By Gary Battenberg
In Part 1 of this series, we looked at the definition of and a brief history of filtration and the importance of removing large solids, particulates and turbidity from a water source with sand separators, sediment traps and screen filters, which are used to improve clarity and aesthetics before boiling the water to ensure potability for drinking and cooking. In Part 2, we are going to look at the finer points of filtration, relative clarification and the remediation of sediment, odor and color removal.
As previously mentioned, an accurate assessment of the problems described by the customer are important not only for effective treatment options but more importantly, to ascertain the level of involvement your water treatment dealership is capable of regarding complete remediation of these kinds of problems. Some states require certification when accessing the well directly for anything related to the pump, discharge piping and casing. If this is the case for your business, it is best to align yourself with a qualified company where downhole service is required, especially when the well is producing sand or heavy sediment.
Now that we are aware of how and why to remove large particulates from a problem water source, let’s look at clarifying water where turbidity, odor and color are present. Each of these conditions may require at least one type of filtration media or as many as three for severe problems. Turbidity may require only a mono-bed filter media specifically manufactured for light sediment/turbidity. This type of filter media only requires a periodic backwash to flush away accumulated sediments. There are several media types to choose from, but care must be exercised when selecting a filter media for a backwash filter.
Pump flow and pressure considerations
First, determine the pump flowrate at 30 PSI (207 kpa) for tank diameters between 6 and 10 inches (154 – 254 mm) and 40 PSI (276 kpa) for tank diameters between 12 and 16 inches (304-406 mm). Caution: Do not exceed the pump rate for backwash, which will determine the maximum tank diameter. Look closely at the recommended backwash flowrate for the media selected. Many filtration media average between 8-12 gpm per square foot (ft2), however there are some that require as much as 20-30 gpm/ft2 to effectively lift and reclassify the media for optimal sediment/turbidity removal.
Finally, pay very close attention to the manufacturers required media bed expansion, which is typically 30 percent of bed depth but for some media, the backwash bed expansion requirements can be as high as 50-70 percent. This is important to consider when selecting the tank height. Where a 10 X 40-inch (254 X 1016 mm) tall tank with one cubic foot (ft3) of filter media is sufficient for a 50-percent bed expansion, it is not sufficient for a 70-percent bed expansion.
Example: One cubic foot of media in a 10-inch diameter tank requires 22 inches (559 mm) of bed depth. A 3-inch (76 mm) underbed (gravel or garnet) would increase the bed depth to 25 inches (635 mm). At 50-percent bed expansion, which is 11 inches (22 X .50 = 33), that means the tank would have to be a minimum of 36-inches (914 mm) high to accommodate the lift. Therefore, a 40- or 44-inch-tall tank is recommended to prevent media loss to drain without exceeding the sideshell height. At 70-percent bed expansion, the same media load would require 15.4 inches (391 mm) of lift (22 X .70 = 37.4), which would require a 44- or 47-inch-tall tank when factoring in the 3-inch underbed.
Tank height should not be overlooked
A question may come to mind here regarding the shorter tanks for these filters, given that most filters are prepared with a 54-inch- (1,372 mm) tall tank. Many manufacturers or assemblers will standardize on one tank size because the economies of scale mean a better price on quantities of one size. Where 1.5 ft3 media load is used, the 54-inch tank will have 33 inches of bed depth and with the underbed, a total bed depth of 36 inches. This leaves a freeboard area in the tank of 18 inches, which provides the required bed expansion. It is not advisable to use this tank for cubic foot media load. This would create a 29-inch freeboard depth, requiring a much greater lift for the sediments to travel to and out the drain of the filter.
Consequently, heavier sediments or chunks of filter cake may not be flushed away. Over time, a buildup of residual sediments will diminish the efficiency of the filter to yield clear water to service. Give some thought to the tank size and speak with your supplier or manufacturer for their recommendations as well and be specific in the type of media being specified. This is especially important where warmer water temperatures are typical, so be diligent in obtaining the necessary water characteristics to ensure accurate filter design.
