Affluent usually describes someone or somewhere wealthy, as in an affluent neighbourhood. Effluent is waste water or sewage flowing out of somewhere, as in treated effluent released into a river. Remember: affluent flows in (wealth pours in), effluent flows out (waste pours out).
The core difference
Affluent means wealthy (or a stream flowing in); effluent means outflowing waste water.
- affluent — having a lot of money or material wealth: Property prices are steep in that affluent part of town.
- effluent — sewage or waste water discharged into a river, lake or the sea: The plant treats effluent before it reaches the river.
How to tell them apart
Both words trace back to the Latin verb fluere, "to flow", which is why they look so alike. Affluent comes from ad- plus fluere, picturing wealth flowing towards someone; the word can also describe a smaller stream flowing into a larger one. Effluent comes from ex- plus fluere, picturing something flowing away or out, and today it almost always refers to waste water or sewage.
The mix-up tends to happen in writing rather than speech, since the words sound similar when read quickly. A quick check: if the sentence is about money, status or a comfortable lifestyle, it is affluent; if it is about drains, treatment plants or pollution, it is effluent.
