Understanding Chemical Coagulants in Water Treatment

Chemical coagulants are essential substances used in water treatment processes to remove suspended solids, turbidity, color, and other contaminants that cause cloudiness or discoloration in water. These contaminants are often very small and remain suspended because they carry a negative electrical charge, causing them to repel each other. Coagulants work by neutralizing these charges, allowing the small particles to clump together into larger, heavier particles called flocs. These flocs can then be easily removed through sedimentation or filtration.

Analyzing the Given Options

Let's examine each option to determine which substance is NOT a chemical coagulant commonly used in water treatment:

1. Aluminium sulfate (Alum): Aluminium sulfate, with the chemical formula $\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$, is one of the most widely used chemical coagulants in water treatment plants worldwide. It is highly effective at clumping suspended particles.
2. Polyaluminium chloride (PAC): Polyaluminium chloride is a group of aluminium-based polymers, often represented as $[\text{Al}_2(\text{OH})_{n}\text{Cl}_{6-n}]_{m}$. PAC is another common and effective chemical coagulant, often preferred over alum in certain situations due to its performance across a wider pH range and lower sludge production.
3. Aluminium chloride: Aluminium chloride ($\text{AlCl}_3$) can also be used as a chemical coagulant in water treatment. Similar to alum and PAC, it provides aluminium ions which are effective in charge neutralization and floc formation.
4. Nitrogen dioxide: Nitrogen dioxide ($\text{NO}_2$) is a gas and a major air pollutant. It is produced from the combustion of fossil fuels. Nitrogen dioxide is NOT used as a chemical coagulant in water treatment. Its chemical properties and typical state (gas) are not suitable for the coagulation process aimed at removing suspended solids from water.

Identifying the Non-Coagulant Substance

Based on the analysis of each option, Aluminium sulfate, Polyaluminium chloride, and Aluminium chloride are all types of chemical coagulants used in water treatment. Nitrogen dioxide, however, is not a chemical coagulant and has entirely different applications and environmental concerns.

Conclusion on Chemical Coagulants

Therefore, Nitrogen dioxide is the substance from the list that is NOT a chemical coagulant used in water treatment.

Revision Table: Water Treatment Chemicals

Additional Information on Coagulation

Coagulation is a critical step in conventional water treatment processes, usually followed by flocculation, sedimentation, filtration, and disinfection. The effectiveness of coagulation depends on several factors, including the type and dose of coagulant used, the pH of the water, the temperature, and the mixing conditions. Proper mixing (rapid mixing during coagulation, followed by slow mixing during flocculation) is crucial for efficient particle destabilization and floc growth. The goal is to produce clear water that is then easier to filter and disinfect, ensuring safe drinking water.