The primary produced water treatment stages on an FPSO — hydrocyclones, gas flotation and coarse filtration — are designed to remove most of the oil and solids from the produced water stream. But in some operating conditions and regulatory environments, they are not sufficient on their own. When the primary treatment train cannot consistently achieve the discharge limit or the injection quality required, a produced water polishing system is added as a final treatment stage to remove the fine residual oil and solids that primary treatment cannot reach. For a full explanation of the primary treatment stages that precede polishing, see What Is a Produced Water Treatment Package and How Does It Work?.
| Key thing to remember A produced water polishing system is not a replacement for primary treatment — it is a final stage added after hydrocyclones and flotation when the primary treatment outlet quality is insufficient to meet the discharge limit or injection quality requirement. The polishing system removes the fine residual oil and solids that primary treatment cannot capture. Without adequate primary treatment upstream, a polishing system will quickly become overloaded and ineffective. |
What Is a Produced Water Polishing System?
A produced water polishing system is a tertiary treatment stage installed at the outlet of the primary produced water treatment package to remove fine residual oil droplets, dissolved hydrocarbons and suspended solids that primary treatment stages — hydrocyclones and gas flotation — cannot adequately capture.
The term polishing reflects the function of the system: it is the final step that brings the produced water quality to the standard required for discharge or reinjection, removing the last traces of contamination that primary treatment leaves behind. The oil droplets that reach a polishing system are typically very fine — often below 5 to 10 microns — and are therefore difficult to separate using the centrifugal and flotation mechanisms of primary treatment.
Polishing systems use different technologies from primary treatment — typically granular media filtration, cartridge filtration or membrane systems — that capture fine particles by mechanical interception or adsorption rather than by gravity separation or flotation.
Why Does Primary Treatment Sometimes Fail to Achieve the Required Outlet Quality?
Primary produced water treatment is designed to remove bulk oil and larger oil droplets from the produced water stream. Hydrocyclones are effective at removing droplets above approximately 10 to 15 microns; gas flotation targets droplets in the 5 to 15 micron range. Together, they handle the majority of the dispersed oil in a typical produced water stream.
However, primary treatment can fail to achieve the required outlet quality in several situations:
Very fine oil droplets
Where the produced water contains a high proportion of very fine oil droplets — below 5 microns — primary treatment stages cannot capture them effectively. Very fine droplets are produced by stable oil-water emulsions, certain crude oil types with naturally small droplet size, or by excessive shear from pumps and valves in the treatment system.
High water cut and throughput
As water cut increases over the field life, the volume of produced water through the treatment package increases. If throughput exceeds the design capacity, residence time in the hydrocyclones and flotation units decreases — reducing separation efficiency and increasing residual oil at the primary treatment outlet.
Stable emulsions from certain crude types
Some crude oil types produce particularly stable oil-water emulsions that resist conventional hydrocyclone and flotation treatment. Where the emulsion is stabilised by natural surfactants, asphaltenes or production chemicals, primary treatment may achieve only modest oil reduction even under ideal operating conditions.
Tighter discharge limits than primary treatment can achieve
Where the field permit specifies a discharge limit that primary treatment cannot consistently achieve — for example, a monthly average below 15ppm in a field where primary treatment produces outlet concentrations of 20 to 30ppm — polishing is required to bridge the gap between primary treatment capability and permit requirement.
What Technologies Are Used in Produced Water Polishing Systems?
Several different technologies are used in produced water polishing systems, each with different performance characteristics, maintenance requirements and space and weight implications for FPSO topsides installation.
| Technology | Type | How it works | Typical outlet quality |
| Walnut shell filter | Granular media filter using crushed walnut shells as the filtration medium | Removes residual oil and fine solids; backwashable offshore without media replacement; widely used in FPSO produced water polishing | Moderate — effective to approximately 5 to 10ppm residual oil depending on feed conditions |
| Sand or anthracite filter | Granular media filter using sand or anthracite as the filtration medium | Removes fine solids and residual oil; backwashable; established technology with long track record in produced water applications | Moderate — similar to walnut shell; performance depends on media specification and operating conditions |
| Cartridge filter | Disposable or replaceable filter cartridges providing fine particle removal | Removes very fine solids and residual oil droplets; suitable as a final polishing stage or as pre-treatment ahead of membrane systems | High — can achieve below 1ppm residual oil with appropriate cartridge specification |
| Membrane filtration | Ultrafiltration or nanofiltration membrane systems | Removes dissolved hydrocarbons, very fine oil droplets and colloidal particles; required for produced water reinjection or very stringent discharge permits | Very high — capable of achieving below 1ppm residual oil and very low suspended solids |
| Flotation polishing | Additional gas flotation stage beyond primary IGF or DGF treatment | Removes fine oil droplets remaining after primary flotation; used where a single flotation stage cannot achieve the required outlet quality | Moderate to high — depends on feed quality and flotation unit design |
The choice of polishing technology depends on the target outlet quality, the volume of produced water to be treated, the characteristics of the produced water stream, the available space and weight allowance on the FPSO topsides, and whether the polished water will be discharged or reinjected.
