil. Downstream wastewaters often contain small droplets (20–100 microns), detergents, and challenging solids profiles.
Drill Cuttings
Rock fragments produced during drilling. Cuttings washdown often contains hydrocarbons requiring solids and oil removal.
Drilling Mud
Fluid used in drilling operations. Waste mud water may contain emulsified oils and solids.
Emulsion (O&G Production)
A stabilized mixture of oil and water. True emulsions often require chemical treatment beyond gravity separation.
Field Tank Battery
A grouping of tanks used for storing oil, produced water, and slop oil. Mercer separators stabilize water quality before transfer or injection.
Flowback Water
Water returning from hydraulic fracturing. Contains high solids and hydrocarbons. Mercer systems remove free/dispersed oil, improving reuse or disposal.
Frac Tank
A large temporary tank used in drilling and stimulation operations. Mercer systems reduce the need for multiple frac tanks by treating water continuously.
Gun-Barrel Tank
A vertical tank historically used for oil-water separation. Inefficient, slow, and prone to solids buildup. Mercer separators frequently replace gun-barrel tanks.
Injection Well (SWD – Saltwater Disposal)
A well used for injecting produced water underground. Cleaner influent reduces injection pressure, formation plugging, and operational cost.
Midstream Operations
Transportation, storage, and transfer of crude oil and natural gas. Terminal runoff, tank bottoms, and transfer wastewater benefit from Mercer separation.
Oil/Water Interface
The boundary between oil and water layers. Mercer designs create a stable interface for controlled oil skimming.
Paraffins / Waxes
Hydrocarbon components that solidify at lower temperatures. May affect separation by changing oil viscosity and droplet behavior.
Pigging (Pipeline Cleaning)
The process of pushing devices (“pigs”) through pipelines to clean deposits. Pigging wastewater often contains oils and solids requiring treatment.
Produced Water
Water naturally occurring in oil reservoirs and brought to the surface during production. Contains hydrocarbons, solids, salts, and sometimes emulsions.
Production Separator
Primary separator at the well site separating gas, oil, and water. The water phase typically requires additional treatment by systems like Mercer.
Refinery Complex
An industrial facility processing crude oil into fuels and chemicals. Wastewater contains small droplets, surfactants, metals, and solids.
Slop Oil
Recovered oil from tank bottoms, drainage systems, or maintenance activities. Mercer systems reduce slop oil generation through upstream treatment.
Sour Water
Water containing hydrogen sulfide (H₂S). May require specialized materials or coatings.
Spill Basin / Containment Basin
Areas designed to capture spills at refineries, well pads, and terminals. Mercer separators prevent oily overflow into storm systems.
Steam-Assisted Gravity Drainage (SAGD)
A heavy-oil recovery method. SAGD water streams may require enhanced gravity separation before further treatment.
Tank Bottoms
Settled solids and sludge found at the bottom of storage tanks. Mercer’s hopper-bottom separator design minimizes tank-bottom buildup upstream.
Terminal Wastewater
Oily water generated at midstream terminals from truck unloading, pipeline transfer, tank cleaning, and stormwater.
Upstream Operations
Activities associated with exploration and production of oil and gas. Wastewater streams include produced water, flowback, and washdown.
Water Cut
The percentage of water in produced fluids. High water cut operations require continuous separation systems like Mercer’s Compliance Master™.
Well Pad Runoff
Stormwater mixed with drilling fluids, oil, and solids. Requires robust separation to prevent sheen discharge.
Wellbore Fluids
Fluids used in drilling and completing wells. Wastewater from these operations may contain hydrocarbons and solids requiring separation.
Workover Fluids
Fluids used during well intervention. Waste streams may contain oils, brine, and solids requiring treatment.
Zero Discharge (ZD)
A practice of reusing or reinjecting all produced water with no surface discharge. Mercer improves influent quality for ZD operations.
ELECTRICAL UTILITY TERMS
Auxiliary Cooling Water
Water used to cool transformers, turbines, or generators. Leaks or seal failures often introduce lube or hydraulic oil, requiring separation.
Boiler Blowdown
Water removed from a boiler to control solids concentration. Blowdown itself isn’t oily, but blowdown areas may receive mixed hydrocarbon runoff that requires separation.
