Why do large cities often discharge treated wastewater into surface water?

In large cities, the discharge of treated wastewater into surface water is the predominant method of disposal because it is often the most logistically and economically feasible option compared to subsurface disposal, which is more common in rural or suburban areas.12

When cities process wastewater, the primary goal is to protect public health and maintain water quality. Before discharge, the wastewater undergoes treatment—typically primary and secondary levels—to remove suspended particulates, biodegradable organics, and pathogens. This treatment is essential because discharging untreated sewage would deplete dissolved oxygen levels in lakes and streams, endangering aquatic life and causing eutrophication.12

While some municipalities explore wastewater reuse for irrigation or groundwater recharge to conserve limited freshwater supplies, discharging treated effluent into surface water remains the standard practice for managing the large volumes of sewage generated by urban populations. This process is strictly regulated by governmental standards to ensure that the effluent does not deteriorate the quality of the receiving water bodies.123