This book describes wastewater treatments of physical, chemical, biological, and electrochemical methods. The contents are detailed enough to understand wastewater processing. This book sends a message for young researchers, professional researchers, postgraduate students, final year students, academicians, and wastewater scientists to hurry them to find a new system or approach to sustain our wastewater treatment that does not harm the ecosystem. All the described methods and systems have major and minor disadvantages. Also, they failed to be sustainable methods.