Why use the R3F Technology for Filtration?

DavidBromleyM.Eng,P.E.

There are a number of reasons why the R3F Technology provides for a better method of filtration than other options that are in the marke tplace.

All filtration alternatives relate to the use of some type of barrier to separate particles from the fluid or gas going through the barrier. The typical methods are a“surface filtration” through the use of a screen, or “depth filtration” where a media is used and the particles are entrapped in void spaces, or a“cross flow filtration” where a barrier membrane or screen is used as a barrier from particles that are traveling across the screen and the fluid is exiting at right angles to the flow of fluid containing the particles.

If one looks at these three areas of filtration, the benefits of the R3F Technology can be highlighted.

1.0  Surface Filtration (Bag Filters,   Meltblown or Pleated Cartridge Filters, or Backwashable Screen or   Disk Filters).

Typically many parts of the filtration industry have used some very simple methods of filtration where a screen or a nonwoven or woven material is used to separate the solids from the fluid or gas. Some examples of these types of filtration products are polyester or polypropylene bag filters, and polypropylene cartridges which can be constructed in a pleated formor melt blown structure. These types of products are single use and disposable.There is, however, asimilarity to the R3F Technology in that they use a circumferential approach to filtration and thereby have amuch higher surface area to footprint ratio than a typical cross section type depth filter like a sand or multimedia filter. Flow through these typesof systems is from the inside - out for bag filters and the outside- in or the opposite way for cartridge or pleated type filters. Companies such as Hayword, Pall, Rosedale, Harmsco and 3M manufacture and sell these types of filters.The level of particle removal in these filters varies with the size of the pore opening in the material or the matrix. Many ofthese companies will rate the filter on a nominal or absolute basis where absolute is supposed to be 99%  removal of a particular particlesize. Some of the filters are rated to an absolute value for 1 micron.The difficulty with this approach, however, is that the matrix of structure always has to be the same to obtain the consistent quality and secondly, the filter will blind off rapidly the smaller the micronrating on the filter.

Another typical approach is a backwashable screen or disk filter. The screen filters use screens such as wedged steel screens. These filters use a variety of methods to

clean the filter but generally include some type of suction type system to remove accumulated particles that have been trapped in the system. Filtration companies such as Ahmiad sel lthese types of systems.Typically these filters will treat down to the 25 to 50 micron size range but one of their difficulties is the high production of backwash water. Manufacturers of these type of filters will suggest that a typical 150 mm (6 inch) diameter unit will achieve very high flow rates of 8 m3 per hr (30 gpm) 

to 50 m3/hr (200 gpm). With regard to disk filters these filters resemble a stack of CD’s where the flow is fromthe outside-in where each disk is comprised of a screen. As the pressure builds up to 7- 10 psi the filter then goes into a backwash mode.In backwash, the water is sent into the systemon a reverse flow. This causes the disks to expand and separate aswell as rotate. This action allows solids to escape in the backwash water. The system is reasonably successful for large particle removal (i.e. 75micron)

The R3F Technology compares well to these types of systems in that a single Martin R3F filter unit can provide up to 99% removal (on a particle count basis) of 2 micron particles and is easily backwashable at backwash levels that represent 0.3

% to 1% of the throughput volumes. As a matter of interest a highly efficient sand filter uses approximately 3% to 5% of the throughput for backwash water. As a result, the MartinR3F Technology allows an operator an alternative to the on-going costs of the disposal and replacement of bag and cartridge filters. With regards to the backwashable screen filters, the Martin R3F Technology with its standard 150 mm (6 inch) size will provide flow rates up to 8 m3 per hour or greater when removing particles in the 25 to 50 micron range. The typical screen backwashable filter would have difficulty accommodating this particle size range especially if the loading was above 50mg/l of total suspended solids. If the particle size was 100micron or greater and the level of solids in the water to be filtered was 30 to 50 mg/l, then it is possible a

$3500 to $4000 backwashable screen filter could be applied.

The Martin R3F Technology is a perfect application at a much lower cost and more efficient filtration system to the bag filters, single use cartridge filters or the backwashable screen filters.

