Pick up the correct statement from the following: A. Hydrogen sulphide gas in excess, may cause corrosion of concrete sewers B. 4 ppm of Dissolved Oxygen (D.O.) is ensured before discharging the treated sewage in river C. Solubility of oxygen in sewage is 95% of that of distilled water D. All the above

The correct answer is D. All the above.

• Hydrogen sulfide gas in excess, may cause corrosion of concrete sewers. This is because hydrogen sulfide gas is a corrosive gas that can react with the concrete in sewers, causing it to break down.
• 4 ppm of Dissolved Oxygen (D.O.) is ensured before discharging the treated sewage in river. This is because dissolved oxygen is essential for aquatic life, and if there is not enough dissolved oxygen in the water, it can harm or kill the fish and other organisms that live in the river.
• Solubility of oxygen in sewage is 95% of that of distilled water. This is because sewage is a mixture of water and other substances, and the other substances in sewage can reduce the amount of oxygen that can dissolve in the water.

It is important to ensure that there is enough dissolved oxygen in the water before discharging treated sewage into a river. This can be done by aeration, which is the process of adding air to the water. Aeration can be done by using aerators, which are devices that add air to the water, or by using natural methods, such as waterfalls or fountains.