Calculator
Enter your filter size and cleaning settings. The calculator estimates backwash flow, volume, drain sizing, and pump power.
Example data table
These sample inputs show how changes in size and time affect water use.
| Filter type | Diameter | Backwash rate | Backwash (min) | Cycles/week | Backwash flow | Weekly water |
|---|---|---|---|---|---|---|
| Media | 24 in | 15 gpm/ft² | 3 | 7 | ~118 gpm | ~2,480 gal |
| Screen | 18 in | 4 gpm/ft² | 2 | 10 | ~39 gpm | ~780 gal |
| Disc | 30 in | 6 gpm/ft² | 4 | 14 | ~221 gpm | ~12,400 gal |
Formula used
- Filter area: A = π × (D/2)²
- Backwash flow: Qbw = A × Rbw
- Backwash volume: Vbw = Qbw × tbw
- Rinse volume: Vr = (0.5 × Qbw) × tr
- Cycle volume: Vcycle = Vbw + Vr
- Pump power: P = ρ × g × Q × H ÷ η
How to use this calculator
- Pick your filter type to load a typical backwash rate.
- Enter the filter diameter and number of filters installed.
- Set backwash and rinse minutes from your controller settings.
- Enter cycles per week based on pressure rise or schedule.
- Add head and pump efficiency to estimate power needs.
- Press Calculate, then export CSV or PDF if needed.
Backwash rate selection and media lift
Effective cleaning depends on reaching the media expansion range for your filter type. Typical targets are 12–15 gpm/ft² for sand, 15–18 gpm/ft² for multimedia, and 5–8 gpm/ft² for disc or screen elements. In colder water, viscosity increases, so the same pump flow produces less lift. If winter operation is common, plan a 10–15% flow margin or verify expansion with a sight glass.
Flow, volume, and water budget impacts
Backwash flow scales with filter area, so diameter changes have a large effect. Doubling diameter increases area fourfold, multiplying required backwash flow and volume. Use the weekly and annual totals to estimate wastewater handling and source drawdown. A single 24-inch sand filter at 15 gpm/ft² needs roughly 118 gpm; a 10-minute backwash plus 2-minute rinse can exceed 1,400 gallons per event.
Cycle frequency driven by pressure rise
Instead of a fixed calendar schedule, set backwash triggers from differential pressure. Many irrigation installations backwash when ΔP rises 7–10 psi above clean conditions, while high-solids water may require tighter thresholds. Frequent short cycles waste water, but long intervals can cause channeling and poor filtration. Compare “cycles per week” scenarios and choose the lowest frequency that keeps outlet quality stable.
Pump head, power, and electrical cost
Power requirements rise with both flow and head. Add static lift, pipe friction, valve losses, and the filter’s backwash head loss to estimate total head. The calculator converts head and flow into hydraulic power and adjusts for efficiency. A 120 gpm backwash at 60 ft head can draw several kilowatts, so running events off-peak or batching zones can reduce operating cost.
Field verification and maintenance checks
Numbers are a starting point; validate with field measurements. Confirm backwash flow using a calibrated meter or timed tank fill. Watch discharge clarity; persistent turbidity suggests under-backwashing, damaged laterals, or worn discs. Track clean-filter ΔP and cleaning recovery; if ΔP never returns near baseline, inspect media grading, screen integrity, and valve timing. Consistent records make troubleshooting faster.
FAQs
1) How do I choose a backwash rate?
Start with the default for your filter type, then adjust for water temperature and manufacturer guidance. If the bed does not expand or discharge stays dirty, increase rate within pump and valve limits.
2) What if my pump cannot reach the required flow?
Reduce the number of filters backwashed at once, shorten piping, or increase line size to cut losses. If flow is still low, consider a dedicated backwash pump or a smaller filter bank per station.
3) Should I include a rinse cycle?
Yes for sand and media systems. Rinse helps settle the bed and prevents a dirty surge to irrigation lines. Disc and screen systems may use a brief flush, depending on the controller and water quality.
4) How often should I backwash?
Use differential pressure whenever possible. Backwash when ΔP rises about 7–10 psi above clean conditions, or when outlet quality drops. Fixed schedules are acceptable only when inlet solids are stable.
5) Why does the calculator ask for head and efficiency?
Head and efficiency estimate electrical demand. Higher head from long runs, elevation, or clogged piping increases power draw. Efficiency varies by pump and motor condition, so use a realistic value from testing or nameplate data.
6) What maintenance improves backwash performance?
Keep valves and actuators synchronized, clean strainers, and verify flow meters. Replace damaged laterals or worn discs, and re-grade or replace media when recovery after cleaning is poor. Record clean ΔP to spot gradual fouling.