In municipal water treatment, which of the following is the most economical primary method for removing suspended solid matter from water?

Introduction / Context:
Water treatment plants must remove suspended solid particles such as sand, silt, and mud from raw water before it is supplied for domestic or industrial use. There are many methods available, ranging from simple gravity based processes to advanced membrane technologies. This question asks which method is the most economical for removing solid matter, especially as an initial primary treatment step in large scale water supply systems.

Given Data / Assumptions:

• The focus is on removing suspended solid matter, not dissolved salts or microorganisms.
• The methods listed include carbon adsorption, electrolysis, distillation, sedimentation, and reverse osmosis.
• We are interested in the most economical and practical method for large volumes of water.
• Standard water treatment practice is being considered.

Concept / Approach:
Sedimentation is a basic physical process where heavier solid particles settle to the bottom of a tank under gravity. It requires relatively simple infrastructure, such as large basins or clarifiers, and minimal energy input compared to advanced processes. It is therefore widely used as an economical primary treatment step to remove suspended solids before filtration or disinfection. Methods like distillation and reverse osmosis are energy intensive and typically reserved for specialised applications such as desalination. Electrolysis is not a usual method for bulk solid removal, and activated carbon is mainly used to remove dissolved organic compounds, not for primary solid sedimentation.

Step-by-Step Solution:
Step 1: Identify that the target is suspended solid matter, such as sand and silt, not dissolved salts. Step 2: Recall that sedimentation involves allowing water to stand undisturbed so that heavy particles can settle at the bottom under gravity. Step 3: Recognise that sedimentation tanks or clarifiers can handle very large volumes of water at relatively low operational cost. Step 4: Consider that distillation and reverse osmosis require significant energy and equipment costs, making them less economical for basic solid removal. Step 5: Note that activated carbon and electrolysis serve other treatment goals, such as removing organic compounds or changing ionic composition, rather than acting as primary solid separators. Step 6: Conclude that simple sedimentation is the most economical method listed for removing solid matter from water.

Verification / Alternative check:
Standard process diagrams for water treatment plants show a sequence such as screening, sedimentation, coagulation and flocculation, filtration, and disinfection. Sedimentation basins are always among the early stages and are used to remove the bulk of the suspended solids before finer filtration. Financial and engineering analyses show that sedimentation is cost effective because it uses gravity rather than high pressure or thermal energy. The more advanced methods like reverse osmosis are reserved for cases where dissolved contaminants must be removed, such as in desalination of seawater, and are much more expensive per unit volume of water treated.

Why Other Options Are Wrong:
- Using activated carbon beds: Activated carbon is expensive and is primarily used to adsorb organic compounds, taste, and odour causing substances, not for first stage solid removal.
- Electrolysis of the water: Electrolysis splits water into hydrogen and oxygen and is not an economical routine method for removing suspended solids in a water supply system.
- Distillation of the entire water supply: Distillation requires heating the water to form vapour and then condensing it, which consumes a great deal of energy and is unsuitable for large scale municipal treatment purely for solid removal.
- Reverse osmosis filtration: Reverse osmosis uses high pressure to push water through a semi permeable membrane and is very effective for desalination, but it is expensive and not used merely to remove ordinary suspended solids.

Common Pitfalls:
Some students may choose advanced sounding technologies like reverse osmosis or distillation, assuming that the most sophisticated process is always the best. However, in real engineering practice, the simplest method that achieves the goal is preferred, especially when cost and scale are critical. Remember that sedimentation is a gravity based physical process that is ideal for removing large amounts of suspended solids in a low cost way before any high technology treatment steps are used.

Final Answer:
The most economical method for removing suspended solid matter from water is Simple sedimentation in tanks.