By Gary Battenberg
In Part 2 of this series, we looked at the finer points of filtration regarding clarification and remediation of sediment, odor and color removal from water to improve the aesthetics and potability. Additionally, the importance of obtaining the hydraulic characteristics of the well pump flowrate at 30 and 40 psi for the purpose of proper sizing of automatic backwash filters that do not exceed the available pump output was also discussed.
Now we are going to consider the diverse types of filtration media commonly used in our industry today, with close attention to hydraulic and temperature requirements to ensure consistent and reliable filtration performance. We left off with tannin removal from water and we will start there with the available treatment options.
As previously mentioned, tannin is the proverbial fly-in-the-ointment when it is present in a water supply and should be removed early in the treatment process. Methods used for many years with good success include oxidizing processes (such as super chlorination or hydrogen peroxide injection) dating back to the mid-1950s and are still among the most effective options to date, especially if the water is complexed with iron, manganese and/or hydrogen sulfide.
The disadvantage with this treatment configuration is the total footprint requirement, which includes the dosing pump and solution tank, contact tank and backwashing carbon filter and softener. In recent years, the hyper-contacting tank has replaced the traditional 80- or 120-gallon (302 or 454- liter) contact tanks with a much smaller tank volume of 25 gallons (95 liters), making the footprint much smaller. The hyper-contact tank utilizes an internal manifold that incorporates a mixing device similar to a static mixer (that increases the rate of mass transfer of gases and chemicals into the water quickly and efficiently) versus injection of a chemical or gas into the water stream and then expose that stream into a large body of water, where slow movement creates the contact time.
Tannin color bodies are high-molecular-weight, microscopic, colloidal particulates. Recent tannin treatment options include membrane hyperfiltration such as reverse osmosis, nanofiltration or ultrafiltration. Because tannins exhibit high molecular weight, membrane separation is highly effective for tannin removal. More recently, electropositive filtration has demonstrated a high affinity for tannin removal from water with good success. (Tannins are colloidal and the electro-adhesion capability of the media works like a particle magnet. This will be discussed further in Part 4.) The advantage over membrane type treatment is there is no wastewater stream to contend with because the electropositive media functions as a dead-end or final barrier filter.
Filter media selection
There are multiple choices and methods available for primary filtration whether it is for sediment capture, or more complex problems such as bacteria and algae remediation. We will consider the most common media including, dissimilar metals, garnet, gravel, sand, calcite and corosex, manganese greensand, manganese-reactive media, birm, Filter-Ag and granular activated carbon. The importance of hydraulics in water filtration is not to be underestimated or taken lightly. The recommended media service flowrates and hydraulic lift requirements for backwash must stay within the hydraulic constraints of the well pump output for consistent, dependable and long-term performance and most importantly, customer satisfaction.
Dissimilar metals
The most common of this type of media is KDF 55, KDF 85 and KDF fines. These media catalyze oxidation-reduction reactions, which means the media require an electrical potential to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant). The electrical potential is created by the TDS (conductivity) in the water. The media requires a minimum of 200 mg/L TDS to create the oxidation reduction potential (ORP). KDF 55 is typically used to treat for free chlorine and heavy metals removal.
KDF 85 is typically used to treat iron, manganese and hydrogen sulfide. This medium can operate at up to 15 gm/ft² with sufficient TDS. KDF 85 is, however, the heaviest of all and has a high apparent density (weight) of 171 lbs/ft³ and requires 30 gpm sq/ft² for backwash to ensure required lift. KDF fines are typically used in carbon blocks and the media has been introduced in block form as well for filter systems. CAUTION: This media is not recommended for use where high turbidity and/or phosphate are present. High turbidity and phosphates will accumulate and build up on the media, which causes surface blinding and reduces the ORP or insulates the media from the water. Phosphates also will cause the media to solidify into a solid mass, which results in high pressure drop.
Garnet
Garnet is the second heaviest medium in our list with an average weight of 144 lbs/ft³ and is typically used as an underbed, where media apparent density is 100 lbs/ft³ or more. Garnet is available in typical mesh sizes of #8, #8-12, #30-40 and #60 mesh. The larger #8 mesh size is typically used as underbed for multi-media, sand and other high apparent density media filters. The smaller #30-40 is usually the second layer in a multimedia filter followed by sand and anthracite. The smallest #60 mesh garnet is used as a polishing medium, which yields particle reduction down to 2-5 microns in professionally designed systems.
Gravel
Gravel is available in multiple effective sizes including 0.5-inch X 0.25-inch, 0.25-inch X 0.125-inch and #20 flint, and has apparent density of 100 lbs/ft³. For residential filters, 0.25-inch X 0.125-inch is the recommended underbed for softening and lighter filtration media with apparent densities less than 100 lbs/ft³. A 3-5-inch depth is typically required to ensure complete coverage of the lower collector to eliminate the potential for a transverse distributor effect. This condition is created when the lower collector is partially exposed on one side and is usually caused by laying systems fully horizontal during transport which allows the media to shift. When the filter of softener is stood upright, the gravel may not return to a previously level condition. Therefore, uneven flow through the resin or filter media will cause accelerated service flow to the exposed collector and uneven backwash, resulting in mediocre quality effluent.
