Question

When treating sewage, which of the following options gives the correct order in which contaminants are removed?

• A. Plastic bags $\to$ bacteria $\to$ suspended solids $\to$ heavy metals
• B. Suspended solids $\to$ heavy metals $\to$ plastic bags $\to$ bacteria
• C. Plastic bags $\to$ suspended solids $\to$ heavy metals $\to$ bacteria
• D. Bacteria $\to$ heavy metals $\to$ suspended solids $\to$ plastic bags

Answer 1

The correct option is C

Plastic bags $\to$ suspended solids $\to$ heavy metals $\to$ bacteria

Larger contaminants are usually the first to be removed. Plastic bags and other garbage in sewage are first removed. Then, suspended solids (which usually are silt and organic waste stirred up in flowing water) are removed.

Then the water is treated using chemicals to remove heavy metal salts. Finally, the water is treated with chlorine or ozone to kill bacteria. Note that bacteria are larger than the metal salts, but bacteria are removed last because they are necessary to treat water.