Conventional systems

Conventional Systems
Systems that have been traditionally used to collect municipal wastewater in gravity sewers and convey it to a central primary or secondary treatment plant prior to discharge to surface waters.
Source: Terms of the Environment ...

And solar water heaters last between 15 and 40 years--the same as conventional systems--so after that initial payback period is up, zero energy cost essentially means having free hot water for years to come.

See also: Waste, Water, Standards, Surface water, Treatment