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.

H2S Control and Removal

Complete removal of H2S can best be accomplished by oxidation by chlorine; conversion of chlorine, resulting colloidal sulfur and polysulfides by metabisulfite or sulfur dioxide to thiosulfates, and then converting to sulfates by rechlorination.

Filtronics series of reaction vessels takes advantage of this mechanism by insuring break point chlorination and carrying a 0.5 mg/l free chlorine residual to the distribution system for disinfection and sulfur bacteria control. The reaction is begun in the first reaction vessel where all the forms of sulfur are produced. Between the first and second reaction vessel, sulfite is added where the thiosulfate is then formed. The thiosulfate oxidizes very rapidly producing a sulfate and a sulfite. Thus another sulfite is available to continue the conversion of any sulfur or polysulfides. By going to breakpoint and carrying a free residual through the system, re-chlorination is not necessary. 

For more information, check out our H2S Technical Bulletin at


NXT-2 Media Extends Bed Volumes

NXT-2 is a patented media which provides users with the lowest treatment costs per thousand gallons for adsorption technology. This is due to the high surface area of the media, which is often times greater than 250 m2/gram.  High surface area increases the medias capacity for arsenic adsorption and extends bed volumes.

In addition to the high surface area, NXT-2 is stable. Once the arsenic is bound to the media, NXT-2 will not release the bound arsenic in the event of pH upset. Release or “spiking” occurs when the pH of the water exceeds the zeta point pH of the media. NXT-2 has a high zeta point pH of 9.5 while many iron based medias have a zeta point of 7.8 pH. This stability reduces the potential to distribute water which has high arsenic concentrations  in excess of the EPA standard, due to loss of pH control.

For your smaller arsenic removal projects, NXT-2 is a cost-effective solution.

Learn more by downloading our technical bulletin here:

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! :)

Filtronics Repstravaganza

Filtronics will be hosting our first Repstravaganza at the WEFTEC show in New Orleans Convention Center on October 2, 2012 at 8AM. All filtronics manufacturer reps, interested engineers, consultants, health department officials, Municipal Water Treatment department personnel are invited to attend. The topics to be covered will be our Industrial line of GE Membrane Products, our adsorptive NXT-2 media, and Filtronics product line including our new EM-mini solution. We will be raffling off several prizes including a new driver by TaylorMade. Coffee and danishes will be served.

If you are interested in attending our event, please contact Jan James from our sales team for more information. If you cannot attend our event, we will be available to answer any questions you may have at booth #2665. The WEFTEC show runs from October 1 through October 3, 2012.