Up RO permeate conductivity: causes and diagnosis (SWRO and 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, y (3) identify whether it is a vessel or general problem. DuPont approaches it as troubleshooting by symptoms (salt passage/flow/ΔP) to get to the cause.
Step 0 (SWRO and BWRO)Make sure that “conductivity” always means the same thing.
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 laptop at the same point?
Step 1 (SWRO and BWRO): REPENTENTINE or GRADUAL rise?
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 affected vessel parts.
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)LOCATED (1 vessel) or GENERAL?
Localise by vessel (most cost-effective)
DuPont recommends (even at start-up) to review 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.
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
vessel-based localisation
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: measure by vessel to identify vessels with leaking O-rings.
Action (today):
Locate vessel → check O-rings/brine seal → correct and re-measure.
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 laptop.
Sudden vs. gradual.
Locate by 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 + compare with laptop.
If it does not fit with the rest → check CO₂/pH (early hypothesis).
Sudden vs. gradual.
Locate by vessel to rule out O-rings.
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/brine seal and the affected 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 y differential pressure and guide the diagnosis without assumptions. - DuPont FilmTec - “Start-Up Sequence”.”
Very useful operational recommendation: check the permeate conductivity per 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 affected vessel parts. - 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.