Two general methods are considered for wastewater treatment planning and design — centralized and decentralized models. Centralized wastewater treatment involves collection of wastewater from a large area to one centralized location where it is most commonly treated and discharged to local surface waters. Decentralized wastewater treatment provides treatment and discharge either directly on site or within close proximity. Advancements in technology allow either method to provide equivalent levels of treatment and protection of public health.
However, centralized and decentralized wastewater treatment systems vary greatly, as do the impact they have on environmental health and potable water supplies. Centralized systems obtain potable water from one location, that water is utilized by the public, then collected as wastewater and transported to the treatment plant following which it is discharged to surface waters. This process short-circuits or bypasses the local water cycle and can cause aquifer depletion.
The decentralized model can collect, treat, and then discharge to the subsurface all within a local area. This has the benefit of replacing the original water resource back to the local aquifer. When adequately designed and installed, decentralized wastewater systems have the capacity to process large quantities of wastewater into the underlying soils, making it one of the most passive, sustainable forms of aquifer recharge.