Required Practical: Water Purification (RP Link/Concepts)

Purpose of Water Purification

Water purification is essential to remove impurities from natural water sources, making the water safe and potable for human consumption. Impurities include suspended solids, harmful microbes, and chemical contaminants. Purifying water prevents waterborne diseases and protects public health, ensuring the water meets safety standards for drinking.

In this practical, the focus is on removing visible solids and killing microbes to produce clean, safe water. This process is vital for health and safety, especially in areas where water sources may be contaminated by natural or human activities.

Steps in Water Purification Practical

Filtration: The first step involves filtering the water to remove suspended solids such as dirt, sand, and organic matter. This is done by passing the water through filter paper, which traps solid particles but allows water to pass through.

Sterilisation: After filtration, the water may still contain harmful microorganisms like bacteria and viruses. Sterilisation kills these microbes, commonly by adding a sterilising agent such as chlorine or by boiling the water. Chlorine kills microbes by disrupting their cell functions, while boiling kills microbes by denaturing their proteins.

Testing: The purified water is tested for clarity and safety. Clarity is checked visually to ensure solids are removed. Safety tests may include checking for the absence of microbes or chemical contaminants using simple indicators or test kits. Note that detailed chemical contaminant testing requires advanced methods not covered in this practical.

For example, after filtration, the water should appear clear rather than cloudy. If chlorine is added, a slight smell may be detected, indicating sterilisation.

Equipment and Materials

• Filter paper and funnel – to perform filtration and remove solids
• Source water sample – such as river water or pond water, which contains impurities
• Sterilising agents – commonly chlorine solution or tablets to kill microbes
• Beakers or clean containers – to collect filtered and sterilised water
• Test tubes and water testing kits (optional) – to check water clarity and microbial safety

Note: This practical does not involve chemical analysis equipment or chromatography, which are covered in other topics.

Observations and Results

During the practical, you will observe changes in the water’s appearance and test results that indicate purification success:

• Appearance changes: Initially, the source water is often cloudy or coloured due to suspended solids. After filtration, the water should become noticeably clearer.
• Effectiveness of filtration: The filter paper traps solid particles, which can be seen as residue on the paper. The clearer filtrate shows solids have been removed.
• Testing for contaminants: After sterilisation, tests for microbes (such as using indicator strips or observing no cloudiness after incubation) show the water is safer to drink.

If the water remains cloudy or shows signs of contamination after sterilisation, the process may need repeating or adjusting.

Learning Example: Calculating Filtration Efficiency

Suppose a sample of river water has a turbidity (cloudiness) measured as 50 NTU (Nephelometric Turbidity Units), which quantifies how cloudy the water is due to suspended particles, before filtration. After filtering, the turbidity reduces to 5 NTU.

The filtration efficiency can be calculated as:

$$\text{Efficiency} = \frac{\text{Initial turbidity} - \text{Final turbidity}}{\text{Initial turbidity}} \times 100\%$$

$$\text{Efficiency} = \frac{50 - 5}{50} \times 100 = \frac{45}{50} \times 100 = 90\%$$

This means the filtration removed 90% of the suspended solids, showing effective purification.