This book provides an overview of biological wastewater treatment and sludge recovery. It presents the main concepts related to treatment processes, as well as the principles of operation, sizing, and implementation of unit operations and processes associated with wastewater and sludge treatment. Wastewater Engineering and Treatment 4 addresses the biological removal of micropollutants present in wastewater, secondary treatment (construction ponds, oxidation ponds and reed beds), and the treatment and reuse of sludge. It presents the characteristics of sludge, thickening, digestion, dewatering, stabilization, the removal of pathogenic microorganisms, recovery through land application, composting, and the formation of byproducts such as struvite or bioplastics. Didactic and combining theory with practice, this book is intended for engineers in engineering and environmental sciences, as well as professionals in wastewater treatment.