Coagulation-flocculation
Student/s: Filip Vávra, Dominik Šafanda
Date: 12.11.2025
Group: T1-I
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Objective of the experiment
The purpose of this experiment is to figure out the best conditions for treating
simulated polluted water using coagulation. Since the success of coagulation
depends on the type of coagulant, the pH of the water, and how much coagulant
is added, the experiment helps us find the ideal combination of these factors.
First, we compare different coagulants to see which one produces the best flocs
and the clearest water. Then, using the chosen coagulant, we test different pH
values to find the pH at which it works best. Finally, we try different doses of that
coagulant to determine the amount that gives the best results. In the end, the
goal is to find the most efficient way to remove impurities from the simulated
polluted water.
Theoretical base
This experiment is based on the principles of coagulation and flocculation, two
processes used in wastewater treatment. Their main purpose is to remove
small, suspended particles that are too tiny to settle on their own.
Coagulation is the first step. Wastewater naturally contains particles that repel
each other because they carry similar electrical charges. As a result, they stay
dispersed in the water. When we add a coagulant—such as aluminum or iron
salts—it neutralizes these charges. Once the repulsion disappears, the particles
can come together more easily.
Flocculation follows coagulation. After the particle charges are neutralized,
gentle stirring helps them collide and stick together, forming larger clumps
called flocs. These flocs are heavier and can settle at the bottom of the beaker.
The pH of the water is an important factor because the effectiveness of
coagulants depends strongly on acidity or alkalinity. At certain pH values, the
coagulant reacts better and forms stronger, more stable flocs.
The coagulant dose is also critical. Too little coagulant won’t neutralize enough
particles, while too much can restabilize them or increase sludge production.
Materials
To carry out this experiment, different types of laboratory equipment and
chemical solutions are required. The materials mainly help us measure, mix,
and control the conditions during coagulation and flocculation. The table 1
below shows the necessary equipment for this experiment.
Table 1: Necessary equipment for the experiment
Labware
3 250 ml beakers
1 Flocculator
3 25ml measuring cylinders
2 Pasteur pipette
1 pH indicator paper
1 chronometer
Chemicals
Wastewater
Ferric Sulfate (30 g / l)
Ferric Chloride (30 g / l)
Aluminum Chloride (30 g / l)
HCl
NH3
Source: created by the authors
Procedure
The work begins by preparing three beakers, each containing 200 ml of the
same wastewater sample. After checking the initial pH of the water as the
picture 1 shows, a different coagulant is added to each beaker—aluminum
chloride, ferric chloride, and ferric sulfate, always using the same volume so the
comparison is fair. In our case we chose to add 5 ml of each coagulant.
Picture 1: Initial pH of the wastewater
Source: created by the authors
The beakers are then placed on the flocculator, where the samples are mixed
first at a higher speed to encourage coagulation and then more gently to allow
the formation of flocs. When the stirring stops, the flocs are left to settle, and the
results are compared visually. At this point, the decision is made about which
coagulant performs best based on the clarity of the water, the size and amount
of flocs, and the speed of sedimentation. The differences of each sample are
showed in picture number 2.
Picture 2: Samples with added coagulant
Source: created by the authors
Once the most effective coagulant has been selected, the next task is to
investigate how the pH influences its performance. The beakers are washed
and prepared again with fresh wastewater samples. Each one is adjusted to a
different pH: one to a lower value using hydrochloric acid, one to a higher value
with ammonia, and the third is kept at the original pH of the sample. When the
pH values are stable, the chosen coagulant (AlCl3) is added to all three beakers
at the same time, and the stirring process is repeated following the same
sequence as before. After the mixtures are allowed to settle, the appearance of
the samples reveals which pH is working best as we see in picture 3.
Picture 3: Beakers adjusted with a different pH
Source: created by the authors
In the final stage, the focus shifts to finding the ideal dose of the coagulant at
the optimum pH. Again, three portions of wastewater are prepared, and their pH
is adjusted to the value determined in the previous stage. This time, the amount
of coagulant added to each beaker is different. The samples are stirred under
the same conditions as before, giving time for both coagulation and flocculation
to occur. When stirring ends, the samples are left undisturbed so the flocs can
settle. By comparing the clarity and sedimentation quality of the water in the
three beakers, it becomes possible to identify which dose leads to the most
efficient treatment.
Results
Show your data. You can include calculations, tables, graphs, drawings,
pictures.
Include here the answers to the questions proposed in the activity.
Conclusion
The objectives of the experiment were successfully fulfilled. First, the
comparison of the three coagulants allowed us to clearly identify which one
produced the most effective coagulation and flocculation under the same initial
conditions. Next, by adjusting the pH and repeating the tests, we were able to
determine the pH value at which the selected coagulant performed best. Finally,
by testing different doses at the optimal pH, we found the amount of coagulant
that gave the clearest water and the most stable sedimentation.