2.1     Depth  Filtration(Sand Filters, Dual or multimedia filters, disposable depth filters).

The most common type of filtration system for potable water drinking water systems and other industrial water systems such as for cooling water reuse systems,washwater systems, and utility water systems are the typical cross sectionas down flow sand filter or dual media (anthracite and garnet)

There are numerous manufacturers of this type of filter.

In addition some of the single use systems discussed above have alsotriedtodevelopa singeuse(disposable) depth filter. These cartridge type filters use a winding of polypropylene or a graduated porous melt blown structure that is designed to provide depth filtration.

The down flow sand or dual media filter is a well tested and trusted filtration process. It is clearly the status quo, however, over the last 20 years, an assessment on these filters has identified a number of issues of concern with the down flow filter.

a.   Backwash rates and volumes have to be carefully monitored simply because the opportunity to send the backwashwater back to the head of the plant has been removed for potable ter treatment facilities. As a result the disposal volume of backwash water has to be carefully considered.

b.   Backwash rates have to be maintained to ensure proper bed expansion and stratification.

c.    Air scouring has been found as an effective tool to improve back wash efficiency but because of the size of the beds, the amounts of air required can be costly.

d.   According to the AWWA’s National Assessment of Particle Removal byFiltration -1998, spiking of particles occur in the effluent during normal operations. These spikes can last for 0.5 hours to10 hours and normally occur during filtration operations. Spiking is a concern because it is during these time periods that cysts (Cryptosporidiumand Giardia) escape from the filtration system. In addition particle removals, is consistent for all particle sizes in a sand filter. In other words if a sand filter removes 95 % of particlesin the 2- 4 micron range, then it is likely the filter will remove 95% of  particles in the 10 to 15 micron size.The typical down flow dual media filter is not very selective. This could be a detriment if the cysts in a particularwater to be treated are in a narrow size range or for that matter, if one wants to have selective filtration of a particular particle size range ,the typical down flow dual media filter will have difficulty satisfying this objective.
 
    

e.   Cross sectional area down- flow filters require a significant amount of floor space whereas the R3F Technology requires significantly less due to an increase in surface area close to 20 times that of conventional cross sectional technology.     .

3.1  Crossflow Membrane Systems

Over the last 20 years, the use of cross flow membrane systems has grown dramatically, particularlybecause of the needto provide ahighqualityin certain waterssuchasthe pharmaceuticalandwafer board industries.However the use of membrane technology has also grown in thetreatmentofpotable watersbecause of the need to remove cysts such as Cryptosporidiumand Giardia.

Crossflow membrane technology has four key levels as defined by the capability toremoveacertain sizeparticle.Ina recent AWWAseminar on “Emerging Treatment Technologies- November 2002”, the four categoriesand the particle size range where removals were greaterthan95%areasfollows;

• Microfiltration – 1 to 10 micronwhichis therangeforcysts (Cryptosporidiumand Giardia) and bacteria
• Ultrafiltration - 0.05 to 1micronwhichistherangeof colloidal particles andviruses.
• Nanofiltration - 0.001 to0.05 micron
• Reverse Osmosis-0.0001to0.001 micronwhichisthemolecularrange for dissolvedsalts.

Wit hsome of the higher purity requirements in pharmaceutical waters or the need to remove dissolved salts, reverse osmosis systems are certainly the technology of preference. For many treatment requirements,however,the removal of the particlesize of 1 to 10 microns is considered as the ideal range of solid separation as it provides a high quality water and for potable water it is the appropriate range to control Giardiaspp. and Cryptosporidiumspp. cyst removal from the water. As a result the most popular cross flow membrane category is microfiltration. One of the benefits of Microfiltration is the fact that the use of pressure to feed the system is much lower than the pressure requirements used in Reverse Osmosis systems. In fact a typical pressure drop across a Microfiltration system is 15 to 35 psi. This is a significant improvement from the early stages of development for this technology when reverse osmosis systems operated at pressures of 800 to 1000 psi. Obviously this made operating costs very high. Many companies now manufacture and sell membrane technology such as Zenon( who were the pioneers in this technology development),U.S. Filter and Pall. A very popular use of the technology is to use it in a high solid content liquid to operate under vacuum at negative 10 to 12 psi and then concentrate the solids even further.The permeate or treatedwater from these systems is usually a very high quality water that can be easily be reused. 

