The sewage treatment units in which anaerobic decomposition of organic matter is used, are called

ANS:B - trickling filters

Trickling filters are a type of biological wastewater treatment process used to remove organic matter and pollutants from sewage or wastewater. They are commonly employed in municipal wastewater treatment plants and are also utilized in industrial and decentralized wastewater treatment systems. Here's how trickling filters work:

1. Basic Design: A trickling filter consists of a bed or tank filled with a medium, typically rock, gravel, or plastic media. The media provides a large surface area for the attachment and growth of microorganisms, which are responsible for breaking down organic matter in the wastewater.
2. Wastewater Distribution: Raw wastewater is distributed over the surface of the trickling filter bed either by a rotating arm or a distribution system. The wastewater then trickles down through the media, coating the surface and creating a thin film of wastewater.
3. Microbial Treatment: As the wastewater trickles through the media, aerobic (oxygen-dependent) microorganisms colonize the surface of the media and form a biofilm. This biofilm serves as a habitat for the microbial community, which includes bacteria, fungi, and protozoa. These microorganisms metabolize and degrade organic pollutants present in the wastewater, utilizing them as a food source.
4. Oxygen Supply: The thin film of wastewater flowing over the biofilm provides oxygen for the aerobic microbial metabolism. Oxygen diffuses from the air into the wastewater and reaches the microbial community on the surface of the media. This aerobic environment promotes the breakdown of organic matter through biochemical reactions such as oxidation and mineralization.
5. Treatment Efficiency: The efficiency of trickling filters in removing pollutants depends on various factors, including the surface area of the media, the retention time of wastewater in the filter, the composition of the microbial community, and the characteristics of the influent wastewater. Generally, trickling filters are effective in removing organic matter, suspended solids, and biochemical oxygen demand (BOD) from wastewater.
6. Effluent Collection: Treated wastewater exits the trickling filter bed and is collected in a collection system or effluent channel. From there, it may undergo further treatment processes such as secondary clarification, disinfection, or tertiary treatment before discharge or reuse.

Trickling filters are known for their simplicity, reliability, and low operational and maintenance requirements. They are suitable for treating a wide range of wastewaters, including domestic sewage, industrial wastewater, and stormwater runoff. However, they may require occasional media cleaning or replacement to maintain treatment efficiency, and proper management is essential to prevent clogging and ensure optimal performance.