This test method is a standardized procedure for determining 5-day Biochemical Oxygen Demand (BOD₅) in water and wastewater. BOD (Soluble) first filters particulate matter from the sample, then measures the amount of dissolved oxygen (DO) consumed by microorganisms while breaking down organic matter under aerobic conditions over a 5-day period at 20 °C. It is widely used to assess the organic pollution load and treatment efficiency of wastewater systems.