Define infiltration and inflow, and describe their impact on plant performance.

• A. Infiltration is groundwater entering through leaks; inflow is rainwater entering via sewers; both increase flow, overload treatment, and degrade effluent quality
• B. Infiltration is groundwater entering through leaks; inflow is rainwater entering via sewers; both decrease flow and improve effluent
• C. Infiltration and inflow are the same phenomenon and have no impact
• D. Only infiltration affects plant performance

Answer: (A)

Infiltration and inflow describe extra water entering the sanitary sewer system beyond the intended wastewater. Infiltration is groundwater seeping in through leaks in pipes, joints, and manholes. Inflow is rainwater or surface water getting into the sewer system via direct connections like downspouts, runoff drains, or misconnected storm drains. Both add to the volume of water that must be treated at the plant.

Because treatment plants are designed for a specific flow and hydraulic load, this additional water raises flows, often dramatically during rain events. The sudden increase can overwhelm treatment processes, shorten detention times, and strain clarifiers and disinfection stages, leading to reduced treatment efficiency and poorer effluent quality. In short, infiltration and inflow make the plant treat more water than planned, which can compromise performance and, if severe, cause bypasses or overflow issues.