Combustion Turbine (CT)
A gas turbine used in power generation. CT lubrication and hydraulic systems can leak oils that enter washdown or stormwater.
Condensate Contamination
Oil contamination in condenser systems due to leaking heat exchanger tubes. Mercer separators help remove leaked oils from condensate drainage.
Cooling Tower Basin Runoff
Water collected from cooling tower drift, leaks, or washdown. May contain oils from fan gearboxes or equipment lubrication.
Cycling Plant
A power plant designed for frequent startups and shutdowns. Cycling increases the likelihood of hydraulic leaks and oil ingress into sumps or drains.
Diked Transformer Containment
A containment area around transformers designed to capture oils in the event of a leak or failure. Mercer separators help treat oily water that collects in these areas.
Environmental Swale
Shallow vegetated channels used to treat stormwater. In substations, swales may receive oily runoff from transformers or breakers.
Excitation System
Controls generator output. Oil-cooled excitation equipment can leak into adjacent drainage systems.
Flushing Water (Turbine Flushing)
Water used to flush lubrication or hydraulic systems during maintenance. Often contains small oil droplets requiring separation.
Genset (Diesel Generator Set)
Backup power systems powered by diesel engines. Washdown water and pad runoff may contain diesel and lube oils.
GE Transformer Oil (Type)
A dielectric insulating oil used in transformers. A common hydrocarbon found in substation runoff during leaks.
Hydraulic Power Unit (HPU)
Equipment providing hydraulic pressure for turbine controls, valve actuators, and mechanical systems. HPU leaks are a major source of hydrocarbon contamination.
Hydroelectric Turbine Pit Water
Water that accumulates in turbine pits, draft tubes, and tailrace areas, often containing lube oils and greases from mechanical systems.
Insulating Oil (Dielectric Fluid)
Oil used in transformers, breakers, and switchgear. A primary contaminant in substation stormwater and sump water.
Lube Oil System (Turbine Lube System)
Circulates lubrication oil for bearings and rotating equipment. Leaks may enter drains or sumps requiring separation.
Main Generator Lube Oil
Lubrication oil for steam or combustion turbine generators. Frequently detected in plant sump water.
Natural Draft Cooling Tower
A large hyperbolic cooling tower. Drift droplets may contain oil from gearboxes or drive systems.
Oil-Filled Transformer
Electrical transformer using mineral or synthetic oil for cooling and insulation. Runoff in containment areas often requires oil-water separation.
Oil Mist Elimination System (OME)
A system used to control oil mist from turbine bearing housings. Collected mist or condensate may contain recoverable hydrocarbons.
Outfall Structure (Utility Setting)
Discharge point for stormwater or treatment effluent. Mercer separators help prevent sheen at outfalls.
PCB (Polychlorinated Biphenyls)
Historically used in transformer oils. Mercer separators do not remove PCBs but may be used in systems handling PCB-free dielectric oils.
Penstock Leakage (Hydroelectric)
Oil leakage from hydraulic gates, turbine shafts, or bearings into penstock water. Collected leakage often requires treatment.
Power Block Sump
A central sump collecting water from turbine decks, boiler rooms, or pump galleries. Often contains oil requiring gravity separation.
Ring Bus (Substation)
An electrical configuration at substations. Equipment drainage from breakers or transformers may require separation.
SF6 Breaker Washdown
Cleaning runoff from gas-insulated (SF6) breakers, which may include residues from nearby oil-filled equipment.
Sluice Gate Hydraulics
Hydraulic systems used to control water flow in dams and hydroelectric stations. Hydraulic leaks commonly enter adjacent water.
Spill Containment Vault
Underground containment structure beneath transformers or oil-filled equipment. Water collected here frequently contains hydrocarbons.
Station Drain Water
General drainage from powerhouse or plant floors. Contains oils from maintenance activities, lubrication systems, and tiny hydraulic leaks.
Substation Runoff
Stormwater flowing through electrical yards. Often contaminated with transformer oil, gear oil, or hydraulic fluid.
Switchyard
Outdoor area containing transformers, switchgear, and breakers. Runoff may contain hydrocarbons requiring separation.
