Effluent sewer
Effluent sewers also currently serve fewer people than septic systems, which also use septic tanks, but simply dispose of the effluent by draining it into a leach field. About one quarter of United States homes dispose of their wastewater with septic tanks. However, effluent sewers are being looked at as a sewage treatment solution in areas where gravity sewer systems are not well-suited or when the high capital cost to build a gravity system is prohibitive. Areas that are less than ideal for gravity systems include areas that are large, but extremely flat and areas that require long-distance pumping, such as where homes are widely spread out or when several small villages or towns connect their sewage systems so that a centralized plant can be built.
Combined sewer
Retention treatment basins or large concrete tanks that store and treat combined sewage are another solution. These underground structures can range in storage and treatment capacity from to of combined sewage. While each facility is unique, a typical facility operation is as follows. Flows from the overloaded sewers are pumped into a basin that is divided into compartments. The first flush compartment captures and stores flows with the highest level of pollutants from the first part of a storm. These pollutants include motor oil, sediment, road salt, and lawn chemicals (pesticides and fertilizers) that are picked up by the stormwater as it runs off roads and lawns. The flows from this compartment are stored and sent to the wastewater treatment plant when there is capacity in the interceptor sewer after the storm. The second compartment is a treatment or flow-through compartment. The flows are disinfected by injecting sodium hypochlorite, or bleach, as they enter this compartment. It then takes about 20‑30 minutes for the flows to move to the end of the compartment. During this time, bacteria are killed and large solid materials settle out. At the end of the compartment, any remaining sanitary trash is skimmed off the top and the treated flows are discharged into the river or lake.