The document provides an introduction to sewage treatment. It discusses the need for sewage treatment to remove pollutants and toxicants from wastewater before discharge or reuse. A typical sewage treatment plant uses a combination of unit operations and unit processes. Unit operations involve physical separation methods, while unit processes use chemical or biological reactions. The treatment process involves preliminary, primary, secondary and sometimes tertiary stages. Preliminary treatment screens and removes large solids. Primary treatment uses sedimentation to remove settleable solids. Secondary treatment uses biological processes like activated sludge to break down organic matter.