Turbine Deck Drainage
Water collected from steam or combustion turbine decks. Often contains turbine lube oils, hydraulic oils, or bearing drips.
Turbine Oil Reservoir
A large tank storing lubrication oil for turbine systems. Overflow or washdown water may require separation.
Vacuum Oil Purification System
Equipment used to purify turbine or transformer oil. Purge water or flushing water may contain free/dispersed oil.
Water-Lubricated Bearings (Hydropower)
Bearings cooled with water rather than oil. In adjacent systems, oil-lubricated components still produce wastewater requiring separation.
🚆 RAIL INDUSTRY TERMS
Ballast Water (Rail Yard Context)
Runoff that drains through stone ballast beneath tracks. Often picks up oils, greases, and diesel residue from locomotives.
Brake Shoe Dust
Fine particles from locomotive and railcar brakes. Often mixed with oils and solids in washdown wastewater.
Car Wash (Rail Wash Rack)
Washdown system for locomotives and railcars. Runoff contains oil, grease, grit, and detergents.
Diesel Fuel Spill Basin
Containment areas at fueling pads. Mercer separators prevent contaminated water from entering storm drains.
Locomotive Maintenance Pit Water
Runoff collected in maintenance pits beneath locomotives. Contains lube oil, diesel, greases, and metal fines.
Rail Yard Runoff
Stormwater flowing across fueling, service, or maintenance areas. Often contaminated with hydrocarbons.
Sand Tower Runoff
Water used to clean locomotive sand-delivery systems. Contains grit, fines, and residual oils.
Switch Lubrication Oil
Oil used to lubricate rail switches. Common contaminant in rail stormwater.
Tank Car Washdown
Washdown or spill cleanup from tank railcars carrying petroleum or chemicals. These streams require robust separation.
✈️ AIRPORT & AVIATION TERMS
Aviation Fuel Spill Runoff
Stormwater contaminated with Jet-A or Avgas from fueling aprons. Requires reliable oil-water separation.
Deicing Fluid (Glycol-Based)
Used for aircraft deicing. Not removed by gravity separation but often mixed with oily washwater in apron drains.
Hangar Washdown Water
Floor washdowns from aircraft maintenance. Contains lube oils, hydraulic fluids, and grease.
Hydraulic Skydrol
A phosphate-ester hydraulic fluid used in aircraft. Mercer identifies Skydrol contamination during audits because it behaves differently from petroleum-based oils.
Runway Rubber Removal Water
Wastewater from rubber-removal operations that can contain hydrocarbons and fine particulates.
Taxiway/Apron Drainage
Stormwater flowing across fueling and maintenance areas. High risk of fuel contamination.
ULSD (Ultra-Low Sulfur Diesel)
Fuel used in ground vehicles and auxiliary power units. Frequently appears in maintenance-area washwater.
🚇 MASS TRANSIT SYSTEM TERMS
Bus Wash Wastewater
Runoff from bus wash stations containing detergents, grease, oils, and road grime.
Electric Rail Lubrication Oil
Oil used in track or wheel lubrication systems for urban rail networks.
Fleet Maintenance Drainage
Wastewater from maintenance shops for buses, streetcars, or metro vehicles.
Hydraulic Lift Pit Water
Collected water beneath vehicle lifts. Often contains hydraulic fluids and lubricants.
Service Bay Runoff
Washdown and spillwater from transit maintenance bays.
Transit Yard Stormwater
Runoff from fueling islands, maintenance zones, and overnight parking areas.
🚢 MARITIME & PORT OPERATIONS TERMS
Bilge Water
Water accumulating in ship hulls that contains oils and fuel residues. Frequently treated at port facilities.
Bunker Fuel
Heavy marine fuel. Spills in port maintenance or fueling areas require separation.
Dockside Drainage
Water drained from piers or wharves after maintenance or fueling operations. Often contains petroleum residues.
Dry Dock Sump Water
Water pumped from dry docks that may contain oils, grease, and paint residues.
Marine Gear Oil
Lubricant used in marine engines. Common in maintenance-area runoff.
Port Washdown Water
Washdowns of vessels, docks, or loading areas. Picks up oils, metals, and grease.
Shipyard Process Water
Industrial wastewater from boat manufacturing or repair including oils, solvents, and paint residues.
