The activated sludge and partially treated sewage pass on to the final settlement tanks, where the sludge settles out.

The liquid from the final settlement tanks is of sufficient quality that it can be discharged into Salmons Brook, from where it makes its way into the Lee Navigation.

They occur naturally in aerobic lagoons, activated sludge processes, in trickling filters and in final settlement tanks and are a significant factor in removing suspended bacterial cells and algae from the water column.

It is not known for certain how the fish got in, although staff at Yorkshire Water think they may have been washed in back in 2007 when a nearby beck burst its banks, engulfing part of the adjacent works which includes the final settlement tank.