Rising RO permeate conductivity: causes and diagnosis (SWRO/BWRO)
When the conductivity of the permeate rises, you don't want to “guess”. The sequence that works best in the field is: (1) confirm measurement and temperature, (2) classify whether the rise was sudden or gradual, and (3) determine whether the problem affects one pressure vessel or the whole system. DuPont approaches it as troubleshooting by symptoms (salt passage/flow/ΔP) to get to the cause.
Step 0 (SWRO and BWRO): ensure “conductivity” refers to the same measurement
Conductivity (EC) vs TDS (ppm)
EC (conductivity)usually in µS/cm (permeate) or mS/cm (feed).
TDS (ppm/mg/L)is usually an approximate conversion made by the equipment (it is not universal, it depends on the water).
To measure “right” in industry, it is recommended to standardise method and calibration (constant cell with KCl) as Standard Methods 2510B (NEMI summary) and equivalent practices.
Most important: EC25 (conductivity compensated at 25 °C)
Conductivity changes with temperature. In quality control it is used “specific conductance at 25°C”(corrected to 25 °C) in order to be able to compare readings. USGS makes it clear: it is reported as specific conductivity at 25 °C (corrected value).
Checklist (5 min):
Does your SCADA/probe show @25°C / EC25 / 25°C?
If it does not compensate for temperature, USGS recommends recording the raw data and converting to 25°C with manufacturer's factors.
Have you compared with a portable conductivity meter at the same point?
Step 1 (SWRO and BWRO): SUDDEN or GRADUAL rise?
Sudden or gradual increase: two diagnostic patterns
| Pattern | Time scale | Priority checks |
|---|---|---|
| Sudden | Minutes or hours | Mechanical bypass at O-rings or brine seals, a leak in a pressure vessel, and faulty seals or adapters. |
| Gradual | Days or weeks | Soiling (fouling), encrustations (scaling), chemical damage or oxidation, and membrane aging. |
The pattern guides the investigation; by itself it does not confirm the cause. Compare operating conditions and verify the instrumentation.
If the rise is SPONTANEOUS (minutes/hours)
It is usually bypass/internal leakage. Toray recommends straightforwardly: if the conductivity of the permeate is much higher than expected, check O-rings, brine seals and components of the affected pressure vessel.
If the rise is GRADUAL (days/weeks)
It is often linked to fouling/scaling, off-design operation, ageing/damage or chemical. DuPont mentions that the solute passage may remain normal at first and increase when fouling becomes “massive”.”, and recommends monitoring ΔP in stages as a sensitive indicator.
Step 2 (SWRO and BWRO): LOCALISED (one pressure vessel) or GENERAL?
Locate the problem by pressure vessel (the most cost-effective approach)
DuPont recommends (even at start-up) to review the permeate conductivity of each pressure vessel to detect, for example, Leaking O-rings or other anomalies.
Conductivity profile (if available)
DuPont explains that a “normal” profile shows a progressive increase towards the concentrate end; a large deviation locates the source of the high salt passage.
High conductivity: what to check first
- Problem localised in one pressure vessel: Check O-rings, brine seal and possible internal leaks or bypasses.
- General problem in several stages: check if the pressure loss increases (ΔP).
- With ΔP increasing: investigate fouling and scaling.
- Without an increase in ΔP: reviewing non-design conditions, possible damage, and bypasses.
- In all cases: check salt passage and profile readings by pressure vessel to locate the source.
These checks complement the conductivity trend of the permeation corrected by temperature; they do not replace the complete diagnosis of the plant.
Don't use SDI as absolute truth. Use it as a signal: SDI + ∆P in filters + cartridges + RO trend is what real history tells you.
SWRO (seawater) diagnostics
What SWRO usually commands
In SWRO, as the feed is very saline, avoid obsessing over the “absolute” number and focus on:
EC25 trend of permeate
salt passage / rejection
localisation by pressure vessel
relationship with ΔP
Typical SWRO causes (practical priority)
1) Mechanical Bypass (O-rings / brine seal / connectors)
Typical signature: sudden and/or localised rise.
Toray: check O-rings and brine seals when permeate comes out very conductive.
DuPont: take readings by pressure vessel to identify vessels with leaking O-rings.
Action (today):
Locate the pressure vessel → inspect O-rings and the brine seal → correct the fault and measure again.
2) Mechanical damage of the element / telescoping / incorrect mounting
DuPont describes mechanical damage (e.g. telescoping) and mentions cases where O-rings are not installed or incorrectly installed after loading.
