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.

Backwash Description

With our filtration systems, the backwash is automatically initiated through a timer or in cases of a differential pressure limit being reached.

The purpose of the backwash step is to expand the filter bed to permit the cleaning of the collected contaminants from the media by introducing water at the bottom and discharging it at the top.  Backwash water must be supplied from a source that is filtered or clean and disinfected.  The rate of flow control valve provides the rate and constant flow necessary to achieve proper backwash.  It is imperative to have the proper rate.  Normal backwash time is four minutes.

The filter will always backwash prior to shutdown to clean the collected contaminants from the media.

Envirowash® - Filtronics Efficient Backwash/Reclaim System

Among the key features to any of the Filtronics Electromedia® filters is the efficient use of water for backwash. For a typical application:

A. Our Electromedia filters only require a four minute backwash.
B. The backwash to filtration ratio is less than one percent.
C. Recover after backwash is short, generally five minutes.

With a raw water turbidity of 15 NTU or less, we conservatively estimate the interval filter run time duration to be 18 hours. The backwash to filtration ratio for a typical 750 gpm application is:

¬ 750 gpm x 18 hours x 60 minutes = 810,000 gallons produced.

¬ The backwash cycle lasts four minutes at 1800 gpm and surface wash lasts one minute at 180 gpm. (The backwash rate assumes 20 degrees Centigrade temperature of the source water).

¬ The total volume of water used for surface wash and backwash total 7,380 gallons.

¬ The backwash to filtration ratio is 7380/810,000 = 0.009 or 0.9% for a 99+% efficiency.

Filtronics systems have a very short recovery after backwash. The filter system produces a filtrate of less than 0.2 NTU in only five minutes after a backwash. Thus, not only adding to the efficiency of the system but producing a high water quality quickly to the potable water system.

Assuring a two minute rinse to waste water cycle, the overall efficiency is 98.9%.

To further reduce the amount of water lost, many applications include our highly efficient Envirowash®, a backwash water reclaim system to recover more than 99.999% of the water used for backwash. That’s water that can be sold to the customer, meeting the same effluent requirements as the raw source.

This video shows a 22MGD plant with a four minute backwash. When you view the video, think about the sheer volume of water saved by such a short backwash cycle. As stated above, this water can be reclaimed and sold to the customer. Efficient AND Revenue generating! :)