Water tools

Normalise RO operating data to standard conditions

Raw permeate flow can stay constant while the membrane fouls, because operators raise pressure to hold it. Normalising to one reference condition strips out temperature, pressure and feed quality and leaves the membrane's own trend.

Measured permeate flowm³/dAny unit; the standardised flow has the same unit.

Feed pressurebar

Concentrate pressurebar

Permeate pressurebar

Feed TDSmg/L

Permeate TDSmg/L

Recovery%

Feed temperature°C

Standard feed pressurebarReference (manufacturer or start-up) condition.

Standard head loss per elementbar

Elements per vessel

Standard feed TDSmg/L

Standard permeate TDSmg/L

Standard recovery%

Standard temperature°C

Feed-concentrate averaging

Standardised permeate flow

10,801m³/d

Compare this figure between dates: a fall of 10 to 15% from the clean baseline is the usual cleaning trigger.

Standardised salt passage

5.72%

Measured salt passage

4.60%

Temperature correction factor

1.513

Net driving pressure, measured

26.54bar

Average feed-concentrate TDS

7,828mg/L

Osmotic pressure in the channel

6.33bar

How it works

Net driving pressure from the measured pressures and the osmotic pressures of the average feed-concentrate and the permeate:

N D P = \frac{P_{F} + P_{C}}{2} - P_{P} - (π_{F C} - π_{P}) C_{F C} = \frac{C_{F}}{r} \ln \frac{1}{1 - r}

(1)

Standardised permeate flow and salt passage, with the temperature correction factor unless the manufacturer supplies one:

Q_{P, S} = Q_{P, M} T C F \frac{N D P_{S}}{N D P_{M}} T C F = {1.03}^{T_{S} - T_{M}}

(2)

S P_{S} = S P_{M} \frac{N D P_{M}}{N D P_{S}} \frac{C_{F C, S}}{C_{F C, M}} \frac{C_{F, M}}{C_{F, S}}

(3)

where

$N D P$: net driving pressure, bar
$P_{F}, P_{C}, P_{P}$: feed, concentrate and permeate pressure, bar
$π_{F C}, π_{P}$: osmotic pressure of the average feed-concentrate and the permeate, bar
$C_{F C}$: average feed-concentrate concentration, mg/L
$C_{F}$: feed concentration, mg/L
$r$: recovery, fraction
$Q_{P}$: permeate flow
$T C F$: temperature correction factor
$T_{S}, T_{M}$: standard and measured temperature, °C
$S P$: salt passage, permeate over feed concentration
$S, M$: standard and measured condition

Standardise each stage separately in multi-stage systems. The 1.03 per degree factor is a viscosity approximation; the standard condition must fix feed flow and feed concentration as well as pressure. Different manuals normalise in slightly different ways; use one consistently.

Normalise RO operating data to standard conditions

How it works

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