Spill Containment Boom Water
Water collected from contamination control zones. May contain diesel or fuel oil.
🚚 TRUCKING & LOGISTICS TERMS
Diesel Island Runoff
Fueling pad stormwater contaminated with diesel, lube oils, and drips from connections.
Fleet Wash Bay
Truck wash systems producing wastewater with detergents, oils, and solids.
Freight Terminal Drainage
Stormwater from loading docks and yards often contaminated with fuel and hydraulic oils.
Hydraulic Lift Oil
Oil used in lifting equipment such as dock levelers or truck-mounted lifts.
Mechanic Bay Washdown
Wastewater from cleaning and maintenance in truck repair shops.
Parking Lot Runoff (Heavy Vehicles)
Runoff with accumulated oils, grease, and fines from parked trucks.
Tank Truck Cleanout Water
Washwater from cleaning tanks that have hauled petroleum products.
Trailer Refrigeration Unit Drips
Oil leaks from refrigerated trailers (reefers). Appears in yard runoff.
🚛 GENERAL TRANSPORTATION TERMS
Fuel Island / Fueling Pad
Designated fueling area for vehicles, locomotives, or equipment. High potential for oil/fuel contamination.
Mobile Equipment Lube Oil
Lubricants used in various transport-sector machinery and equipment. A frequent contaminant in shop runoff.
Pad Grading (Drainage Grading)
Slope and drainage design in fueling and maintenance pads. Improper grading can send oily runoff to storm drains.
Railcar Spill Containment
Containment structures for loading/unloading. Mercer separators manage oily water collected from these zones.
Runoff Containment System
Infrastructure designed to collect and convey stormwater to a treatment system.
Trackside Drain
Drainage channels running parallel to rail lines. Frequently receive oil drips from locomotives.
Vehicle Service Pit
Underground service area beneath vehicles. Washdown and spills from these areas often feed directly into separators.
SECTION G: STORMWATER & ENVIRONMENTAL TERMS
Best Management Practices (BMPs)
Standard procedures and physical controls used to minimize pollutants in stormwater discharges (e.g., oil-water separators, spill prevention, housekeeping). Mercer systems help facilities meet hydrocarbon-related BMPs.
Bioretention Basin
A landscaped stormwater treatment feature using soil and plants. Usually installed downstream from oil-water separators to polish stormwater.
Catch Basin
A drainage structure that collects stormwater and debris. Often the first point where oily runoff is observed before treatment.
First Flush
The initial surge of stormwater during rainfall that carries the highest concentration of pollutants, including oils. Mercer separators are engineered to capture free and dispersed oil during first flush events.
Green Infrastructure (GI)
Sustainable stormwater management systems like bioswales, rain gardens, and permeable pavement. Mercer systems often serve as pretreatment to protect GI from hydrocarbons.
Hydrocarbon Sheen
A thin, reflective layer of oil on water. A regulatory violation if present in stormwater outfalls. Mercer separators are designed specifically to prevent sheen release.
Impervious Surface
Concrete, asphalt, and other surfaces that do not absorb water. These areas often generate oily runoff requiring treatment.
Industrial Stormwater Permit (MSGP)
A federal permit regulating stormwater discharges from industrial facilities. Mercer systems are commonly used to meet MSGP hydrocarbon limits.
Infiltration Basin / Infiltration Trench
Stormwater systems that allow water to soak into the ground. These systems must be protected from oil loading; Mercer separators provide upstream protection.
Non-Point Source Pollution
Pollution collected from large areas (roads, yards, parking lots) via stormwater. Oil and grease are common NPS pollutants addressed by Mercer.
Oil & Grease (O&G) Benchmark
A regulatory stormwater benchmark for hydrocarbon concentration. Mercer separators help keep O&G below required limits.
Outfall
The point where treated stormwater is discharged to a ditch, channel, or waterbody. Sheen at the outfall is a compliance violation.
Overflow Bypass
A stormwater overflow condition that bypasses the separator due to extreme flow. Properly sized and baffled Mercer systems reduce bypass risk.
Pavement Runoff
Stormwater that collects oils, fuels, metals, and sediments from paved surfaces. A primary application for Mercer separators.