As referenced at the AWWAEmergingTechnologies Seminar typical capital costs for the microfiltration equipment only are $0.20 to $0.45 per gallon per day ($300 per gpm to $650 per gpm) and full installation of such a system would be $0.75  to  $1.25 per gallon per day or $1100 to $1800 per gpm. Membranes typically last 4 to 6 years and cleaning under a clean – in – place ( CIP) takes place approximately once every 30 days and has a duration of approximately 6 hours. The membranes are typically back washed every 15 to 30 minutes for a period 45 to 75 seconds. Backwash water is usually chlorinated to 3 to 8 mg/l and at pressures of 35 psi. Some backwash systems such as with the Zenon systems use air pulsing as part of the backwash procedure. According to a study undertaken by Farahbakhsh and Smith, February 2001, backpulsing results in the loss of productivity since water production is halted and significant quantities of finished water are used during this period.On the other hand backwashing every 15 minutes had a significant effect on the length of run as the run time was doubled compared to a backwash of 30 minutes.

Pretreatment using alum (2 to 10 mg/l) or PACl ( 4 to 8 mg/l) of the waters prior to cross flowmembrane filtration is also typically required to reduce biofouling. The Farahbaksh et al 2001 study confirmed that fouling was a result of the smaller particles (2 to 15 microns). As a result pretreatment with the alum or PACl  improves run time significantly and the effectiveness of backwash. Typically membranes used to treat water after clarification for a typical surface water treatment system can achieve 90% and greater removal of turbidity,and particle counts less than 2 per ml (except after backwash) for particles in the range of 2-15 microns. TOC removals are generally very low. (Farahbakhsh et al, February, 2001). Again according to the same study,energy usage for a membrane system used to treat water after clarification in a potablewater treatment plant would be approximately 0.45 kWh to 0.50 kWh/ 1000 litres (1.7kWh to 1.9kWh /1000 gallons).

Another major component to the use of membrane technology is the ability to monitor the integrity of the membrane such that a particle release does not occur and potentially the discharge of cysts. Many municipal drinking water authorities now require continuous membrane monitoring for operations that use membrane technology for potablewater treatment.

4.1   COSTING & TECHNOLOGY 

        COMPARISONS

According to a 2001 paper prepared by G Finlayson of GHD Engineering Consultants in Sidney Australia entitled“ Real World Implementation of  Microfiltration” ,  the author suggested that when considering the cost of a particular filtration alternative where such alternative is capable of satisfying the water quality objectives, the following 

parameters should be considered;

5.1  Some Test Results and
       Conclusions





After extensive testing by the University of Alberta, Edmonton, Alberta, Canada and the EPA, Cincinnati, Ohio it was found that the R3F Filtration Technology provides some interesting similarities in performance to microfiltration technology but also provides significant cost and operational benefits when compared to microfiltration, as well as typical downflow dual media sand filtration and the disposable cartridge and bag filtration systems. The R3F Filtration Technology tested at the U of Alberta used glass beads as fine as 20 microns in diameter whereas the EPA tested a fine garnet media.

1. In most of the tests at the University, the glass bead size for the filtration media was in the order of 40 to 80 micron diameter resulting in a void space size of 2.5 to 5 microns. It was found that when testing the R3F Technology, it  was evident that as the media became smaller the greater percentages of smaller particles were separated from the water being filtered. As indicated earlier this was different than with sand filters which seem to obtain the same percentage removal for all particle sizes but is very similar to membrane filtration which is very selective on particle size based on the pore size ofthe membrane. As a result the R3F Technology provides the benefit of selective removal which can be used to remove something like cysts/ pathogens (Giardia spp. andCryptosporidiumspp).
   