When Is a Produced Water Polishing System Required?
A produced water polishing system is not required on every FPSO. The decision to include polishing in the treatment train depends on the gap between what primary treatment can achieve and what the discharge limit or injection quality requires. The table below summarises the main situations that trigger the need for a polishing system.
| Trigger for polishing | What it means in practice | How polishing addresses it |
| Stringent discharge permit limit | Field permit specifies a discharge limit below 15ppm, or requires a monthly average that standard primary treatment cannot reliably achieve | Add polishing stage to reduce residual oil to within permit limit consistently across all operating conditions |
| High water cut in mature field | Rising water cut increases produced water volume beyond treatment package design capacity, reducing residence time and treatment efficiency | Polishing stage compensates for reduced primary treatment efficiency as throughput increases |
| Produced water reinjection quality | PWRI requires tighter oil and solids specifications than overboard discharge — typically below 10ppm oil and below 2 to 5mg/l total suspended solids | Polishing and filtration stages are required to achieve injection water quality from a produced water stream |
| Challenging emulsion chemistry | Stable oil-water emulsions from certain crude types or chemical dosing regimes that primary treatment cannot adequately resolve | Polishing removes the fine emulsified oil droplets that resist conventional hydrocyclone and flotation treatment |
| Ecologically sensitive area | Operating in a marine protected area, OSPAR area or other ecologically sensitive location where regulators require best available technique | Polishing demonstrates that the operator is applying BAT to minimise the environmental impact of produced water discharge |
| Zero discharge requirement | Field permit or operator policy requires zero overboard discharge of produced water | Polishing provides the water quality required for full produced water reinjection, eliminating overboard discharge entirely |
The most common triggers are a stringent field permit discharge limit that primary treatment cannot consistently meet, and the requirement for produced water reinjection quality where tighter oil and solids specifications apply. For a full explanation of when reinjection quality specifications are more demanding than discharge limits, see What Is Produced Water Reinjection and When Is It Used?.
Where Does the Oil-in-Water Monitor Sit in a Polishing System?
In a produced water treatment train that includes a polishing system, the position of the oil-in-water monitor is critical. The monitor must be positioned after the polishing stage — at the outlet of the complete treatment train — not at the outlet of primary treatment and not between treatment stages.
Monitoring at the outlet of the polishing system is important because:
- The polishing system provides the final quality of the water that will be discharged or reinjected — only a reading taken after polishing reflects the actual quality of the discharge
- Monitoring before the polishing system would show primary treatment outlet quality, which may be higher than the discharge limit — using this reading as the compliance record would be misleading
- The polishing system may itself introduce changes to the produced water characteristics — for example, filter media may release fines during backwash — that the monitor at the final outlet will detect
- Where the polishing system is bypassed for maintenance or backwash operations, the monitor at the final outlet will detect the change in quality and can trigger an alarm or discharge suspension
The OCD Xtra is well suited to monitoring at the outlet of a produced water polishing system. Its 0 to 200ppm measurement range and ability to be calibrated on multiple crude oil types provides reliable measurement of the lower oil concentrations expected at the polishing system outlet — where readings may be in the 2 to 10ppm range rather than the higher concentrations typical of primary treatment outlets.
How Does a Polishing System Affect the Sample Conditioning Requirements for the Monitor?
The characteristics of produced water at the polishing system outlet are different from those at the primary treatment outlet — which has implications for the sample conditioning system design for the oil-in-water monitor.
