Hydraulic Loading Rate Calculator
Determine the volume of wastewater applied per unit area of a treatment system.
Understanding Hydraulic Loading Rate (HLR)
The Hydraulic Loading Rate (HLR) is a fundamental parameter in environmental engineering used to design and evaluate the performance of wastewater treatment components such as septic drain fields, sand filters, constructed wetlands, and clarifiers. It represents the depth of liquid applied to the surface area of a biological or physical filter over a specific time period.
The HLR Formula
The calculation for HLR is straightforward, dividing the total volume of water by the surface area of the receiving medium:
HLR = Q / A
- Q: Flow Rate (e.g., Gallons per Day or Cubic Meters per Day)
- A: Surface Area (e.g., Square Feet or Square Meters)
Why HLR Matters
Monitoring the hydraulic loading rate ensures that a system is not "flooded." If the HLR is too high, the wastewater passes through the treatment medium too quickly (lowering retention time) or causes surface ponding and system failure. Conversely, a loading rate that is too low may lead to the drying out of biological films necessary for treatment.
Typical HLR Ranges
| System Type | Typical HLR (GPD/ft²) | Typical HLR (m/day) |
|---|---|---|
| Slow Sand Filter | 0.05 – 0.15 | 0.002 – 0.006 |
| Rapid Sand Filter | 2.0 – 10.0 | 0.08 – 0.40 |
| Constructed Wetland | 0.5 – 1.5 | 0.02 – 0.06 |
| Septic Drain Field (Loam) | 0.4 – 0.6 | 0.016 – 0.024 |
Example Calculation
If a small community wastewater system processes 10,000 gallons per day (GPD) and utilizes a drainage area of 5,000 square feet (ft²):
HLR = 10,000 GPD / 5,000 ft² = 2.0 GPD/ft²
This result would then be compared against local regulatory standards to ensure the soil or media can handle that specific volume without overflowing or failing.