Reverse osmosis recovery and concentrate: read the whole water balance

What recovery actually measures

Reverse osmosis divides the feed into permeate, the product stream, and concentrate, the stream carrying a greater share of the retained dissolved constituents. Recovery describes the proportion of feed water that becomes permeate. It is a volume or flow relationship, not a percentage describing how many pollutants have been removed.

For a simplified example, suppose a system receives 100 cubic metres per day and produces 75 cubic metres of permeate. If there are no other flows within the selected boundary, the remaining concentrate is 25 cubic metres per day. Recovery is 75 divided by 100, or 75 percent. These are illustrative numbers, not a recommended operating point for a particular water.

Why less concentrate can be harder to manage

If dissolved salts were completely retained and no salts precipitated, that example would place almost the same salt mass into a quarter of the feed volume. The resulting concentrate would be approximately four times as saline as the feed. Real calculations must account for salt passage, dosing, precipitation and the actual system boundaries.

Increasing recovery can reduce concentrate volume while increasing dissolved concentrations and scaling pressure. It may change energy needs, operating margins and cleaning requirements. The highest advertised recovery therefore does not automatically produce the most practical or economical project.

Choose the concentrate route early

Concentrate is not simply water that can be sent to any nearby drain. Assess its composition, volume and variability, together with the proposed receiving system. Depending on location and chemistry, options may involve an authorised sewer connection, an approved discharge route, further treatment or off-site management. Each option requires its own technical and regulatory assessment.

A useful comparison should show:

• Permeate production and concentrate flow over the operating year.
• Predicted concentrate composition under the chosen design assumptions.
• Other residual streams, including pretreatment and cleaning wastes.
• Storage, monitoring and contingency arrangements.
• Capital and operating costs for the complete management route.

Do not transfer a concentrate solution from a coastal project to an inland site without checking the receiving conditions and permissions.

Compare like with like

Distinguish membrane-stage recovery from overall plant recovery. Backwashing, flushing, cleaning and any additional treatment can change how much usable water the entire site obtains. Define the measurement boundary before comparing supplier figures.

Request alternatives based on the same feed analysis and product-water specification. Compare annual useful output, reliability and total operating cost alongside recovery. A sound concept shows both where the product water will be used and where every residual stream will go. If the concentrate route remains unresolved, it is still a major project decision rather than a minor connection detail.