influent

Influent is a term used primarily in environmental engineering and fluid mechanics to describe the liquid that enters a process, unit, or treatment facility. It denotes the feed or incoming stream to systems such as wastewater treatment plants, biological reactors, clarifiers, or filtration units. The word is often used as “influent stream” or “influent wastewater,” and it contrasts with effluent, which is the liquid that exits the process after treatment.

In wastewater engineering, influent quality and quantity influence the design, operation, and performance of treatment processes. Key characteristics routinely monitored include flow rate, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total suspended solids (TSS), nutrients, pH, and temperature. Influent variability—due to diurnal patterns, weather events, and diverse industrial discharges—affects the sizing of equalization tanks, screening, grit removal, and subsequent treatment steps.

In other contexts, influent can describe the feed stream to any unit process, such as tanks, reactors, sedimentation basins, or adsorption columns. Understanding influent properties is essential for achieving target effluent quality and for regulatory compliance.

Etymology and usage: influent comes from Latin influere, meaning “to flow into” (in- meaning into and fluere meaning to flow). While most common in engineering discourse, the term may appear in hydrological or process engineering literature to specify the entry point of liquids into a system.