RO Rejection — Industry Standard (Window Washing)

Acceptance threshold:

• ≥80% rejection is acceptable per manufacturer.
• (Typical when new: ~80–95%. Below 80% → troubleshoot/plan replacement.)

Common factors that impact rejection:

• Water temperature: Colder water reduces rejection and output rate.
• Pressure at the membrane: Low/unstable pressure lowers rejection. (Aim for strong, steady pressure.)
• Prefilter condition: Clogged sediment/carbon reduces performance; replace on schedule.
• Chlorine breakthrough: Inadequate carbon allows chlorine to damage the membrane.
• Source TDS variability: Seasonal or site-to-site changes affect the math.

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Window Washing Pure Water Industry Standard

For a spot-free finish, the industry standard is to clean with pure water under 20 ppm TDS, measured at the brush.

• How to verify: Run the system for 60–90 seconds, then test TDS at the brush head with a handheld meter.
• If it’s 20 ppm or higher: Replace the DI resin and re-test before cleaning.
• Why it matters: At this level, there isn’t enough dissolved mineral content to leave visible spotting when the water dries under normal conditions—assuming good technique (adequate rinse, don’t over-wet frames, etc.)..

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Before You Start

• For multi-stage systems, close the flush valve during testing.
• Let water run 30–60 seconds before taking readings at each point. Test again until you get a stable reading.

What Each Reading Means

• TAP PPM – Source water TDS from the faucet.
• RO PPM – TDS after RO membrane(s).
• Flush PPM – TDS from the RO flush line (should be higher than RO PPM).
• DI Removed PPM – Sanity check; should be within ±2 of RO PPM.
• DI PPM – Final water after DI resin.

Pass/Fail Targets

• RO Efficacy = ((TAP − RO) / TAP) × 100
  • ≥ 80% = Normal
  • 75–79% = Low → plan to replace
  • < 75% = Critically low → replace now
• DI PPM
  • ≤ 15 ppm = Good
  • 16–20 ppm = Marginal → replace soon
  • > 20 ppm = Replace DI

Diagnostic Flow

• No flush flow? Slightly open the flush valve:
  • If flow appears → replace the flush valve.
  • If still no flow → replace the RO membrane.
• Low pressure? Check for leaks at carbon/DI housings and fittings. If none, replace the carbon/combo filter.
• Single Stage:
  • Compare Out System → target ≤ 15 ppm (≤ 20 ppm marginal).
  • Out Tubing should not exceed Out System; if higher, replace tubing.
  • Out Brush should not exceed Out Tubing; if higher, inspect brush tubing.

Tips

• When DI PPM > RO PPM, resin is exhausted and releasing ions — replace DI.
• For refillable DI, ensure proper packing and correct orientation (brine seal on inlet side).
• High TAP (> 100 ppm) will consume DI faster; consider RO system or expect frequent DI changes.