Sand
Sand is one of the oldest filtration media and like gravel, has an apparent density of 100 lbs/ft³. For mono bed sand filters, #8-12 garnet is recommended as an underbed to prevent fluidization of the sand. Keep in mind that the underbed must always be heavier when the filter media is 100 pounds or more. Sand filters typically yield clarity to the 10-micron level @ 5 gpm/ft² service flowrate. A healthy human eye can see 40 microns. Backwash flowrates are between 10-15 gpm/ft², depending on water temperature and require a minimum 35-percent bed expansion. For example, water that is 10⁰F colder will provide approximately 40-percent bed expansion versus 35-percent bed expansion during backwash. Conversely, water that is 10⁰F warmer will decrease lift from 35 percent to approximately 21 percent. NOTE: For water treatment where potability is a requirement, make sure garnet, gravel, sand and anthracite coal meet AWWA B100 purity standards for arsenic leaching.
Calcite and corosex
These two media are sacrificial and elute (dissolve) into acidic water to elevate low pH conditions. Calcite is a hard calcium carbonate (CaCO₃) that dissolves slowly to elevate low pH (5-6) to neutral, which reduces the potential for corrosion and the leaching of copper, lead and other metallic piping and fixture metals of service plumbing systems. Corosex is granular magnesium oxide (MgO) and is used to elevate extremely low pH (4-6) waters and elutes at a higher rate than calcite. It is not recommended to use this medium where pH values are less than 4.0 because solidification may render it inoperable. Corosex is commonly mixed with calcite to prevent over correction of pH.
Service flowrates for these media range between 3-6 gpm/ft², while backwash flowrates are 8-12 gpm, depending on water temperature and require a minimum bed expansion of 35 percent. NOTE: These media require periodic replenishment due to their sacrificial characteristics. Additional hardness will be added to the effluent and should be combined with the raw water hardness to calculate the total compensated hardness where a water softener is specified.
Manganese greensand and manganese reactive media
Manganese greensand has been used in water treatment since 1925 for the removal of iron, manganese, hydrogen sulfide, arsenic and radium. It is regenerated with potassium permanganate (KMnO4) at 0.25 lbs (4 ounces) /ft³ periodically. More recently, the original manganese greensand was improved upon with the development of Manganese Greensand PlusTM. Both media had/have an apparent density of 85 lbs/ft³. The original manganese greensand, now obsolete, had a temperature limitation of 85⁰F because of the glauconite core substrate, whereas the newer product has a silica sand core substrate and no temperature limit. Service flowrates range between 2-12 gpm/ft² and the backwash rate requirement is 12 gpm/ft² at 55⁰F, with a bed expansion of 35-50 percent. Pay close attention to the pH limits for this media The pH should be no lower than 6.2 nor greater than 8.5.
Technical Tip: Field tests for a standalone manganese greensand iron filter without a softener have shown that a 75-percent bed expansion (where a 75-percent freeboard in the media tank is available) virtually eliminates the ferric and manganese oxide residues, which react with chlorine and may produce a tea-colored tint to the water. (In this application, the media tanks were 10-inch diameter X 60 inches tall with 1 cu/ft³ of media, which provided a 35-inch freeboard area and easily allowed for the 75-percent bed expansion without losing media to drain while backwashing at 14gpm. Additionally, the filters were regenerated counter currently to prevent dilution of the potassium permanganate.) This was proven for a swimming pool application where the water was treated with manganese greensand and chlorine-dosed effluent water and no discoloration resulted.
Manganese-reactive media function well for oxidation of iron, manganese and hydrogen sulfide and can either be regenerated with potassium permanganate, chlorine, ozone, or air injection to aid oxidation. They can function as catalytic media requiring only periodic backwash. These media range in apparent density of 120-125 lbs/ft3. Service flowrates are recommended at 5 gpm/ft² and backwash rates of 22-30 gpm/ft² with bed expansion of 15-30 percent depending on water temperature. The pH values should be between 6.5- 9.
Birm
Birm is a lightweight media with an apparent density of 46-50 lbs/ft³. It is primarily used for iron and manganese reduction and requires no regeneration chemicals. The water to be treated, however, must have a minimum pH above 6.8 for iron alone, with a desirable pH between 8.0 and 9.0 for best results where iron and manganese are combined. Additionally, a minimum dissolved oxygen (DO) content of 15 percent of the combined iron and manganese is required to ensure the oxidation reaction, which converts ferrous (clear water) iron and manganese to ferric iron and manganese oxides. For example: 1.5 mg/L iron + 0.7 mg/L manganese equals 2.2 mg/L combined X 0.15 = 33mg/L DO. Hydrogen sulfide and oil must not be present in the water and organic content must be below 4 mg/L. Service flowrates range between 3.5-5 gpm/ft² and backwash rates between 10-12 gpm/ft², with bed expansion between 35-50 percent, depending on water temperature.
