Test Procedure: Filterability Index


The Filtrability index is used to determine the filtrability of a water sample as compared to a blank. It is essential to run a control sample with the water treatment plant’s existing treatment protocol. The sample should be drawn from the effluent water of settling basins. If a comparative filter index can be achieved using the proposed treatment versus the existing treatment, then chances for a successful treatment modality are good.


a. Collect several samples of the plant’s filtered water and measure the turbidity.

b. Using a control sample conduct a filterability index with a 0.8 micron membrane, collect the filtrate and measure its turbidity. Compare this turbidity to those measured in (a). If the plant filtered turbidity is lower, use the 0.45 micron membrane. If it is higher, then use the 1.2 micron membrane.

Establishing a correlation between the control jar and plant operating conditions would enable one to determine whether or not comparable data could be obtained with the proposed treatment.

  1. Place a membrane filter on the supporting plate on the bottom half of the filtration unit, attach the top half and screw it on tightly.
  2. Wet the filter with distilled water.
  3. Attach vacuum device and regulate vacuum at 15” of mercury.
  4. Measure out a 200 ml portion of supernatant.
  5. Pour the sample into the funnel, start the vacuum pump, and start the stopwatch as soon as the water passes through the filter and hits the bottom of the filtration unit.
  6. Stop the watch at the moment the last drips of water pass through the filter.
  7. Divide the number of seconds required for filtration under these conditions by the number of seconds required for filtration of 200 ml of distilled water at 20°C to obtain the filterability index or number. This number is dimensionless.

Note: A blank is essential for all evaluations. Recheck the blank whenever using membranes obtained from different boxes. Variations up to 30% in the filter times of distilled water have been found between different batches of filter pads. Also recheck the blank whenever the source of distilled water is changed.


The optimum dosage selection should have equivalent or slightly lower filterability index and filtered turbidities in comparison to a control samples and/or actual plant operation. A high filtrate turbidity and a low filterability index could represent breakthrough. Inversely a low filtrate turbidity and a high filterability index could be an indiction of filter blinding.


F.I. = seconds sample/seconds blank

Example: blank required 33 seconds while treated water sample required 43 seconds. F.I. = 43/33 = 1.30


1 – Millipore filitration system

1 – Vacuum pump

Filter membranes

1 – Stopwatch – Any type

1 – 250 ml graduate

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