Sewage treatment works and flow to full treatment

Flow to full treatment (FFT) is a measure of the volume of wastewater (sewage) that a sewage treatment works is treating.

Maximum flow to full treatment is the maximum permitted level of wastewater, in litres per second, that a sewage treatment works can treat before it can divert the waste to storm overflows.

Under normal (dry weather) conditions, sewage entering a works should undergo primary settlement (separating solids and liquids) and secondary treatment (micro-organisms break down organic matter) before being discharged into a river or sea.

However, in storm conditions, the volume of wastewater increases, and the treatment works runs the risk of becoming overloaded.

At a certain level of flow (maximum flow to full treatment), the treatment works is permitted to divert the wastewater into storm tanks, where it may be stored for a time. If these also become full, the wastewater is discharged directly into streams, rivers or the sea via storm overflows.

More information from Ofwat: Flow to Full Treatment (FFT) explainer.

How the sewage system works

How sewage works infographic — A representation of the main stages of wastewater treatment commonly used in the UK, including screening and grit removal, primary settlement, biological treatment and secondary settlement. Sewage sludge collected in settlement steps is digested and dried before it is used in agriculture. During heavy rainfall, storm tanks capture high flows beyond the treatment works capacity for a duration determined by the size of the tank.
Adapted from the NEPC testing the waters report