Effluent is the outflow of water from a natural body of water or from a man-made structure.

Effluent in the man-made sense is generally considered to be pollution. This could entail outflow from a sewage treatment facility, the wastewater discharge from industrial facilities or water from a detention or process tank. The water can be of varying quality – ranging from heavily polluted to completely clean.

As an example, an effluent sump pump pumps waste from toilets installed below a main sewage line. In the context of wastewater treatment plants, effluent that has been treated is sometimes called secondary effluent or treated effluent. This cleaner effluent can then be directed to other processes or to the recipient depending on the step in the wastewater treatment plant.