What is hydraulic loading rate?

• A. Pressure of soil
• B. Temperature of effluent
• C. Volume flow per unit area of drainfield per unit time
• D. Depth of trench

Answer: (C)

Hydraulic loading rate measures how much wastewater is applied to the drainfield relative to the area available for absorption, over a given period. In practical terms, it’s the volume of effluent per unit area per unit time. This rate is what the soil and drainfield must be able to absorb and treat; if the input exceeds the soil’s infiltration capacity, water can pond at the surface, drive up groundwater, or cause system failure. That’s why the concept is defined as volume flow divided by the drainfield area and time.

Think of it as matching the daily flow to the size of the leach field: you’d take the total daily wastewater flow and divide by the area of the drainfield to get a rate like gallons per day per square foot (or liters per square meter per day). The other options describe different properties—soil pressure, effluent temperature, or trench depth—which aren’t about how much water is being applied per area over time.