Process diagram: Wastewater treatment

The diagram illustrates how sewage is cleaned at a wastewater treatment plant, a process with eight stages.

Overall, the water passes through a series of screening, settling and treatment stages before it is discharged, clean, into the river. The solid waste is dealt with separately as sludge. The first three stages remove solid materials, the next two use bacteria and settling to deal with the remaining waste, and the final three disinfect and release the water and treat the sludge.

First, the sewage is screened so that large objects are removed. It then passes through a channel in which sand and stones settle, in the stage called grit removal. In primary settling, solids sink to the bottom and form sludge.

Next, air is pumped into the water in the aeration stage, so that bacteria can digest the waste. Any remaining particles are then allowed to settle out in secondary settling. The water is disinfected with chlorine or ultraviolet light, which kills germs.

Finally, the clean water is discharged back into the river. The sludge that was removed earlier is treated separately, and it is dried and used as fertiliser.