Operator terms · Wastewater
What is influent in wastewater treatment?
Influent is the untreated wastewater flowing into a treatment plant. It is what the plant receives before any treatment, carrying the solids, organics, and pollutants the process removes. Comparing influent to effluent, the treated water leaving the plant, shows how much the plant removed.
Last reviewed 2026-08-03
Where it enters
Influent arrives from the collection system and passes first through preliminary treatment, where screening and grit removal take out the coarse material before the biological steps.
Measuring performance
Operators test influent and effluent for BOD and TSS. The drop between them shows how well the plant is treating the wastewater.
Common questions
- What is the difference from effluent?
- Influent enters the plant untreated; effluent leaves it treated.
- Where does influent come from?
- The collection system that carries wastewater from homes and businesses.
- Why test the influent?
- To measure the incoming load and compare it to the treated effluent.