Water Filters

Anna S. Nadia M. & Youree C.

Physics Concepts

A carbon filter uses a bed of activated carbon to remove contaminants and impurities, using chemical adsorption. Chemical adsorption is the process in which atoms, molecules, or ions are adhered to a surface. These atoms, molecules, or ions are trapped inside of the porous structure of the carbon substrate, thereby allowing less contaminated water flow through the carbon substrate, purifying it, or at least reducing the amount of particles in the sample. Water purification is the most prominent use of adsorption. In order to purify water, remove contaminant particles from it, activated carbon, which can be manufactured from a carbonaceous material, is used. It is the most widely used adsorbent because its physical properties, such as its pore size distribution and surface area that can be adjusted in order to make the filter more efficient for what it is used for.

Adsorption results primarily from interactions between the surface of the carbon in the filter, and the molecules passing through the filter. In the case of carbon filters, the process of adsorption that takes place is physical adsorption (rather than chemisorption), in which weak van der waal forces hold the surface molecules of the filter to the molecules of the particles being filtered. These forces are weak attractions that result from the difference in polarity of molecules due to molecules being classified as dipole.

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Currently, carbon filters are most effective at removing chlorine, sediment, volatile organic compounds, taste, and odor from water and the typical particle size that is able to be removed by a filter ranges from 0.5 to 50 micrometers. Furthermore, when water is allowed to flow through a filter at a lower rate, more particles are able to be removed. Yet, carbon filters are incapable of removing minerals, salts, and dissolved organic compounds, making them not completely effective.

The two types of carbon filters used most commonly are powdered block filters and granular activated filters. Carbon block filters are generally more effective at removing particles from a water sample because it has an increased surface area of carbon. Many carbon filters can use silver or Kdf-55 which prevents bacteria growth in the filter.

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Carbon filters have been used for several hundreds of years and are considered one of the oldest means of water purification. There is evidence that carbon filters were used by ancient Egyptian cultures in order to purify and use as a medical agent. Furthermore, a Sanskrit text from 2000 BC references filtering water through charcoal. However, the use of a carbon filter to purify potable water on a large scale was not until the 19th century in England. Today, carbon water filters are used in individual homes, groundwater remediation, landfill leachate, industrial wastewater, and in municipal water treatment facilities.

Water filters operate by trapping particles or chemical substances, while allowing water to continue to flow through pores within the filter. As Brita filters are designed to remove extremely small particles, it is assumed that they can successfully trap slightly larger particles that are frequently found in pond water. While it may not perfectly remove all particles, it is thought that one filtration of a sample of pond water will be sufficient enough to remove a significant amount of particles.


When comparing filtered pond water to non-filtered pond water, the p value returned from a t-test performed on both sets of data will be less than 0.05, signifying that the filter has removed a significant amount of particles from the water.


The goal of this experiment is to find out if there is a significant difference between the number of particles found in filtered pond water and non-filtered pond water. A t-test will be run in order to determine the level of significance between the two groups.


  1. Brita Water Filter

  2. Pond water

  3. Compound microscope

  4. Two Containers

  5. Pipette

  6. Microscope slides

  7. Microscope slide covers


  1. Fill one container with pond water. The amount does not matter, but more than half of a liter should be plenty.

  2. Filter half of the water through the Brita filter into the second container.

  3. Stir the unfiltered water, ensuring the visible particles in the water are distributed evenly through the sample.

  4. Place a few droplets of the unfiltered water on a microscope slide and slip a cover sheet over it

  5. Focus on one field in the microscope

  6. Count the total number of particles found in the slide

  7. Repeat steps 5 and 6 four more times, to have a total of 5 counted slides

  8. Place a few droplets of the filtered water on a microscope slide and replace the unfiltered slide with the new one.

  9. Repeat steps 7 and 8 with the new sample

Throughout the entire experiment, the water was only filtered one time. This was done because under normal conditions water is only filtered once when used at home.

Only one sample of water was taken from the pond. This helped eliminate sources of error because it ensured water was not being taken from different spots of the pond which might contain different concentrations of particles.

Sources of error were also minimized by taking multiple pictures of different fields under the microscope, and averaging the number of particles from each field.


Number of Particles in the Filtered vs. Unfiltered Sample
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The t-test return value between filtered pond water and non-filtered pond water was 0.041. This means that there is a significant difference between the number of particles found in unfiltered pond water and filtered pond water.

Overall there was a decrease in particles by 39.11 percent.


Improving the Experiment:

  • Testing other forms of water purification; such as boiling, distillation, reverse osmosis, or desalination, could have improved the experiment by showing the most effective way to get ride of particles in a water sample. This would have provided more insight on the best methods of water purification.
  • The experiment could be furthered by putting water through the filter more than one time. Each time the water goes through a filter, more particles are removed as it has more time to be adsorbed into the carbon substrate. Therefore, putting the pond water sample through a filter more than once, and comparing the difference in particles found in the sample each time could provide insight into the diminishing effectiveness of the carbon filter.
  • Another variable to test could be the different brands of water filters. In this case a Brita filter was used, however other brands could be used to test and compare the difference in their ability to filter the water.

Improving Carbon Filters

  • Carbon filters are not currently able to completely purify water from a source such as a lake. However, the developing of carbon filters so that they are able to filter a greater amount of particles and target different threats, could make water more accessible around the world.
  • Carbon filters can be improved through the addition of chemical substances which are able to target certain particles, such as dissolved organic compounds, that are not normally targeted by filters. This would result in a greater effectiveness of carbon filters overall.