Water tools

Filter run volume, backwash volume and net water recovery

Every backwash and every filter-to-waste period costs product water. Expressed per square metre of filter they tell you how much of the throughput the plant actually keeps.

Filtration ratem/h

Filter run lengthh1 to 4 days common; at least a day is desirable.

Backwash ratem/h

Backwash durationmin

Low-rate stage rate (with air scour), if anym/h

Low-rate stage durationmin

Filter-to-waste durationmin

Target recovery%

Net recovery

98.2%

Unit filter run volume

900m³/m²

At least 200 m³/m² is needed for 95% recovery with typical wash volumes.

Unit backwash volume

10.0m³/m²

Unit filter-to-waste volume

6.3m³/m²

Target

Run volume needed for the target

325m³/m²

Run length needed for the target

26.0h

Washwater as share of production

1.8%

How it works

U F R V = v_{F} t_{F} U B W V = \sum_{i} v_{B W, i} t_{B W, i} U F W V = v_{F} t_{F T W}

(1)

r = \frac{U F R V - U B W V - U F W V}{U F R V} U F R V_{n e e d e d} = \frac{U B W V + U F W V}{1 - r^{\cdot}}

(2)

where

$U F R V$: unit filter run volume, m³/m²
$v_{F}$: filtration rate, m/h
$t_{F}$: run length, h
$U B W V$: unit backwash volume, m³/m²
$v_{B W, i}, t_{B W, i}$: rate and duration of backwash stage i, m/h and h
$U F W V$: unit filter-to-waste volume, m³/m²
$t_{F T W}$: filter-to-waste duration, h
$r$: net recovery, fraction
$r^{\cdot}$: target recovery

Typical wash volumes are about 8 m³/m² per wash, so runs shorter than about 200 m³/m² drop below 95% recovery. Excessive backwash lengthens ripening, which lowers recovery further. The recovery counts only backwash and filter-to-waste, not other plant losses. Filters are sized to pass plant flow at the design rate with one unit out for washing.

Filter run volume, backwash volume and net water recovery

How it works

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