Week 5 – Water Treatment (Sedimentation & Filtration)

Recalling Week 2’s activity, we learnt about the first two stages of the water treatment process – coagulation & flocculation.

This week, Week 5, we moved on to complete the final two stages of the water treatment process – sedimentation & filtration.

The objectives of this activity:

  • To describe in detail the theory of sedimentation
  • To compare the experimental settling velocity with the theoretical settling velocity
  • Design, build & test water filtration units
  • Apply the hypothesis testing when comparing filters

*Every group was assigned to collect five bottles of 500ml dirty swamp water before the lesson.

Materials needed for the sedimentation experiment:

  1. Small balls
  2. A bottle of canola oil
  3. A bottle of clear water

Materials needed for the filtration experiment:

  1. 3 cups of dirty swamp water
  2. Charcoal
  3. One translucent 500ml plastic mineral water bottle


During sedimentation, individual particles settle based on their size and density and do not interact with one another. Particles settling vertically in a fluid are influenced by three main forces, namely gravity, buoyancy and drag. Initially, there is a net force downwards, but after travelling over a certain distance, particles attain terminal or settling velocity, travelling at a constant speed when the net force is 0. In other words, when the buoyant and drag forces are equal to the gravitational force, the forces are said to be in equilibrium.

In this experiment, we had to find out the theoretical & practical settling velocities of small plastic balls provided.

Firstly, we found out the following parameters:

Theoretical Calculation for Settling Velocity.jpg

Using the formula shown below,

Settling velocity formula

we calculated that the theoretical value of the settling velocity of the plastic ball in water is 10.1 m/s & in oil is 0.150 m/s.

To obtain the experimental settling velocity, we had to find out the time taken for the ball to fall freely through the respective fluid and recording the time at each height interval (3cm) using a slow motion video recording app, HUDL Technique.

Experiment for Settling Velocity in Oil.jpgUsing Technique AppsExperimental Calculation for Settling Velocity through excel.jpg

*We repeated the experiment twice per fluid, Trial A & B, and calculated the average time for increased accuracy.

Experimental Settling Velocity Data

We found that our mean settling velocities calculated from the experiment did not match up with our theoretical settling velocities at all. A reason for this could be because the balls were not given enough distance to travel such that they were able to reach settling velocity. We can conclude that even though there wasn’t enough distance for them to reach settling velocity, over 14.2cm, the ball in water reached a greater final velocity than the ball in oil.

Through this experiment, we have learnt to be aware of potential problems and improvising solutions on the spot. We also realised that the readings we took could only be so accurate as there was still some room for human error due to the lack of automatic measuring devices.


Our team was assigned to build a water filter system using charcoal & a plastic water bottle.

We had to measure the effectiveness of our rapid water filter and compare the turbidity, pH & conductivity of water samples before & after undergoing filtration. The experiment was repeated three times to obtain more reliable and accurate data. 

Obtain parameter valueCharcoal Filter system.jpg

*The charcoal filter system was rinsed to remove impurities before pouring in the dirty swamp water as the impurities would have affected the properties of the filtered water.


Here’s the table of data that we collected:

Filtration Data Table.JPG

We observed that the water was much cleaner after filtration. This is because during filtration, the larger particles were trapped in the gaps between charcoal particles. Although the water seemed cleaner, it was still unsafe for consumption as the water still contained bacteria. Filtration is a largely physical filtration process. In order to produce clean drinking water, the water would have had to be chemically treated.

We compared our results with group 2 (6cm of charcoal) and group 5 (7cm of sand) while our filter used 7cm of charcoal. The reason we chose to compare with these groups is because in each instance, only one variable was different, either the height of the filter material used or the filter material itself.

By inspecting our data, we found that the biggest change is the conductivity of the water samples. While the turbidity of each water sample decreased a decent amount, we were unable to obtain accurate initial values as the turbidity surpassed 1000 NTU, which was the maximum value the turbidity meter could measure. Hence, the parameter we chose to compare is the percentage reduction in conductivity.

Hypothesis Testing

Hypothesis.JPGHypothesis 2

Through this experiment, we learned to make our own simple filtration system and observe the difference in quality of the water passed through the filter. During the experiment, we realised samples A and B appeared even dirtier after filtration. The reason behind this was because the charcoal was not totally clean from the first rinsing. Therefore, we repeated the experiment again for sample A and B. The difference between this week’s activity and the other activities in the last few weeks is that we had to do 2 activities in 1 lesson but despite that, we were able to complete both activities as we delegated the work fairly to everyone. The roles assigned at the start helped us complete both tasks in ample time and efficiently.

“Although time was very limited, especially for the filtration experiment, our team was able to complete the experiment well. I am really grateful towards Ajay for guiding the team well throughout the whole activity, going through it step by step, allowing us to understand and keep us on track with the work we had to complete. Furthermore, Thomas and Ryan were always willing to help out in anything once they were done with their parts (eg. building the rapid filter system while Ajay and I were completing the last part of the sedimentation activity) Overall, I feel that my team really works well & for me, I need to get myself more involved in the activity and offer my assistance whenever needed.” – Hwee Peng


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