At the polishing system outlet, the produced water typically has:
- Lower oil concentration — the polishing system has removed most of the residual oil, so the monitor must be capable of accurate measurement at lower concentrations than at the primary treatment outlet
- Lower solids content — filtration in the polishing system removes most of the fine solids, reducing the risk of solids interference in the monitor measurement cell
- Potentially higher dissolved hydrocarbons — membrane and cartridge filtration removes particulate oil but may leave dissolved hydrocarbons that optical monitors may not detect, depending on the measurement technology
- Different pressure and temperature — if the polishing system operates at different pressure or temperature from the primary treatment package, the sample conditioning system must be designed for the conditions at the polishing outlet specifically
The SMART PFM 107’s optical microscopy technology differentiates oil particles from gas bubbles and solids — making it robust against interference from the low-level gas and fine particles that may be present at a polishing system outlet. For a full explanation of how measurement technology choices affect monitor performance at low oil concentrations, see the Produced Water Discharge Monitoring for FPSOs explainer page.
What Should EPC Teams Consider When Designing a Polishing System?
For EPC process and instrumentation engineering teams, the key design decisions for a produced water polishing system include:
- Target outlet quality — confirm the discharge limit or reinjection quality specification that the polishing system must achieve, including oil concentration, total suspended solids and particle size where applicable
- Primary treatment outlet quality — confirm what the primary treatment package delivers to the polishing system inlet, which defines the polishing duty and the appropriate technology selection
- Produced water volume and variability — the polishing system must be sized for the maximum produced water flow rate expected over the field life, including the impact of increasing water cut
- Polishing technology selection — based on target outlet quality, feed characteristics, space and weight constraints, and maintenance access on the FPSO topsides
- Backwash and regeneration — granular media polishing systems require periodic backwash to restore performance; backwash water handling must be included in the system design
- Monitor position and sample conditioning — confirm the monitoring point is at the polishing system outlet and that sample conditioning is designed for the conditions at that specific point
- Monitoring technology selection — the oil-in-water monitor must be capable of accurate measurement at the lower concentrations expected at the polishing outlet; the SMART PFM 107 and OCD Xtra are both suitable depending on the application requirements
| Polishing system performance depends on primary treatment A polishing system that receives produced water with high residual oil from underperforming primary treatment will quickly become overloaded and will fail to achieve the target outlet quality. Polishing is a final quality step, not a substitute for adequate primary treatment. If primary treatment is underperforming, the root cause must be addressed — adding a polishing stage to compensate for primary treatment failure is not a sustainable solution. |
Frequently Asked Questions
What is a produced water polishing system?
A produced water polishing system is a tertiary treatment stage installed after the primary treatment package on an FPSO to remove the fine residual oil droplets and suspended solids that hydrocyclones and gas flotation cannot adequately capture. Technologies include walnut shell filters, cartridge filters and membrane systems. Polishing is used when the primary treatment outlet quality is insufficient to meet the discharge limit or the produced water reinjection quality specification.
When is a produced water polishing system required on an FPSO?
A polishing system is required when the primary treatment package cannot consistently achieve the field permit discharge limit, when produced water will be reinjected and a tighter quality specification applies, when high water cut reduces primary treatment efficiency, when stable emulsions from certain crude types resist conventional treatment, or when the operating location requires best available technique to minimise environmental impact.
What is the difference between primary produced water treatment and polishing?
Primary produced water treatment uses hydrocyclones and gas flotation to remove bulk dispersed oil and larger oil droplets — typically above 5 to 15 microns. Polishing uses granular media, cartridge or membrane filtration to remove the fine residual oil and solids that primary treatment cannot capture — typically below 5 microns. Polishing only works effectively when primary treatment is performing well; it is not a substitute for adequate primary treatment.
Where should the oil-in-water monitor be installed in a treatment train with a polishing system?
The oil-in-water monitor must be installed at the outlet of the polishing system — after all treatment stages are complete — not between treatment stages or at the primary treatment outlet. Only a reading taken after the polishing system reflects the actual quality of the water that will be discharged or reinjected. Monitoring at an earlier point in the treatment train would not constitute a valid compliance record for the discharge.
What oil-in-water monitor is suitable for a produced water polishing system outlet?
The OCD Xtra is well suited to polishing system outlet monitoring, providing accurate measurement across 0 to 200ppm and calibration on up to six crude oil types — capable of reliable measurement at the lower concentrations expected after polishing. The SMART PFM 107 is suitable where gas bubble interference is a concern at the polishing outlet. Both products are detailed on the Produced Water Discharge Monitoring for FPSOs explainer page.