3) Fouling / scaling (gradual) + ΔP
If conductivity rises and also ΔP tends to rise, it reinforces the fouling hypothesis; DuPont addresses the increase in solute passage associated with fouling and recommends instrumentation of ΔP per stage.
4) Off-design operation (recovery/flow/pressure)
DuPont recommends operating at design flow and recovery, and using simple mass balances to validate instruments.
SWRO Checklist (30 min)
Confirm EC25 + compare with a portable conductivity meter.
Sudden vs. gradual.
Localise by pressure vessel if possible.
If sudden/localised → O-rings/brine seal.
If gradual → crosses with ΔP/operation and decides fouling/scaling vs. operation/damage.
BWRO diagnosis (brackish water)
Most confusing in BWRO: CO₂ and pH (conductivity “rises” without being salts passing through).
Hydranautics/Nitto explains a key point: CO₂, being a gas, is neither rejected nor concentrated by the membrane., concentration can be similar in feed/permeate/concentrate; and in addition the accuracy of high quality readings benefits from in situ measurement because CO₂ can vary when exposed to the atmosphere.
What it means in practice: in BWRO, a rise in conductivity can be influenced by carbon-inorganic balances (CO₂/bicarbonate/carbonate) and sampling conditions, before it is “membrane failure”.
Typical BWRO causes (practical priority)
1) CO₂/pH and sampling conditions
Typical signature: conductivity changes without ΔP/flux indicating clear deterioration.
Action (today):
Confirms EC25.
Add pH (and if possible alkalinity) to the event log.
2) Mechanical bypass (O-rings / brine seal)
It still applies exactly the same whether it is sudden or localised.
3) Fouling/scaling (gradual)
DuPont: solute passage increases as fouling progresses; ΔP per stage is sensitive indicator.
4) Off-design operation / instruments
DuPont: using mass balances to validate instruments and operate at design conditions
BWRO Checklist (30 min)
Confirm EC25 and compare with a portable conductivity meter.
If it does not fit with the rest → check CO₂/pH (early hypothesis).
Sudden vs. gradual.
Localise by pressure vessel to rule out O-ring leaks.
Cross with ΔP/operation to close cause.
Simple formulas for comparison (SWRO and BWRO)
- Rejection (%) = (1-Cp/Cf)×100
- Salt passage (%) = (Cp/Cf)×100
Always use the same basis (ideally EC25) for the trend to be real.
Frequently Asked Questions (FAQ)
What is EC25 and why do I need it?
Because conductivity is temperature dependent; USGS reports specific conductivity corrected to 25°C to compare measurements.
If it goes up suddenly, what do I look at first?
O-rings, the brine seal and the affected pressure vessel. Toray explicitly recommends this.
In BWRO, can it be CO₂?
Yes: Hydranautics/Nitto explains that CO₂ is not rejected by the membrane and can distort quality interpretation if conditions are not controlled.
For those who want to get down to the nitty-gritty
If you are serious about comparing performance, use standardisation (temperature, net pressure, etc.). DuPont recommends validating instruments and design conditions; there are standardisation methodologies widely used in the industry.
Recommended external sources
- DuPont FilmTec - “Symptoms of Trouble, Causes and Corrective Measures”.”
Symptom diagnosis guide (salt passage / flow / ΔP) with typical causes and corrective measures. - DuPont FilmTec - “Troubleshooting Grid”.”
Practical matrix for cross-referencing changes in permeate flow, salt passage and differential pressure and guide the diagnosis without assumptions. - DuPont FilmTec - “Start-Up Sequence”.”
Very useful operational recommendation: check the permeate conductivity by pressure vessel to detect anomalies (e.g. leaking seals/rings). - DuPont FilmTec - System Tests“
System testing and approach to profiling (conductivity profile) to locate the source of a salt passage increase. - Toray - “RO Operation, Maintenance, and Handling Manual”.”
Direct reference for mechanical failures: if the conductivity of the permeate is much higher than expected, check O-rings, brine seals and components of the affected pressure vessel. - USGS - “Specific Conductance (A6.3)”.”
Measuring and reporting basis: specific conductance corrected to 25 °C (EC25), calibration, procedures and troubleshooting. - EPA - Method 120.1 (Specific Conductance @25 °C)
Classical and often quoted method: report specific conductance at 25 °C and measurement guidelines. - NEMI - Summary of Standard Methods 2510B (Conductivity)
Good “method” link: cell constant, calibration with KCl and range of the test.