Which of the following is a primary treatment for water pollution?

Correct answer: (a) Sedimentation

Explanation

Treatment of sewage and polluted water has three stages:

• primary (physical): screening out large objects and sedimentation, in which heavy solids settle as sludge in large tanks;
• secondary (biological): microbes break down the dissolved organic matter, in the activated sludge method or on trickling filters;
• tertiary (chemical or advanced): removal of nitrates and phosphates and other substances, which would otherwise cause eutrophication.