Filtronics Daily Backwash Volume Calculation

The formula to calculate (and compare) daily backwash volume is:

(((Design Flow / Filtration Rate) * Backwash Duration * Backwash Rate) + (Rinse To Waste Time * Design Flow)) * Number of Backwashes Per Day

The Filtronics system does backwash more frequently than a comparable greensand plant BUT our backwash duration is significantly shorter producing far less waste water, even when taking the more frequent backwashes into consideration. Greensand backwashes can range in duration from 15 minutes at best, and sometimes 45 minutes or more (30 minutes being an approximate average) depending on water quality and how close the bed is to depletion. Filtronics systems have a 4 minute, high efficiency backwash event and a media bed that never needs to be replaced or regenerated. The end result is far less waste water production using the Filtronics system than traditional coagulation & filtration technologies. Our backwash to filtration ratio (without reclaim) is in the 2-3% range whereas greesand systems can be as high as 20%.

For example, using 1,400 gpm as design flow and assuming a WORST CASE Filtronics scenario and a BEST CASE greensand scenario, we can calculate the daily backwash volume of the Filtronics unit as 37,800 (three backwash events of 12,600 gallons each) and a corresponding greensand plant’s daily backwash volume as 64,400 (only one backwash event).

Worst Case Filtronics Assumptions:

Filtration Rate: 10 gpm/ft2 (norm – can be as high as 15 gpm/ft2)
Backwash Duration: 4 minutes (norm)
Backwash Rate: 20 gpm/ft2 (18-20 norm)
Rinse To Waste Time: 1 minute (norm)
Number of Backwashes Per Day: 3 (based on 8 hours/run in 24 hours)

Filtronics Worst Case Calc:
(((1400/10) * 4 * 20) + (1 * 1400)) * 3
(11200 + 1400) * 3
37,800 daily backwash gallons

Best Case Greensand Assumptions:

Filtration Rate: 5 gpm/ft2 (norm – can be as low as 1 gpm/ft2)
Backwash Duration: 15 minutes (ideal circumstances)
Backwash Rate: 15 gpm/ft2 (norm)
Rinse To Waste Time: 1 minute (ideal circumstances)
Number of Backwashes Per Day: 1 (based 24 hour run time)

Greensand Best Case Calc:
(((1400/5) * 15 * 15) + (1 * 1400)) * 1
(63000 + 1400) * 1
64,400 daily backwash gallons

An added benefit, because of the low volume of backwash water produced per event (12,600 gallons in this example), it is more feasable to store Filtronics backwashes in a reclaim tank and reprocess clarified supernatant to the filter. This gives us a backwash to filtration ratio well below 1%.

Also, because of the frequency of backwash, we don’t have the schmutzdecke issues or mudball formation so common with greensand systems where backwash intervals are frankly too infrequent. Hence, we do not require air scour or surface wash systems in our filters. This provides significant advantage in terms of reduced maintenance but, more importantly, gives us consistent performance year after year.