Precipitation Event (Storm Event)
Any rainfall or snowmelt event producing stormwater. First flush frequently carries hydrocarbons requiring separation.
Primary Treatment (Stormwater)
Initial removal of oils, solids, and debris before water flows into downstream green infrastructure or detention basins.
Retention Pond / Detention Basin
Large basins used to manage stormwater storage and release. Mercer separators serve as pretreatment to reduce hydrocarbon loading.
Silt & Sediment Load
The solids carried by stormwater. Excessive solids reduce separator efficiency and require hopper-bottom design — a Mercer advantage.
Source Control
Measures taken to prevent pollutants from entering stormwater at the source (e.g., drip pans, maintenance). Mercer is part of end-of-pipe source control.
Spill Response Water
Water captured during cleanup of spills (fuel, hydraulic fluid, lube oil). Mercer systems often treat spill response wastewater.
Storm Drain Inlet
Entry point for stormwater collection. If located near fueling or maintenance areas, these inlets may be fitted with pretreatment devices or routed to Mercer separators.
Storm Surge (High-Flow Event)
A sudden increase in stormwater flow that can overload poorly designed separators. Mercer baffles handle surge flow better than typical CPI units.
Stormwater Pollution Prevention Plan (SWPPP)
A regulatory document required under MSGP permits. Mercer systems form part of the hydrocarbon control measures outlined in SWPPPs.
Swale (Grass or Bioswale)
A shallow vegetated channel designed to filter stormwater. Often used in combination with upstream oil-water separation.
Vegetated Filter Strip
A landscaped area that removes sediment from stormwater before it enters a drain or basin. Mercer serves as hydrocarbon pretreatment.
Water Quality Unit (WQU)
A stormwater structure designed to remove sediment, debris, and hydrocarbons — often incorporating oil-water separation. Mercer separators can be used as standalone WQUs.
Water Quality Volume (WQV)
The volume of stormwater requiring treatment during a design storm. Mercer engineers consider WQV when sizing separators for stormwater systems.
Watershed
An area where all water drains to a common point. Industrial sites must manage hydrocarbons to protect downstream waterbodies.
Wet Weather Flow
Storm-driven flow that frequently carries oils and debris. Mercer systems are engineered to maintain predictable separation during wet weather.
Zero Sheen Requirement
A regulatory standard requiring no visible sheen in stormwater discharge. Mercer separators help facilities meet zero-sheen requirements reliably.
MERCER-SPECIFIC TERMS (BRANDED)
Chimney Zones™
Mercer’s proprietary vertical flow channels integrated into the coalescer pack that allow solids to fall freely without bridging or clogging. Chimney Zones™ maintain laminar flow and prevent the solids entrapment common in CPI and plastic media systems.
Compliance Master™
Mercer’s flagship enhanced gravity oil-water separator engineered to meet or exceed API 421 design standards for droplet size capture, velocity control, laminar flow, and solids management. A compact, high-performance system used across industrial, O&G, utility, and transportation markets.
Enhanced Gravity Plate Pack™
Mercer’s engineered plate assembly designed to maximize coalescence while maintaining laminar flow. Features flat plates, herring-bone patterning, chimney zones, and removable, field-adjustable modules.
Field-Adjustable Plate Pack™
A modular plate system designed so operators can adjust plate spacing in the field to optimize performance for solids-heavy or oil-light wastewater conditions.
Flow Distribution Baffles™
Mercer-engineered inlet and internal baffles that regulate velocity, straighten flow, and eliminate short-circuiting to ensure that all wastewater meets the coalescer plates under laminar conditions.
Full-Width Weir Plate™
Mercer’s effluent weir design spanning the entire separator width to maintain equal hydraulic load and eliminate side-channeling — a common cause of sheen and bypass in lesser designs.
Herring-Bone Plate Design™
Mercer’s unique flat-plate geometry using a herring-bone pattern to promote droplet coalescence, prevent solids entrapment, and maintain predictable performance even under variable loading.
Hopper Bottom Design™
A Mercer-engineered sloped-bottom tank design that concentrates settleable solids for easy removal. Prevents solids from spreading across tank floors and reduces maintenance frequency.
