Work out ro reject volume instantly with clear inputs, formula shown and shareable results.
Reject flow is feed flow times one minus recovery. At 75% recovery a quarter of the feed leaves as concentrate carrying four times the feed salinity. Disposal is often the binding constraint on inland desalination, since the concentrate cannot generally be discharged to a watercourse or sewer without consent.
Reject flow
Reject = feed flow x (1 - recovery)
Disposal cost
Annual cost = reject flow x operating hours x 365 x disposal cost per m3
RO concentrate discharge is subject to environmental permitting because of its salinity and antiscalant content. Confirm consent conditions before commissioning.
Higher recovery with antiscalant and pH control, a second-pass brine concentrator, or zero liquid discharge with evaporation and crystallisation. Each step raises capital and energy cost sharply.
Sometimes for cooling tower make-up, dust suppression, salt-tolerant irrigation or industrial washing, depending on its composition and local rules. Beneficial reuse usually needs a specific permit.