Problem 82 - Chemistry The Central Science

Question

The water supply for a midwestern city contains the following impurities: coarse sand, finely divided particulates, nitrate ion, trihalomethanes, dissolved phosphorus in the form of phosphates, potentially harmful bacterial strains, dissolved organic substances. Which of the following processes or agents, if any, is effective in removing each of these impurities: coarse sand filtration, activated carbon filtration, aeration, ozonization, precipitation with aluminum hydroxide?

Step-by-Step Solution

Answer

The most effective processes or agents for removing each impurity are: 1. Coarse sand: Coarse sand filtration 2. Finely divided particulates: Activated carbon filtration, Precipitation with aluminum hydroxide 3. Nitrate ion: None of the mentioned processes 4. Trihalomethanes: Activated carbon filtration, Aeration (partially), Ozonization (partially) 5. Dissolved phosphorus (in the form of phosphates): Precipitation with aluminum hydroxide 6. Potentially harmful bacterial strains: Ozonization 7. Dissolved organic substances: Activated carbon filtration, Aeration, Ozonization

Step 1: Coarse Sand Filtration

Coarse sand filtration removes larger particles from water. It is effective for removing: 1. Coarse sand However, it is not effective against other impurities mentioned in the list.

Step 2: Activated Carbon Filtration

Activated carbon filtration uses adsorption to remove contaminants from the water. It is effective for removing: 1. Finely divided particulates 2. Trihalomethanes 3. Dissolved organic substances However, activated carbon filtration is not effective against nitrate ions, dissolved phosphorus, and potentially harmful bacterial strains.

Step 3: Aeration

Aeration creates an environment where dissolved gases and volatile organic compounds can evaporate from water. It is effective for removing: 1. Dissolved organic substances 2. Some amounts of trihalomethanes However, aeration is not effective against the other mentioned impurities.

Step 4: Ozonization

Ozonization is a process where ozone gas is bubbled through water to disinfect, remove contaminants and improve taste and odor. It is effective for removing: 1. Potentially harmful bacterial strains 2. Dissolved organic substances 3. Some amounts of trihalomethanes However, ozonization is not effective against coarse sand, finely divided particulates, nitrate ions, and dissolved phosphorus.

Step 5: Precipitation with Aluminum Hydroxide

Precipitation with aluminum hydroxide is a method where aluminum hydroxide reacts with impurities to form insoluble compounds that can be removed by sedimentation. It is effective for removing: 1. Finely divided particulates 2. Dissolved phosphorus in the form of phosphates However, this method is not effective against coarse sand, nitrate ions, trihalomethanes, potentially harmful bacterial strains, and dissolved organic substances. In conclusion, the effective processes or agents for each impurity are: 1. Coarse sand: Coarse sand filtration 2. Finely divided particulates: Activated carbon filtration, Precipitation with aluminum hydroxide 3. Nitrate ion: None of the mentioned processes 4. Trihalomethanes: Activated carbon filtration, Aeration (partially), Ozonization (partially) 5. Dissolved phosphorus (in the form of phosphates): Precipitation with aluminum hydroxide 6. Potentially harmful bacterial strains: Ozonization 7. Dissolved organic substances: Activated carbon filtration, Aeration, Ozonization

Key Concepts

Activated Carbon Filtration

When it comes to purifying water, activated carbon filtration stands out for its versatility and efficiency.

At its core, this method relies on activated carbon, which is a form of carbon processed to have small, low-volume pores. These pores increase the surface area available for adsorption or chemical reactions.

Activated carbon works like a sponge that absorbs contaminants such as finely divided particulates, trihalomethanes, and dissolved organic substances from the water as it passes through.

• Finely divided particulates are trapped in the pores of the carbon.
• Trihalomethanes, which are byproducts of chlorination and can be cancerous, adhere to the surface of the carbon.
• Dissolved organic substances, contributing to taste and odor issues, are also captured by the carbon's vast surface area.

Despite its effectiveness, activated carbon filtration isn't a one-stop solution.

It doesn't remove nitrate ions or dissolved phosphorus effectively, and while it can capture some bacteria, it is not a reliable method for disinfection. Therefore, it is often used in conjunction with other purification methods to ensure a comprehensive treatment of water.

Ozonization

Ozonization is a powerful water purification technique that employs ozone, a molecule composed of three oxygen atoms.

Ozone is a potent oxidant and functions exceptionally well as a disinfectant. It targets a broad spectrum of microorganisms, including harmful bacterial strains.

• Ozone disrupts bacterial cell walls, leading to cellular damage and eventual death of the bacteria.

Besides its disinfecting capabilities, ozonization also helps in decomposing dissolved organic substances and can reduce the concentration of some trihalomethanes.

However, its effectiveness against other impurities like coarse sand, finely divided particulates, nitrate ions, and dissolved phosphorus is limited.

For this reason, ozonization is typically a part of a larger water treatment process. After ozonization, other methods may be applied to address contaminants that ozone alone cannot remove efficiently.

Precipitation with Aluminum Hydroxide

The process of precipitation with aluminum hydroxide is an important step in water treatment, especially when it comes to dealing with finely divided particulates and dissolved phosphorus like phosphates.

This method involves adding aluminum salts to the water, which react to form aluminum hydroxide precipitates.

• These precipitates can effectively attract and bind with particulates and phosphates, forming larger particles that can be easily removed by sedimentation.

However, this technique has its limits.

It is not suitable for addressing impurities such as coarse sand, nitrate ions, trihalomethanes, and harmful bacterial strains. Furthermore, significant operational control is required to ensure the proper formation and removal of precipitates without negatively impacting water quality.

In practice, precipitation with aluminum hydroxide is combined with other purification methods to tackle a wider array of contaminants and ensure safe, clean drinking water.