Enjoyable drinking water is that which is clear, colorless and odorless. When even a slight odor is detected, one is tempted to seek an alternative such as bottled water or another prepackaged beverage to satisfy their thirst. Odor in water may be caused by many issues and controlling the problem may not be as simple as installing an activated carbon filter. Chlorine taste and odor is easily removed with carbon and generally will last for many years when the filter is installed at the point of entry.
Filters range in size from a single cartridge filter to an automatic backwash filter. Many water utilities across our country use chlorine and/or chloramine to maintain bacterial and biological potability for the consumer. These, however, can have a negative effect on the flavor of drinking water, coffee and tea, juices and the like. Carbon has long been the workhorse of our industry for removing offensive tastes and odors from water effectively.
Odor in water can be caused by hydrogen sulfide gas, characterized by the oft referred to rotten egg smell. Interestingly, it is the smell that people object to instead of the taste and hydrogen sulfide odor is one that you never get used to because it constantly offends the olfactory receptors in the nasal cavity and sends a signal to the brain that the odor is unpleasant. Those in the water treatment industry that provide treatment services in their markets can relate to this fact.
Activated carbon alone is not a good choice for removing hydrogen sulfide because the adsorption sites are quickly fouled and the odor returns in a very short time. The development of catalytic carbons, however, has made a single bed filter possible whereas previously, a dosing pump was used to inject chlorine or hydrogen peroxide into the water stream to oxidize the hydrogen sulfide gas and convert it to elemental sulfur, which was then filtered out with activated carbon. This process typically required a large contact tank to provide sufficient residence time for the chlorine to fully oxidize the hydrogen sulfide before entering the carbon filter for dechlorinating and filtering out the sulfur.
Oxidizing filters such as manganese greensand have been in use for many years and are still specified to treat troubled water containing, iron, manganese and hydrogen sulfide. Iron and manganese bacteria can be effectively oxidized and filtered out to yield clean water for domestic and commercial uses. Manganese greensand has also been proven to remove or reduce arsenic and radium from groundwater sources where sufficient iron is present to facilitate the grab on these contaminants. Be aware of regional waste discharge regulations when using any technology for the remediation of level-one contaminants. The more recently developed Greensand Plus doesn’t require potassium permanganate for regeneration yet exhibits all the benefits of the original Greensand as well as having increased operating temperatures.
Color in water is a regional problem in groundwater especially where conifer (evergreen) trees are the dominant growth species or decayed vegetation is present. Typically, the groundwater will have a light yellow to tea-colored cast that is indicative of tannins. Color in water may also be from dissolved organic matter or dissolved mineral matter. Color is not removed by mechanical filtration because it is completely soluble and therefore requires adsorptive filtration such as carbon or other specialized color removal media.
Tannins can be removed from water with activated carbon if the pH value is below 5. A low pH, however, is typically corrosive and pH adjustment is required to bring the water to a basic condition that is not destructive to plumbing and fixtures. Where the pH is above 6, anion exchange resin is typically the method of choice for tannin removal. It is important to note that when tannin is present in a water supply, it is the first contaminant that must be removed where downstream treatment steps are included in the equipment array.
Tannin is the proverbial fly-in-the-ointment in water treatment and, left untreated will eventually foul other filtration media because of the slight, oil-like characteristic that coats other media and renders it non-functional. If you are uncertain whether tannin is present in the water or you don’t have a test kit, sprinkle some black pepper in a glass containing the water. If the pepper migrates to the outer perimeter of the glass, this is a good indication that tannin is present. Also, tannin can be clear and not visible, and the pepper hack is a good stand by to help ascertain what may be in the water. If, however, you are in a region where tannin is a problem, be professional and obtain a good tannin test kit.
When color is to be removed from water and removal is proving to be very difficult, a water analysis is required to determine what is binding the color and preventing its removal. It may take several attempts with different treatment techniques to establish a consistent remediation process. When faced with these difficulties, contact a water testing laboratory, and obtain their testing capabilities and pricing for their services.
In Part 3 of this series, we will look at the different types of filtration media that are commonly used in our industry and why hydraulic and temperature characteristics of ground water are critical to consistent and reliable filtration performance. 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.