- Enter the feed flow rate in liters per hour (L/hr) reaching the RO unit.
- Enter the Pass 1 recovery percentage (typically 50–75% for brackish/high-TDS feed, higher for low-TDS municipal feed).
- Check "Include Pass 2 (dual-pass)" if your system polishes Pass-1 permeate through a second RO pass, and enter the Pass 2 recovery (typically 85–90%+).
- Read the permeate, reject, and overall recovery results below.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this whenever you are planning water logistics for a brackish or high-TDS source (reject volume affects feed-water consumption, trucking, and disposal), evaluating a vendor's dual-pass RO proposal, or want to understand how much of your feed water actually becomes usable product water.
Appropriate use
Feed-water logistics planning for coastal/island facilities, vendor-proposal sanity checks, and understanding the water-economy trade-off of adding a second RO pass.
When NOT to rely on it
This calculates flow volumes only — it does not determine whether a given feed water's TDS actually requires dual-pass RO, and it does not model optional reject-recirculation (Pass-2 reject blended back into Pass-1 feed), which needs a facility-specific engineering review to confirm it won't reconcentrate problem species.
Pearls & Pitfalls
Dual-pass recovery multiplies, it doesn't average
Overall recovery for a dual-pass system is Pass 1 recovery × Pass 2 recovery — not their average. A 65% Pass 1 feeding an 88% Pass 2 gives roughly 57% overall recovery, not 76.5%. This is why dual-pass systems, despite each pass individually recovering a majority of their feed, can still show markedly lower overall recovery than a single-pass system on cleaner water.
Reject volume is a real logistics number, not just an engineering abstraction
For a small coastal or island facility trucking in feed water, the reject volume this calculator reports is water that must be sourced, paid for, and disposed of — every day, indefinitely. Low overall recovery on a brackish source is a genuine operating-cost driver, not just a design detail.
Pitfalls
(1) Entering an unrealistically high Pass 1 recovery for a high-TDS feed — pushing recovery too high on brackish water increases scaling/fouling risk at the membrane. (2) Forgetting that Pass 2 feed is Pass 1's permeate, not the original feed — Pass 2 recovery should be higher because it's working on already-clean water. (3) Assuming reject-recirculation is a free water-economy win without verifying it won't reconcentrate a specific contaminant for your feed water.
RO Recovery Calculator
Enter feed flow and Pass 1 recovery. Add Pass 2 for a dual-pass system. Permeate, reject, and overall recovery appear below.
⚕ Calculates flow volumes only — does not determine whether dual-pass RO is required for your source water, or model optional reject-recirculation. See the Water Treatment Systems reference §4.3 and §6.3. For biomedical engineers and RO commissioning engineers; not a substitute for a validated facility engineering assessment.
Next Steps
Use the reject volume and overall recovery to plan feed-water logistics and evaluate whether a vendor's dual-pass proposal is reasonable.
- Size feed-water supply (trucking, cistern volume, well capacity) around the total feed flow (product + reject), not just the product water figure.
- Confirm reject-disposal capacity can handle the calculated reject volume, especially for coastal/island facilities with limited drainage options.
- If overall recovery seems unexpectedly low, verify the Pass 1 recovery assumption against your actual feed-water TDS — very high-TDS feed may need an even more conservative Pass 1 recovery to protect the membrane.
- See the full Hemodialysis Water Treatment Systems guide for brackish-water design and dual-pass RO facility layout.
Evidence & References
Formula
| Quantity | Formula |
|---|---|
| Pass 1 permeate (L/hr) | feed flow × (Pass 1 recovery / 100) |
| Pass 1 reject (L/hr) | feed flow × (1 − Pass 1 recovery / 100) |
| Pass 2 permeate (L/hr), if enabled | Pass 1 permeate × (Pass 2 recovery / 100) |
| Pass 2 reject (L/hr), if enabled | Pass 1 permeate × (1 − Pass 2 recovery / 100) |
| Overall recovery | final product water ÷ feed flow × 100% |
Typical Pass 1 (50–75%) and Pass 2 (85–90%+) recovery ranges reflect commonly cited brackish-water RO engineering practice; always verify against your specific membrane manufacturer's design software and your facility's actual feed-water TDS.
References
- Kasparek T, Rodriguez OE. What Medical Directors Need to Know about Dialysis Facility Water Management. Clin J Am Soc Nephrol. 2015;10(6):1061-1071.
- Humudat YR, Al-Naseri SK, Al-Fatlawy YF. Assessment of microbial contamination levels of water in hemodialysis centers in Baghdad, Iraq (referencing AAMI and ISO 23500 series limits). Water Environ Res. 2020;92(9):1325-1333.
