There really is no contest. A sewage treatment plant wins hands down.
The main reason you purchase a sewage treatment plant or septic tank is to treat the sewage and wastewater from your dwelling. A septic tank is basically just an empty tank. The only form of treatment that occurs is basic settlement.
In contrast, a sewage treatment plant actively treats the incoming sewage by aerating the wastewater, either by a small air blower, or naturally via ventilation.
So a septic tank has a treatment efficiency of approx 25%. In contrast, our sewage treatment plants have a treatment efficiency of almost 99%. Big difference.
Low quality septic tank effluent means pollution of groundwater, pollution of streams and clogging of soakaways with effluent with very high amounts of suspended solids.
So to protect the environment, to protect your drinking water and to keep your soakaway working well – a sewage treatment plant is a must have.