Urban lifestyles and industrial processes produce large amounts of waste water that must be treated before it is released into the environment. Sewage and agricultural waste water need organic matter and harmful microbes removed. Industrial waste water may need organic matter and harmful chemicals removed.
Sewage treatment has four main stages. First, screening and grit removal takes out large objects and grit. Second, sedimentation lets solids settle to make sewage sludge, leaving a liquid called effluent. Third, the sludge is broken down by anaerobic digestion, which happens without oxygen. Fourth, the effluent goes through aerobic biological treatment, where microbes use oxygen to break down the remaining organic matter.
Obtaining potable water from different sources varies in ease. Ground water is easiest, because it is already fresh and needs only filtering and sterilising. Waste water is harder, because organic matter, microbes and chemicals must be removed. Salt water is hardest, because desalination needs large amounts of energy.
