Paths to Improve Boron Rejection using 2-Pass RO

Typical Boron RO Rejection Expectations

Without pH adjustment, a 2-pass RO may not reliably achieve low boron (especially if target is <100 ppb).
With interpass pH adjusted >9.5 and a high-boron-rejection 2nd-pass RO membrane like ExcellPureRO, many systems see substantial improvement (a several-fold reduction in boron permeate concentration).

Raise feed pH to the 2nd-pass RO into the ~9.5–9.8 range so boric acid converts to borate ion (B(OH)₄⁻) which RO rejects much better. CO2 is also minimized.
Use a tighter/high-rejection membrane for the 2nd pass (low MWCO / high-boron rejection type).
Operate at higher RO flux in the 2nd pass (improves rejection). 2nd pass can run at 30+ GFD.
Operate the EDI at higher than normal voltage to encourage water splitting. Minimize FCE fed to the EDI. CO2 is minimized because of interstage pH.
Add a polishing resin (boron-selective media) downstream. This also affords boron security against RO-EDI upsets.

Boric acid pKa ≈ 9.2. Below that most boron is neutral boric acid → passes the membrane. Above that it converts to borate ion → much higher RO rejection.
Practical target: dose NaOH so the feed to the 2nd pass is 9.5–9.8. Use an accurate pH sensor like pHure from Mettler-Toledo. At pH 9.5 boron is 66% ionized. At pH 9.8 boron is 80% ionized. With a boron-specific RO membrane that rejects both ionic and uncharged boric acid the RO is optimized.
Tradeoffs: raising pH increases NaOH passage, so slightly increases 2nd Pass RO permeate conductivity.
Read more about boron in water here.

Use membranes optimized for boron/high salt rejection in the 2nd pass (higher silica/boron rejection chemistry, lower MWCO). This often helps without huge operational changes.
Higher flux (increase permeate flow per element in 2nd pass) — increase rejection by driving transport conditions favorable to rejection.
Run slightly higher feed pressure if needed to maintain permeate but be mindful of membrane limits.
Increase stage recovery distribution rather than forcing a single stage high recovery. Two-stage staging can improve permeate quality.

Without pH adjustment, a 2-pass RO may not reliably get very low boron.
With pH adjusted >9.5 and a high-rejection 2nd pass membrane like ExcellPureRO, many systems see substantial improvement (often several-fold reduction in boron permeate concentration).