Start with an unambiguous water balance
Overall recovery is Y = Qp/Qf, where Qp is permeate and Qf is external feed. Distinguish overall recovery from stage conversion and internal recycle. Counting a stream returning to the skid as new feed can produce a misleading percentage and obscure the actual source-water demand.
For 10 m³/h of feed, ideal recovery of 75% gives 7.5 m³/h permeate and 2.5 m³/h concentrate. At 80%, those values become 8 and 2 m³/h. The product increase looks modest, but the smaller concentrate stream must carry nearly the same dissolved-salt load.
| Ideal recovery | Permeate from 10 m³/h feed | Concentration factor |
|---|---|---|
| 75% | 7.5 m³/h | 4 |
| 80% | 8.0 m³/h | 5 |
| 85% | 8.5 m³/h | 6.67 |
Check concentration and chemistry at the limiting condition
Assuming complete rejection and no precipitation, the ideal concentration factor is 1/(1−Y): it rises from 4 at 75% recovery to 5 at 80%. This is a first approximation, not a replacement for an ionic projection, salt passage or concentration at the membrane surface.
Assess carbonate, sulphate, silica and other relevant constraints using representative pH and temperature. Any chemical needs a documented efficacy envelope. Seasonal variation may change which salt limits the plant. Include margin against an unfavourable scenario rather than designing only around a convenient laboratory sample.
Review individual elements and train hydraulics
An apparently acceptable overall recovery can conceal excessive loading of lead elements or insufficient concentrate flow at the tail. The projection should show flow, pressure, quality and concentration by stage and, where necessary, by element. Vessel quantity and stage arrangement materially affect these conditions.
Available pressure, pumping power, permeate backpressure and pretreatment capacity also matter. Recycle can maintain feed-channel flow, but it changes salinity, energy use and local recovery. It does not remove the chemical limit that prompted the proposed modification, and its effect needs to be included in the full balance.
Request recovery with defined acceptance conditions
Specify usable flow, product quality, permitted feed range, design temperature and consumptions, using the same supply boundary for all quotations. Clarify the response when feedwater falls outside that envelope and how operation can be adjusted. Annual availability matters as much as a steady-state percentage.
For an upgrade, retain comparable before-and-after references and track cleaning frequency and pressure drop. A higher setting that works only briefly before CIP may reduce annual usable water production. Evaluate recovery together with available operating hours and the cost of the complete treatment system.
Common questions
Does recovering more water eliminate the salt?
No. RO separates streams and concentrates retained constituents into a smaller volume. That load needs a destination and affects whether the proposed recovery is viable.
Do more stages guarantee higher recovery?
No. They may improve hydraulic distribution, but do not remove solubility, pressure, fouling or minimum element-flow limits.