Filter Loading & Backwash
Calculate operating filtration rate, required area and backwash water use.
Calculator inputs
Results
Equations and method
Filtration rate = total flow ÷ online filter area. Required online area = flow ÷ selected maximum rate. Backwash flow = area per filter × backwash rate. Wash volume = wash flow × duration.
Worked example
At 500 gpm with two 50 ft² filters: service rate = 5 gpm/ft². A 15 gpm/ft² backwash for 10 minutes uses 7,500 gal per filter.
Scope and assumptions
Backwash one filter at a time. Rates are user-selected design values, not recommendations for every medium. Check supplier limits, water temperature, bed expansion and available wash-water supply. Daily volume excludes rinse and filter-to-waste.
Related water-treatment calculators
Equation values and input guide
Enter service flow and the number and area of filters actually online. Choose filtration and backwash rates appropriate to the media and temperature. Count individual filter washes when estimating daily water use.
- Online area
- Area per filter multiplied by the number in service. Exclude filters isolated for washing or maintenance.
- Filtration rate
- Total service flow divided by online area, in gpm/ft² or m/h. It is a hydraulic loading rate.
- Maximum filtration rate
- User-selected design limit. Required online area equals service flow divided by this limit; the tool does not determine the correct limit for the media.
- Backwash rate and flow
- Selected upward hydraulic loading and that rate multiplied by one filter’s area. Use manufacturer criteria for temperature and bed expansion.
- Wash duration, daily washes
- Minutes per individual filter wash and count of individual washes each day. Wash volume is wash flow times duration; daily volume multiplies that by the count.
Use the unit selectors beside the inputs. Units shown in the equations must be consistent when checking the math by hand.