Relatively low-energy centralized approaches for nonpotable wastewater reuse are also available,
such as waste stabilization ponds (WSPs), in which sewage is directed through a series of openair shallow ponds where physical processes (flocculation and gravitational sedimentation), microbial processes (algal growth, aerobic and anaerobic
heterotrophic metabolism, nitrification, and denitrification), and exposure to sunlight jointly remove pathogens, organic contaminants, and nitrogen (31).Effluent from WSPs can irrigate crops (Fig. 2) or recharge groundwater aquifers, and the ponds themselves may provide a much needed quasiwetland habitat for waterbird conservation (18).The world’s largest WSP system, the Western Treatment Plant in Melbourne, produces 40 Gl
year−1 of treated wastewater, equivalent to 11%