Determining When a Water Filter Needs to be Changed

Water filters have a finite capacity to adsorb contaminants before they need to be changed.    The amount of water that can pass through a filter before it needs to be replaced depends on such factors as: the type and size of the filter, the substance being filtered,  the concentration of the substance in the water, and the flow rate of the water.   A brealthrough test can be performed to determine whan a filter needs replacement.  The breakthrough test  system used here is shown in the figure below.  After the filter has been saturated with clean water, the salt solution is pumped from the tracer reservoir to the top of filter. A portion of the salt tracer overflows into a tank in order to maintain a constant liquid level above the filter which helps maintain a constant flow rate through the filter. The flow rate of tracer flowing through the filter is adjusted using a discharge valve. The tracer discharges to a small graduated cylinder. Inside the cylinder is a electrical conductivity (EC) probe  which monitors the salt tracer concentration eluting from the filter as a function of time. Recall the electircal conductivity of water is a function of the total dissolved solids which  in this case is the salt tracer concentrtion.  The EC level is shown on the EC meter screen.  You will be recording the EC as a function of time and graphing the data.  Your graph should look like the curve shown below ( a breakthrough curve).  The EC will rise unti it reaches a near steady maximum level.  Using the breakthrough curve, you will determine the time when the solution eluting the filter was at 10% of the maximum EC level.  You will then determine the flow rate of the tracer through the filter by timing the discharge into a graduated cylinder.   The volume of water  (VOL) that can be filter before the filter needs to be changed  is then calculated by: Vol = flowrate (cc/sec) * time at 10% maximum (sec).  {Why 10% and not 100% Maximum? To be conservative filter manufactures use 10% of the maximum since the factors that influence the life of a filter are variable from household to household)

Breakthrough Data Collection Spreadsheet



  1. Open Breakthrough Data Collection Spreadsheet
  2. While watching the video below fill n the spreadsheet (time and EC value).  Collect data for about every minute until the maximum EC value is reached
  3. Determine the time at 10% Maximum EC value from the graph.
  4. Determine the discharge rate of  water through the column
  5. Calculate the  volume of water that can be filtered before the filter needs replacement
  6. Fill in the form below.


Breakthrough Test

  • Vol = discharge rate*10% Maximum Time
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