During municipal water treatment, coagulation is primarily designed to remove which type of impurity from the water supply?

Introduction / Context:
Water treatment plants use several processes to convert raw water from rivers, lakes, or reservoirs into safe drinking water. One important step is coagulation followed by flocculation and sedimentation. Coagulation involves adding chemicals that help very fine suspended particles and colloids clump together so that they can be removed more easily. This question asks what type of impurity coagulation is mainly intended to remove, highlighting the role of this process in water purification.

Given Data / Assumptions:

• Coagulation is part of conventional water treatment processes.
• It is usually followed by flocculation, sedimentation, and filtration.
• The options include large particles like sand, bad smells, microorganisms, and suspended particles.
• We assume a typical municipal drinking water treatment plant.

Concept / Approach:
Raw water often contains very fine suspended solids and colloidal particles such as clay, organic matter, and microscopic debris. These particles are too small to settle quickly by gravity and may pass through simple filtration. Coagulant chemicals such as alum or ferric salts neutralise the charges on these particles, causing them to aggregate into larger flocs that can settle out or be filtered more easily. This process specifically targets fine suspended and colloidal matter. Sand and gravel are usually removed earlier by screens and grit chambers. Pathogenic microorganisms are reduced later by filtration and disinfection. Bad smells are addressed by other treatments such as activated carbon.

Step-by-Step Solution:
Step 1: Identify that coagulation is used to treat water that contains very fine suspended and colloidal impurities which do not settle readily. Step 2: Recall that chemicals such as alum are added to destabilise these particles by neutralising their surface charges. Step 3: Recognise that once destabilised, particles clump together during flocculation and form larger flocs. Step 4: Understand that these larger flocs can then be removed by sedimentation and filtration, thereby clearing the water of turbidity. Step 5: Conclude that the primary purpose of coagulation is removal of fine suspended particles and colloidal matter.

Verification / Alternative check:
Descriptions of conventional water treatment outline steps in order. Screening and grit removal take out large debris such as sticks, leaves, sand, and gravel. Coagulation and flocculation then deal with very small particles that cause turbidity but are too small to settle easily. Sedimentation basins allow the flocs to settle, and filtration removes remaining fine particles and many microorganisms. Disinfection with chlorine or other agents specifically targets pathogens. This flow of processes confirms that coagulation focuses on physical suspended particles and colloids rather than large debris, smells, or microbes directly.

Why Other Options Are Wrong:
Sand and gravel are largely removed by simple screening and grit removal processes before coagulation, so option sand and gravel is not the main purpose of coagulation. Bad smells and tastes are treated using adsorbents like activated carbon and by controlling organic content, not mainly by coagulation, although coagulation may indirectly help. Pathogenic microorganisms are targeted primarily during filtration and disinfection steps, particularly chlorination or ultraviolet treatment, rather than by coagulation alone. Therefore, these options do not correctly describe the specific function of coagulation in water treatment.

Common Pitfalls:
Some learners may assume that each water treatment step removes all types of impurities equally and may pick microorganisms, thinking coagulation is a general cleaning process. Others may confuse coagulation with disinfection or aeration. To avoid such mistakes, remember that coagulation is a physical chemical process aimed at improving the removal of particles that cause turbidity. Linking coagulation with floc formation and suspended solids will help you choose the correct answer in similar questions.

Final Answer:
Coagulation during water treatment is designed mainly to remove fine particles and colloidal matter suspended in water.