Question

The water supply for a midwestern city contains the following impurities: coarse sand, finely divided particulates, nitrate ions, 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?

Short answer

The following processes are effective in removing the specified impurities from the midwestern city's water supply: 1. Coarse sand: Coarse sand filtration (A) 2. Finely divided particulates: Coarse sand filtration (partially, A) and precipitation with aluminum hydroxide (E) 3. Nitrate ions: None of the provided methods 4. Trihalomethanes: Activated carbon filtration (B) 5. Dissolved phosphorus (phosphates): Precipitation with aluminum hydroxide (E) 6. Harmful bacterial strains: Ozonization (D) 7. Dissolved organic substances: Activated carbon filtration (B) and ozonization (partially, D)

Step-by-step solution

Coarse sand filtration effectiveness

Coarse sand filtration is effective at removing large particulates, such as coarse sand and some finely divided particulates. Therefore, it can be effective in removing impurities 1 and partially effective in removing impurity 2.

Activated carbon filtration effectiveness

Activated carbon filtration is effective when it comes to removing dissolved organic substances, trihalomethanes, and some taste and odor causing compounds. Thus, it is effective in removing impurities 4 and 7.

Aeration effectiveness

Aeration helps in removing dissolved gases, volatile organic substances, and improving the taste and odor of water. However, it is not directly effective in removing any of the mentioned impurities.

Ozonization effectiveness

Ozonization is a powerful disinfectant and oxidizing agent. It can effectively remove harmful bacterial strains and break down some dissolved organic substances. Hence, it is effective in removing impurity 6 and partially effective for impurity 7.

Precipitation with aluminum hydroxide effectiveness

Precipitation with aluminum hydroxide is effective in removing phosphate ions and some finely divided particulates. Alum (aluminum sulfate) is added to form aluminum hydroxide precipitate, which entraps impurities and can be removed through sedimentation and filtration. Therefore, this process is effective in removing impurities 2 and 5.

Summary of effectiveness:

1. Coarse sand: Coarse sand filtration (A) 2. Finely divided particulates: Coarse sand filtration (partially, A) and precipitation with aluminum hydroxide (E) 3. Nitrate ions: None of the provided methods 4. Trihalomethanes: Activated carbon filtration (B) 5. Dissolved phosphorus (phosphates): Precipitation with aluminum hydroxide (E) 6. Harmful bacterial strains: Ozonization (D) 7. Dissolved organic substances: Activated carbon filtration (B) and ozonization (partially, D)

Key concepts

Coarse Sand Filtration

Imagine trying to remove pebbles from sand at the beach with a sieve. Similar to this, coarse sand filtration is used to remove large particulate matter from water, such as coarse sand, gravel, and other large debris. This process is a fundamental initial step in water purification as it prevents larger particles from damaging or clogging the finer filters used in later stages. The way it works is that water is passed through beds of coarse sand or gravel where the physical barrier traps the large particles.

As outlined in the textbook solution, this filtration method effectively removes the larger 'chunks' from our water, providing the first line of defense in the water purification process. However, it's important to note that it's not very effective against smaller particles or dissolved substances, which is why it usually precedes other, more refined filtering techniques.

Activated Carbon Filtration

Think of activated carbon filtration as a powerful sponge that absorbs tiny impurities you can't even see. Activated carbon, often derived from charcoal, has an extensive porous surface that captures micro-contaminants, including chemicals that cause unpleasant tastes and smells. It works through a process called adsorption, which is different from absorption. It's like how a magnet works: impurities are attracted and held on the surface of the carbon particles rather than being soaked up.

In the case study from the textbook, activated carbon filtration is excellent for snagging dissolved organic substances and trihalomethanes, which are byproducts of chlorine treatment in water. These are often the culprits for bad tastes or smells, so this step is crucial for ensuring that our water isn't just safe, but also pleasant to drink.

Ozonization

Ozonization is a bit like using a high-powered sanitizer to zap away unwanted microorganisms. Ozone (O₃) is a powerful oxidizing agent that destroys bacteria and viruses without leaving behind any harmful residues as ozone quickly decomposes back into oxygen. During this process, water is infused with ozone, which reacts with and breaks down many types of microorganisms and complex chemicals.

From our exercise, we can gather that ozonization is particularly valuable for annihilating harmful bacterial strains. It even has a partial effect on breaking down dissolved organic substances that activated carbon might miss. This makes it a crucial step in purifying water to make it safe for consumption. Ozone effectively ensures that our water is free from pathogens that could cause waterborne diseases.

Aluminum Hydroxide Precipitation

Finally, aluminum hydroxide precipitation is a chemical ninja that sneakily removes dissolved substances by making them solid so they can be filtered out. When chemicals like aluminum sulfate are added to the water, they react to form aluminum hydroxide, which is a kind of sticky solid. This gunky precipitate captures and pulls down finely divided particulates and phosphate ions as it settles to the bottom. This settled matter, known as sludge, can then be filtered out or removed.

As indicated in the solution, precipitation with aluminum hydroxide is a smart way to tackle the presence of dissolved phosphates, which can lead to problems like algal blooms if released into the environment. It's also a helper for getting rid of some of those finer particles that made it through the initial coarse sand filtration step. Overall, this process contributes to clearer, safer, and more environmentally-friendly water.