Industrial Truth-Teller™ Approach
Mercer’s commitment to transparent, honest engineering — telling customers upfront what will and will not work. This includes the guarantee that if Mercer’s solution will not deliver meaningful improvement, the customer is informed before committing to anything.
LaminarFlow Control Architecture™
Mercer’s integrated baffling and plate design used to ensure laminar conditions throughout the separator, minimizing turbulence and maximizing droplet rise rate.
Mercer Advantage™
The combination of proprietary features and engineering principles — including flat-plate coalescence, chimney zones™, hopper-bottom design™, adjustable plates™, and full-width weirs™ — that consistently outperforms API, CPI, and media-based separators.
Multi-Pack™ Coalescer System
Mercer’s proprietary coalescer assembly featuring removable flat plates arranged for maximum surface area, optimized flow paths, and easy cleaning. Designed to withstand solids-heavy streams without clogging or fouling.
Predictable Performance Design™
Mercer’s engineering philosophy ensuring that separator performance remains stable even under fluctuating flows, solids loading, and variable wastewater quality.
Quick-Clean Plate Modules™
Removable plate modules designed for fast access during cleaning, reducing downtime and operator labor compared to CPI or mesh media systems.
Real-World API Compliance™
Mercer’s applied interpretation of API 421 guidelines to achieve realistic, consistent oily wastewater performance — not merely theoretical or best-case lab conditions.
Request a Free Site Audit™
Mercer’s no-cost evaluation program that reviews wastewater characteristics, hydraulic behavior, solids loading, and existing equipment. Includes sample analysis, engineering review, and upgrade recommendations with a guarantee of honesty.
Solids-Release Plate Geometry™
Mercer’s flat-plate design engineered to minimize solids adhesion and encourage solids to fall cleanly into the hopper bottom — unlike CPI sinusoidal plates that trap solids.
Streamlined Sludge Management™
Mercer’s approach to predictable solids handling using hopper bottoms, chimney zones™, and automation to minimize operator involvement and sludge hauling.
Velocity Control Baffles™
Baffles engineered to maintain API-specified <3 ft/min horizontal velocity throughout the separator, reducing turbulence and preventing re-entrainment.
Zero-Bypass Baffle System™
Mercer’s internal baffling configuration ensuring that all influent water passes through the plate pack — eliminating side, bottom, or top bypass routes.
GENERAL ENGINEERING & PROCESS TERMS
Absorption Chiller
A cooling system using heat to drive refrigeration. Drainage or maintenance wastewater may contain oils from pumps and seals.
Actuated Valve
A valve operated by pneumatic, electric, or hydraulic actuators. Mercer integrates actuated valves into automated solids or oil removal systems.
ANSI Flange
A standardized flange connection used in piping systems. Mercer equipment is built with ANSI-compliant connections.
Backflow Preventer
A device ensuring water flows only in one direction, preventing contamination. Often installed upstream of treatment systems.
Bar Screen
A coarse mechanical screen used to remove debris from wastewater before treatment. Recommended upstream of separators.
Batch Processing
A treatment or industrial operation that runs in batches rather than continuously. Requires special consideration for surge handling.
Bearings (Rotating Equipment)
Components supporting rotating shafts. Lube oil leaks from bearings are a common source of oily wastewater.
Cavitation
Formation of vapor bubbles in a pump due to pressure drop. Causes surging that can create turbulence in separators.
Coefficient of Expansion
The rate at which materials expand with temperature. Relevant for tank and baffle engineering.
Composite Sampling
Collecting multiple samples over time to represent average wastewater quality. Often used in compliance reporting.
Control Panel (PLC/HMI)
The electrical system controlling pumps, valves, and instrumentation. Mercer systems can integrate with plant controls via PLC or HMI.
Corrosion Allowance
Extra steel thickness added to a tank to account for corrosion over its lifespan. Mercer designs incorporate proper allowance based on application.
Dead Load / Live Load
Static and dynamic forces considered in structure design. Important for separators installed on elevated platforms or skids.
Differential Pressure (DP)
Pressure difference across a component. In separators, DP changes can indicate fouled or obstructed flow paths.
Discharge Head
The height a pump must overcome to move water. Impacts flow stability and separator performance.
Drain-Down Frequency
How often a tank or system is drained. Affects solids accumulation and maintenance planning.
