Most wastewater treatment system are Conventional Septic Systems. This includes a septic tank and a trench (leach field or drainfield. Conventional Septic Systems are normally found at a single-family home. However, older commercial properties are known to utilize septic systems.
The gravel/stone leachfield, or drainfield has been around for a very long time. Effluent (nicer term for sewage waste) is dispersed from the septic tank to an underground trench of stone or gravel. In Maricopa County, Leach Rock is used. On top of the trench is sand and/or dirt.
The effluent (sewage waste) is filtered through the leach rock and is treated by microbes in the soil below the gravel/stone trench.
Leach systems are can take up quite a bit of space. They may not be suitable for all residential properties.