By Gary Battenberg
What is Filtration?
By definition, ‘filtration is that process whereby fluid passes through a porous substance to remove or separate solid particulates, sediments, turbidity, colloids, impurities, taste, odor, color, iron, algae and microorganisms including viruses, bacteria and cysts.’ Filters range from coarse gravel, sand and charcoal type to felt bag filters. Loose media loaded in pressure vessels with automatic or manual controls for backwashing or regenerating media that function as adsorptive, neutralizing, oxidizing and catalytic processes are the most common types currently in use. Filters may also include media packed in cartridges for smaller applications and/or specially prepared cartridges that target specific contaminants such as lead, arsenic and other emerging contaminants of concern.
A brief history of early filtration
Filtration occurs in nature by clarifying turbid water as it seeps or flows through earthen layers on its way to underground aquifers, streams or pools, sometimes referred to as a freshwater lens. Early Egyptian, Chinese, Greek, Roman, Persian and other ancient cultures all understood the need to filter water to improve the clarity and aesthetics of water, before boiling it to ensure it was safe for drinking and cooking. The first filter patent was issued in 1790 and credited to Mrs. Johanna Hempel in England for drinking water, which was constructed of pottery containing four parts tobacco pipe clay and mixed with five parts of sea, river or drift sand for what we know as a gravity filter. Robert Thom, a Scottish civil engineer, invented and received a patent for his slow-sand filtration process in 1827.
Two years later, civil engineer John Simpson developed a similar filter, which was installed and commissioned at the Chelsea Public Water works in London. His system was subsequently adopted for use around the world. This process produced potable water by using the naturally occurring complex biological film (biofilm) that grows on the surface of the sand substrate. In 1885, the first slow-sand filtration was installed in the United States and by 1900, there were 10 operating systems in the US. Since that time, municipal water utilities have adopted more modern methods for treating large volumes of water (which include aeration, coagulation and flocculation, precipitation, chlorination and other disinfection processes) to produce potable water deemed safe for drinking, cooking and oral hygiene.
Types of filters
In this part of the series, we will look at methods for removing sand and large particulates from municipal, public and/or private groundwater sources. This first step is crucial because if sand and grit are not removed from the water, the collateral damage to virtually all water related fixtures and appliances could result in unexpected repair or replacement costs. There are cases where sand, sediment and large particulates enter the service plumbing of a residence or commercial building due to several factors. Examples of this include water main breaks where rocks, gravel, dirt and sand enter the water lines prior to and during repair. After repairs are completed, the water main(s) is/are flushed to purge these conditions until the water is clear and detectable chlorine is present.
Other instances include smaller community of mutual domestic water systems drawing water from surface sources or shallow ground water wells. Turbidity is typically the most common aesthetic inconvenience, especially where water is drawn from a surface water source. Sand and gravel migration from groundwater sources may be caused by a failing gravel pack in a well or sand may become present after a subterranean seismic event such as an earthquake or tremor that fractures the earth strata.
Many municipal and public water utilities across our nation still have black iron water pipe infrastructure in use. These systems regularly notify their customers of flushing water mains to clear settled sediments out of the distribution system. After flushing is completed, customers are counseled to open an outside hose bib and run the water until it is clear to make sure any sediments or particulates do not migrate into the service plumbing. Despite this process, there are times when large particles and even small pebble-size deposits break loose from the piping and migrate to the aerators of the fixtures and reduce water flow. This is even possible where the residence or commercial building is serviced by a water well. In other cases, sediment or large particulates may enter the piping infrastructure because of a water main break. After repairs are made, the utility personnel do their best to flush away and transient sediments or heavy solids by flushing through nearby fire hydrants to eliminate migration to the service connections of residential or commercial buildings.
Sand separators and sediment traps
Some ground waters will yield sand, which ideally should be addressed when the well is jetted and flow tested. If the well produces sand after it is stabilized, the driller will usually recommend a pump-mounted sand separator. Pump-mounted separators are designed to remove sand and grit that is destructive to submersible or turbine water pumps. The centrifugal action prevents sand from entering the pump inlet by spinning or ‘tossing’ the sand to the perimeter of the separation chamber where it discharges out the bottom and back into the well. Sand-free water enters the low-pressure area of the separator and flows up to the pump inlet, thereby protecting the impellers, bearings and other parts of the pump rotating assembly. Dropping the sand back into the well helps slow or even stop sand intrusion into the well.
Sediment traps have been in use before the advent of the centrifugal or cyclone/cyclonic separator. The typical sediment trap for removal of fine sand and abrasive sediments consisted of a galvanized tank with three ports, including the inlet, outlet and flush or blowdown port. The tanks were typically 12-16-inch diameter X 36- 48 inches high (30.48-40.64 cm X 91.44-121.92 cm).
Sediment-laden water enters the tank in the lower one-third of the sideshell, with an elbow rotated upward at 220, creating a spiral vortex that allows the sand and heavy sediment to settle out of solution to the bottom of the tank. Clarified water exits the top of the tank sideshell and into the service plumbing (or additional pretreatment such as a cartridge filter or automatic backwash sediment filter to remove lighter sediments that remained in suspension). The flush valve at the bottom (commonly referred to as the blowdown valve) is opened weekly either manually or automatically with a timer and solenoid to purge the trap of collected sand and sediment.
Sediment traps require a much larger footprint versus the cyclone separator, which is very compact and is typically mounted to the wall for either submersible or jet pump installations thereby reducing the overall treatment footprint. It is very important to properly size a cyclonic separator based on the pump rate. Too often, the separator is selected based on pipe size and not flowrate. Typically (but not always) the cyclone separator is sized smaller than the main waterline. For example, where the main water pipe is 0.75-inch (1.90-cm) diameter, the separator will typically be selected with 0.5-inch (1.27-cm) ports because of the available pump rate on the well. Cyclonic separators work best when the flow velocity is optimized to force the solids to the outer perimeter of the separation chamber quickly for high efficiency and cleaner effluent water. Always use the sizing/selection chart to avoid oversizing the cyclone type separator.
Screen filters may be used and are recommended where plumbing code compliance requires installation when water pressure exceeds 80psi. Screen filters range in size and configuration depending on needs. The basic screen filter requires turning off the water to remove and clean the screen when debris is captured. Others are self-cleaning and require manually opening a flush valve or will clean automatically with an electronically controlled valve.
In areas where groundwater tables are just below the surface, quality sometimes is affected by heavy rains or storm runoff because of a shallow activity zone typically in sandy soil areas. Sand and grit may be present in these areas shortly after these rain events and many residences have screen filter devices like the spin-down type that are easily purged via a flush valve mounted at the base of the housing. This feature makes cleaning simple without having to turn off the water supply, especially with the clear housing that provides visual confirmation of sand or grit capture and a quick purge flush away of the solids.
This first crucial step is something to be aware of during the initial call to a prospective customer who is experiencing these kinds of problems. Listening to what the customer has to say about their problems and water quality concerns is very important so that no real or potential issues are overlooked. Accurate assessment of the water aesthetics combined with a complete water analysis and hydraulic characteristics of the water source will ensure competent treatment options may be presented to the customer. In Part 2, we will look the finer points of filtration and the options that are available to remediate sediment and color issues. Stay tuned.
1. The Encyclopaedia Britannica: A Dictionary of Arts, Sciences and …, Volume 9, Page 168.
2. Hendricks, David W. Water Treatment Unit Processes: Physical and Chemical, Page 664.
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 firstname.lastname@example.org or by phone (407) 488-7203.