Erosion-Corrosion
Metal loss caused by fluid friction and corrosive chemistry. Baffling and velocity control reduce erosion risk.
Feed Line (Influent Line)
Piping that conveys wastewater to the separator. Proper hydraulics prevent turbulence.
Flow Equalization
Balancing or moderating flow to prevent surges. Used upstream of separators for highly variable processes.
Footprint (Equipment)
The physical space required for a system. Mercer designs have a compact footprint compared to API separators.
GPM (Gallons Per Minute)
A standard flow rate measurement used in sizing separators.
Head Pressure
The pressure exerted by the height of a liquid column. Influences pump behavior and separator hydraulics.
Heat Exchanger Leakage
Leaks from heat exchangers that may contaminate cooling water with oils. Mercer separators address the contaminated drainage.
HMI (Human-Machine Interface)
The touchscreen or user interface allowing operators to monitor and control a system.
Hydraulic Gradient
The slope of energy or pressure along a fluid path. Critical in designing inlet/outlet piping.
Industrial Coatings
Protective coatings applied to tanks and internals. Mercer uses coatings designed for hydrocarbon and solids exposure.
Instrumentation
Sensors and measurement devices used to monitor system performance.
Level Transmitter
A device that measures the liquid level in a tank. Used for automation and alarms.
Load Variation
Changes in hydraulic or solids load over time. Mercer systems are engineered to maintain performance under load variation.
LOTO (Lock-Out Tag-Out)
A safety procedure for isolating equipment during maintenance. Required for pump or motor service.
Mechanical Integrity Testing
Inspection to ensure equipment is structurally sound. Mercer tanks are designed for long-term integrity.
Modular Construction
Designing equipment in modules for easy installation and maintenance. Mercer plate packs use modular construction.
NEMA Rating
A classification for electrical enclosures (e.g., NEMA 4X). Relevant for control panels in harsh or wet environments.
Open Channel Flow
Flow in an unpressurized, gravity-driven channel. Many separators discharge via open channel weirs.
Overflow Alarm
A sensor alerting operators when water level is too high. Prevents spills and hydraulic overload.
Perforated Plate
A plate with holes to regulate flow or capture debris. Used in some baffling configurations.
pH Adjustment (Not a Mercer Process)
Chemical modification of pH to prepare wastewater for downstream treatment. Mercer identifies pH issues during audits.
PLC (Programmable Logic Controller)
Industrial computer used to automate pumps, valves, and sensors.
Pressure Washing (Plate Cleaning)
A method for cleaning removable Mercer plates. Quick, thorough, and low-effort compared to cleaning CPI or media.
Process Safety Management (PSM)
OSHA framework for managing hazardous chemicals. Mercer equipment supports PSM programs by reducing spill and hydraulic risks.
Pump Curve
Graph showing a pump’s flow vs. head performance. Critical when troubleshooting surges or cavitation.
Recirculation Loop
A loop that returns water to the head of a system for stabilization. Sometimes used for flow dampening.
Retention Basin
A basin designed to hold stormwater. Mercer separators may serve as pretreatment.
Scour / Scouring Flow
Flow strong enough to mobilize settled solids. Mercer baffling prevents scour.
Settling Velocity
The speed at which particles fall in water. Important in hopper-bottom design.
Specific Gravity (SG)
The density ratio of a liquid or solid to water. Oil rises because its SG is typically <1.0.
Static Head
Vertical distance affecting pump pressure when the system is not flowing.
Surge Tank
A tank that controls pressure or flow variation. May be used upstream or downstream of a separator.
Thermal Expansion
Material expansion due to heat. Considered in tank and piping design.
Valve Actuator
A device that controls valve position. Used in automatic oil and solids removal.
Velocity Head
Energy in a fluid due to motion. Excess velocity head creates turbulence and bypass.
Vortexing
Circular flow pattern that can draw oil downward. Mercer baffling prevents vortex formation.
Water Hammer
A pressure surge caused by sudden changes in flow. Can damage piping and cause turbulence.
Wetted Surfaces
Portions of equipment in constant contact with water. Important for coating and corrosion considerations.
Zero-Flow Condition
When no influent enters the separator. Allows solids to settle but may require special consideration upon restart.