Optimum pH for Double-Pass RO Operation
Recommended interpass pH: 8.4–8.7
Introduction:
Ultrapure water (UPW) systems used in power generation and semiconductor manufacturing require very low levels of silica and boron. These specifications are typically achieved using double-pass reverse osmosis (RO) followed by electrodeionization (EDI).
In EDI, dissolved carbon dioxide competes with silica removal because carbonic species consume ion-exchange capacity. Therefore, effective removal of inorganic carbon across the two RO passes is essential for high silica removal efficiency downstream.
This document describes the chemical basis for selecting the optimum interpass pH to maximize CO2 removal and silica and boron rejection.
First Pass RO:
The first pass typically uses a low-energy, high-rejection thin-film composite (TFC) polyamide membrane. Its primary function is removal of:
- dissolved salts (conductivity)
- hardness ions (Ca²⁺, Mg²⁺)
- sulfate and other divalent ions
The main species that passes through the first pass is inorganic carbon (CO2) and silica (SiO2)
The first pass can be a low-energy, high-rejection RO. It is designed to remove the bulk of the conductivity and the divalent ions (hardness, sulfate). The problem is CO2 and HCO3–.
Carbonate Chemistry Limitation
TFC polyamide membranes reject anions primarily through charge exclusion associated with deprotonated surface carboxyl groups (–COO⁻).
Carbonate speciation:
CO2+H2O ⇌ H2CO3 ⇌ HCO3− ⇌ CO32−
At approximately pH 6.5, about half of the dissolved inorganic carbon exists as uncharged dissolved CO₂. Neutral CO₂ permeates the membrane readily, while bicarbonate (HCO₃−) is rejected.
Therefore, the first pass removes salts efficiently but removes CO₂ poorly.
pH to the first pass, therefore, should be much higher than 6.5 for good rejection.
Second Pass RO:
The second pass should use a high-rejection membrane optimized for silica and boron removal and should be designed to operate at standard (high) pressure (150 psi / 10 bar).
pH adjustment with NaOH between the first and second pass (interpass) converts dissolved CO₂ into ionic species ( HCO₃– ) that will be highly rejected.
Effect of Interstage pH
1. Membrane Charge and Anion Rejection
Increasing feed pH increases membrane surface ionization, strengthening electrostatic repulsion, and improving rejection of anions including HCO₃− and silica and borate species.
2. Carbonate Speciation
Above pH ~8.2: CO2+H2O → H2CO3 → HCO3−
Bicarbonate and carbonate ions are strongly rejected by TFC membranes at higher pH. Thus, raising pH converts permeable CO₂ into rejectable ions.
3. Silica Rejection
Silica rejection increases with pH because a larger fraction exists as negatively charged silicate species. Very high pH (e.g., HERO-type processes) maximizes silica rejection but introduces other costs.
4. Hydroxide Leakage
At elevated pH, hydroxide becomes the dominant anion. TFC membranes reject OH⁻ poorly; therefore sodium hydroxide permeates, increasing permeate conductivity.
This effect becomes significant above approximately pH 9.
5. Scaling
Because hardness is removed in Pass-1, moderate pH elevation does not promote calcium carbonate scaling. However, excessive pH (> ~9.2) increases scaling risk and operational instability.
Optimum Interstage pH
The interpass pH operating window balances four competing effects:
| pH Region | Result |
| < 7.5 | Some CO₂ remains uncharged → passes through membrane |
| 8.4–8.7 | CO₂ converted to bicarbonate/carbonate and rejected |
| > 9.0 | OH⁻ leakage increases permeate conductivity |
| > 9.2 | Scaling risk increases |
Therefore, the recommended interpass pH is 8.4–8.7
Summary:
The interpass pH of 8.4–8.7 provides the best overall performance for double-pass RO:
- Converts dissolved CO₂ to rejectable carbonate species
- Maximizes second-pass silica rejection
- Maintains low permeate conductivity
- Avoids scaling because hardness is removed in the first pass
Operating below this range allows CO₂ leakage.
Operating above this range increases hydroxide passage and scaling risk.
Contact SnowPure for more information.
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