Filter-Ag
Filter-AG is a lightweight replacement for sand with an apparent density of 24-26 lbs/ft³. This medium typically reduces suspended solids down to 20-micron range at 5 gpm/ft² service flow and down to the 40-micron range at 8 gpm/ft² service flow. Because of its light weight, it can also be used a filter cap for granular activated carbon filters where light sediment may negatively impact the optimal performance of the carbon. It remains stratified above the carbon during backwash. A 5–8-inch depth is recommended when applied as a filter cap. Backwash rate requirements are 8-10 gpm/ft², depending on water temperature with a bed expansion of 20- 40 percent and depending on water temperature.
Granular activated carbon
Granular activated carbon (GAC) is touted as being the work horse of the water treatment industry because of its affinity for free chlorine, chloramines, tastes and odors, color, some natural and synthetic organic chemicals. NOTE: Not all organics can be removed with GAC.) It is available in several mesh sizes with the most popular being 12 X 40 and 8 X 30 mesh. Use care when designing activated carbon filtration and pay close attention to the backwash rates. Both are good for average service flowrates of 4-6 gpm/ft² when used for protection of downstream treatment processes. Consider the required empty bed contact times (EBCT). EBCT is equal to the volume of the empty bed, divided by the flowrate. One cubic foot of activated carbon is equal to 7.48 gallons capacity (conversion factor) divided by the tank/ft², divided by the flowrate. For example, chloramine requires 3-4 gpm/ft³. Therefore, 2 cubic feet in a 12-inch (.78/ft²) diameter tank = 7.48 x 2/.78/4 gpm = 4.8 minutes EBCT. for organic reduction where flowrates range from 0.7 gpm/ft² for toxic organics, 1.0 gpm/ft² for organics, 6 gpm/ft² for chloramine and up to 15 gpm/ ft² for free chlorine. Carbon has many uses, but caution is urged when designing an activated carbon system and this is where a comprehensive water analysis is invaluable. Backwash rates for activated carbon varies based on mesh size. While 8 X 30 mesh activated carbon requires 16 gpm/ft² backwash flowrate @ 55⁰F, a 12 X 40 mesh carbon requires only 9 gpm/ft² @ 55⁰F. Either mesh size requires a minimum of 35-percent bed expansion, up to 50-percent expansion, depending on water temperature.
Follow the rules
Rule #1 – accurate water analysis. An accurate and comprehensive water analysis is required to ascertain with certainty the treatability of any problem water source. Failure to obtain a water analysis is a certain recipe for disaster.
Rule #2 – confirm hydraulics. Establish the available water flow at both 30 psi and 40 psi. If the difference is more than 25 percent, counsel the prospective customer to have the well pump motor evaluated for amperage draw. If the amperage is too high, the pump motor may require replacement. If the difference is less than 25 percent, a good rule of thumb is to work with 70 percent of the available flow at 30 psi. For example, if the pump produces 10 gpm at 30 psi, design your filtration system at 7 gpm. This will allow for better system performance because the system is not working at the maximum output which will diminish over time as the pump system ages.
EXAMPLE: An 8-inch-diameter softener can easily provide a flow of 7 gpm at an acceptable pressure drop of 6-8 PSID for an average residential application. That same 7 gpm for sediment removal using Filter-Ag would require a 16-inch-diameter tank. Then consider that the backwash rate for that softener is only 1.5 gpm while the backwash requirement for the Filter-Ag is 13 gpm @ 55⁰ F. Next consider that a water softener can operate at 20 psi, whereas a backwash filter requires a minimum of 30 psi for sufficient pressure to lift and clean the filter media. Finally, to make this application work within the hydraulic constraints of the pumping system, consider a triplex 10 X 47 media tank configuration that backwashes at 5 gpm. Using three tanks allows a clear water backwash from the online twins to provide clean water to flush away the sediments of the offline filters, followed by sequential backwash of the other filters. The benefit is faster media bed cleaning with clear water and less water to waste. Typically, the same amount of water used to clean a simplex filter using raw water will clean all three filters with the same volume of clear water.
Rule #3 – Specify. Design the system for optimal performance with the simplest configuration for ease of service and maintenance while not compromising the long-term reliability and consistent water quality your customer expects.
Conclusion
Always bear in mind that any filtration system will quickly fail for lack of sufficient water flow and pressure to lift and expand the media bed required by the media manufacturer for optimal water quality. In the final part in this series, we will look at the finer points of filtration. Stay tuned.
About the author
Gary Battenberg is a Business Development Manager-Senior for Argonide Corporation. Previously, he was Technical Manager, Water Treatment Department of Dan Wood Company. Prior to that, Battenberg was Technical Support and Systems Design Specialist with Parker Hannifin Corporation. His nearly four decades of experience in the water industry include a proven, successful track record in areas of sales, service, design and manufacturing of water treatment systems. Battenberg’s technology base covers mechanical and adsorptive filtration, ion exchange, UV sterilization, RO and ozone technologies. He has worked in the domestic, commercial, industrial, high-purity and sterile water treatment arenas. A contributing author to WC&P International magazine and a member of its Technical Review Committee since 2008, Battenberg was voted one of the magazine’s Top 50 most influential people in the water treatment industry in 2009. He can be reached by email at [email protected] or by phone (407) 488-7203.