This lecture covers the fundamentals of biological nutrient removal in wastewater treatment, focusing on the removal of nitrogen and phosphorus. Topics include the activated sludge process, nitrification, denitrification, phosphorus removal by waste sludge, and the implementation of biological nutrient removal. The lecture also discusses the concentrations of different forms of nitrogen and phosphorus in wastewater, the influence of temperature on growth rates, and the metabolism of phosphate-accumulating organisms. Additionally, it explores the importance of recovering phosphorus from waste sludge and the environmental impact of nutrient pollution.