What is the difference between a septic tank and a sewage treatment plant?

A septic tank provides primary treatment, separating solids from liquid. The liquid effluent still requires a drainage field to treat it further before it enters the ground. Septic tanks, however, cannot discharge to a watercourse under the General Binding Rules. 

In comparison, a sewage treatment plant provides secondary biological treatment, producing a cleaner effluent that can, in mostcases, be discharged directly to a watercourse under the General Binding Rules (or with an Environment Agency permit for larger discharges). 

Older septic tanks that were already discharging to a watercourse before the 2015 General Binding Rules came into force had to be upgraded or replaced by 1 January 2020, or on change of property ownership – whichever came first.