Before discharging the foul sewage into rivers, it is generally treated by

ANS:E - all the above.

Before discharging foul sewage into rivers or other water bodies, it is essential to treat it to remove pollutants and pathogens, protecting the environment and public health. Treatment processes aim to reduce the organic and inorganic content, remove solids, and disinfect the wastewater. Let's discuss each of the treatment methods mentioned:

1. Screening: Screening is often the first step in wastewater treatment. It involves the removal of large debris and solids from the sewage using screens or grates. This process prevents large objects such as sticks, stones, plastics, and other debris from entering the treatment system, which could cause damage to pumps, pipes, and other equipment downstream. Screening helps to protect pumps, prevent blockages, and improve the efficiency of subsequent treatment processes.
2. Sedimentation: Sedimentation, also known as settling, is a physical process in which suspended solids and particles in wastewater settle out under the influence of gravity. This can occur in sedimentation tanks or clarifiers, where the flow velocity of the wastewater is reduced, allowing particles to settle to the bottom. Sedimentation removes settleable solids, such as sand, grit, and organic matter, from the wastewater, producing a clearer effluent that is easier to treat in subsequent processes.
3. Oxidation: Oxidation processes, such as biological oxidation or chemical oxidation, are used to break down organic pollutants in wastewater. Biological oxidation involves the use of microorganisms to consume organic matter and convert it into simpler, less harmful substances through aerobic (with oxygen) or anaerobic (without oxygen) processes. Chemical oxidation involves the addition of oxidizing agents, such as chlorine or ozone, to chemically react with and degrade organic compounds. Oxidation processes help to reduce the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of the wastewater, improving its quality before discharge.
4. Sludge Digestion and Disinfection: After primary treatment processes, sewage often undergoes secondary treatment, which may include processes such as sludge digestion and disinfection. Sludge digestion involves the biological decomposition of organic solids present in wastewater sludge by anaerobic bacteria, producing methane gas and stabilized sludge. Disinfection is the final step in wastewater treatment, where pathogens such as bacteria, viruses, and protozoa are destroyed or inactivated to ensure that the treated effluent meets regulatory standards for discharge. Common disinfection methods include chlorination, ultraviolet (UV) irradiation, and ozonation.

• All of the above.