The treatment of water for domestic use may involve each of the following stages: sedimentation flocculation filtration chlorination fluoridation pH adjustment State the purpose of four of these stages and describe how the water is treated in each of the four stages you have chosen - Leaving Cert Chemistry - Question 8 - 2008

Flocculation's purpose is to aid in the coagulation of small suspended particles, helping them to clump together and eventually settle. This is achieved by adding a flocculating agent, such as alum, which causes these tiny particles to combine and form larger aggregates that can be easily removed.

The purpose of filtration is to remove remaining suspended solids and to clear the water of contaminants. This is done by passing the water through layers of sand and gravel beds that trap particulate matter, resulting in cleaner, clearer water.

Chlorination aims to kill bacteria, viruses, and other pathogens present in the water, thereby disinfecting and sterilizing it. Chlorine, in the form of chloride ions, is added to the water to eliminate harmful microorganisms, ensuring the water is safe for domestic use.

In the primary stage, screening is conducted to remove large suspended solids. This step involves allowing solids to settle and be collected, effectively reducing the load entering further treatment processes.

The secondary stage involves biological digestion, where organic nutrients in the wastewater are broken down by microorganisms, turning them into simpler compounds. This process reduces the organic content and improves the quality of the effluent.

In the tertiary stage, additional treatment occurs to remove any remaining nutrients, such as nitrates and phosphates. This helps in polishing the effluent before discharge, further enhancing the overall water quality.

1. Algal blooms can occur, leading to eutrophication, which depletes oxygen in the water and harms aquatic life.
2. The introduction of pathogens can contaminate the water supply, posing health risks to humans and wildlife.