Water tools

Permeate blending to a target TDS

If the permeate is cleaner than the product needs to be, part of the raw water can bypass the membranes. The blend ratio is a single mass balance; the savings are real.

Permeate TDSmg/L

Raw (bypass) water TDSmg/L

Target product TDSmg/L

Product flowm³/h

RO recovery%

Permeate share of product

72.5%

Bypass share of product

27.5%

Permeate flow needed

108.7m³/h

RO feed flow needed

144.9m³/h

Membrane capacity saved

27.5%

Relative to treating the whole product flow through the RO.

How it works

Mass balance on the blend of permeate and bypassed raw water:

f_{p e r m} = \frac{C_{r a w} - C_{t a r g e t}}{C_{r a w} - C_{p e r m}} Q_{F} = \frac{f_{p e r m} Q_{p r o d}}{r}

(1)

where

$f_{p e r m}$: share of permeate in the product
$C_{r a w}, C_{p e r m}, C_{t a r g e t}$: TDS of the raw water, the permeate and the product, mg/L
$Q_{F}$: feed flow to the RO, m³/h
$Q_{p r o d}$: product flow, m³/h
$r$: recovery, fraction

Bypass water is unchanged raw water, so pretreatment, disinfection and any other constituent (hardness, silica, pathogens) must be checked separately before bypassing. Blending only reduces membrane area when the permeate quality sits well below the target; near the target the permeate share tends to one and nothing is saved.

Permeate blending to a target TDS

How it works

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