   
   
   
2. It was found that using a media of 40 to 80 microns and one pass through a 27mm(1.1 inch) bed depth, the R3F Filtration Technology could remove 95 to 99%  of 2 to 15 micron particle size and approximately 95%  removals of B subtilis spores which was used as a surrogate for cysts/pathogens. The actual testing for spore removal showed over all log spore removals on Mil-Spec12 glass bead and Mil-Spec13 glass bead media were 1.21 ±0.06 and 1.63± 0.07, respectively. Due to the low standard deviation, the importance of repeatability of those runs demonstrates the consistency of R3F technology using the glass bead media for B. subtilis spore removal.
   
   
   
   
3. It was also found that to restore the media after use, a backwash to volume throughput was as lowas 0.3% and as high as 3% depending on the water quality being filtered. (i.e. the lower the influent particle count, the lower the backwash water requirements.) The filter was tested with influent particle counts has high as 30000 particles/ ml and as low as 200 particles/ml
   
   
   
   
4. In testing done by the EPA using 40 micron garnet in place of glass bead media it was found even finer levels of filtration took place with removals of Cryptosporidiumoocysts of 3.47 log. In similar testing using 3.0 micron PSL beads (a non-biological surrogate for Cryptosporidium) there was a 3.30 log removal for the R3F system compared to an average log removal of 0.20 for the multimedia filter when both systems were operated without the use of any chemistry. Both studies above can be provided in full upon request.
   
   
   
   
5. With the simplicity of the R3F Technology, manufacturing costs are very low, giving the ability to achieve microfiltration levels at capital and operating costs that are lower than that for conventional sand filtration.
   
   
   
   
6. The significant benefit of the R3F technology over sand filtration is the much lower need for floor space and infrastructure as well as much lower backwash requirements. The ability to achieve high removals of Cryptosporidium and Giardia without the need for chemistry is an obvious major enhancement over conventional media filtration. These benefits combine to provide significant cost savings over sand or typical cross sectional area filtration systems. Another significant benefit is the ability to change out media because of a change in filtration needs or a situation where the media has been contaminated. In the R3F Technology, the media would be fluidized to the upper end of the column and discharged out the media outlet. Media can be added the same way.This is a far simpler procedure than having to vacuum out or shovel out the media as is the case with sand or dual media filters.
    
    
   
   
7. To accomplish high levels of filtration and added redundancy, the R3F Technology can easily be used in series and by using this approach, the operator can obtain a number of safe guards in the integrity ofthe systemby monitoring between filter columns and sampling effluent. In membrane systems, continuous membrane integrity monitoring is very costly.Withthe low cost of each filter column in the R3F Technology, redundancy is not a significant issue and the abilityto monitor effluent streams with back up filtration is very economical.
   
   
   
   
8. Finally the significant benefit of the R3F Filtration Technology over bag filters and disposable cartridges is the fact that the Technology is backwashable and will result in a significant operational cost saving and quick paybacks.

References

Ai-Ani, M.Y. et al, 1986. Removing Giardia Cysts fromLow-turbidity Waters by Rapid Rate Filtration. J. AWWA. 780:5:66.

AWWA2002. Emerging Treatment Technologies, Basic sand Applications of  UF/MF Membrane Technology Dr. Gil Crozes

Farahbaksh etal., 2001, Investigation of Ultrafiltration Performance on B.C. Drinking Water Quality, University of Alberta, Environmental Engineering Technical Report 02- 01

Finlayson, GHD Consultants Sidney Australia, 2002. Real World Implementation of Microfiltration

Horn, J.B. et al, 1988. Removing Giardia Cysts and Other Particles from low-turbidity Waters Using Dual-stage Filtration. J. AWWA, 80:2:68.

Hijnen,W.A.M.,Wouldemsen-Zwaagstra,J.,Hiemstra,P., Medema,G.J.,andVan der Kooij D. 2000. Removal of Sulphite-Reducing Clostridia Spores by Full-Scale Water Treatment Processes as a Surrogate for Protozoan (oo)cysts removal.

Huck, P. M., Coffey, B.M., Emelko, M.B., Maurizio, D., Slawson, R.M., Anderson, W.B.,Van Den Oever,J., Douglas ,I.P., O’Melia, C. R. 2002. J. AWWA, 94(6): 97- 111.