Filtration systems remove impurities from water by passing it through granular material, such as sand. Although their performance varies, these systems can be used to treat Giardia and Cryptosporidium, bacteria, and viruses, as well as address turbidity and color problems.
Traditional filtration begins with the addition of a pretreatment chemical coagulant to the source water, such as iron or aluminum salts. The liquid is then agitated gently to cause small suspended particles to coalesce and form bigger, more readily removed clots, or “flocs.”
Following that, these systems go through a sedimentation process. Particles in the water, including the floc generated by flocculation, are allowed to naturally settle out of the water by gravity’s pull in this process. These impurities collect as “sludge” at the bottom of the system, which is regularly removed.
Following these steps, water is run through filters to ensure that any leftover particles are physically attached to the filter material. The coagulant destabilizes the suspended particles, allowing them to adhere more readily to the filter material.
Conventional filtration, like other filtration technologies, improves a wide range of source waters significantly. It works best with sources that have a consistent flow and minimal quantities of algae, which can block filter systems.
To get the intended outcomes, coagulation chemicals demand skilled handling, therefore filter treatment facilities must be managed by qualified employees.
Filtration systems remove impurities from water by passing it through granular material, such as sand. Although the effectiveness of filtration varies, these systems can be used to reduce turbidity and color problems, as well as treat Giardia and Cryptosporidium, germs, and viruses.
Direct filtration begins with the addition of a chemical coagulant, such as iron or aluminum salts, to the source water. After that, the liquid is slowly swirled to cause tiny suspended particles to coalesce and form bigger, more readily removed clots, or “flocs.”
After these operations are completed, the source water is run through filters to ensure that any leftover particles adhere to the filter material. The coagulant destabilizes the suspended particles, allowing them to adhere more easily to the filter.
Sedimentation is used in traditional filtering methods to allow particles to settle out of water for removal. Direct filtration removes this step by allowing the filter material to strain impurities on its own.
Direct filtration is a reasonably simple and cost-effective filtration method. The method significantly improves source water quality—but it works best with relatively good quality source waters that have steady flows and minimal turbidity. High quantities of algae, in particular, can block filtering systems.
Because filtration removes all particles, direct filtration cannot cure high turbidity waters. As a general rule, direct filtration is acceptable for source water with turbidity less than ten ntu.
Coagulation chemicals require skilled management to produce the required effects, hence filtering systems must be managed by qualified staff.
Diatomaceous Earth Filtration
Diatomaceous earth filtration is used to physically remove particles from source water that are simply strained. The treatment removes Giardia, Cryptosporidium, algae, and, depending on the grade, certain germs and viruses.
The filter in this system is constructed of diatomaceous earth, a floury, chalky material composed of the crushed, fossilized remnants of one-celled marine life forms known as diatoms.
Pumps either push pressured water through the cake from the source intake or utilize vacuum suction to pull it through from the exit side to flow water through a diatomaceous earth filter system.
Coagulation chemicals, unlike many other types of filtration, are seldom employed to improve the agglomeration of contaminating particles. Diatomaceous earth filtration is better suited to higher-quality source water that is free of inorganic impurities due to this constraint.
The procedure may readily be scaled down to accommodate small-scale operations. In reality, the U.S. Army initially developed this filtration technology during World War II to meet a requirement for portable water treatment facilities. Diatomaceous earth filtering systems are simple to use and cost effective. These characteristics make them useful for interim disaster assistance and in places where more expensive infrastructure is not available.
This filtering system is used in numerous manufacturing processes, including the production of syrups, oils, antibiotics, chemicals, and alcoholic drinks, in addition to water.
Slow Sand Filtration
Many towns used slow sand filtration as their original treatment method in the eighteenth century. These filters may efficiently remove germs that cause waterborne disease, such as protozoa like Giardia and Cryptosporidium, as well as bacteria and viruses, as evidenced by disease rates plummeting in the European cities that pioneered the therapy.
These systems allow water to gently move through a sand bed that is two to four feet (0.6 to 1.2 meters) deep. The water is filtered and impurities are removed along the way using a mix of physical and biological methods.
The sand bed becomes home to a variety of bacteria, algae, protozoa, rotifers, copepods, and aquatic worms with frequent use. These bacteria help the filtering process by eliminating impurities, while water temperatures below 10 degrees Celsius may slow them down. Sand that has “ripened” to house these organisms is preferable to clean or virgin sand. Sand can take weeks or months to ripen, depending on the amount of water present and the temperature. The process ultimately clogs the sand bed and decreases flow rates to the point where it must be unclogged, which is usually done by “backwashing,” or reversing the flow.
Slow sand filtration systems may be unable to handle chlorinated water because chlorine can harm the filter’s microbial population. As a result, after going through the filtering process, water that has to be disinfected with chlorine can be treated in storage facilities.
Storage also contributes to the flexibility of a system’s water production. Slow-acting sand filter systems cannot manage increasing water volumes during peak demand, and they should not be run at less-than-optimal flows during low-demand periods.
Slow sand systems function best with water that has minimal turbidity and algae levels, as well as no color pollution. These systems are particularly vulnerable to excessive algal or clay concentration, which can choke sand beds. Nutrient-rich source water, on the other hand, may help slow sand filters clean by increasing their biological component.
Slow sand systems are often simple, low-maintenance, and low-cost to operate.
Water is treated in filtration systems by passing it through porous materials that remove and retain pollutants.
Bag and cartridge filters are simple and easy-to-use devices that physically strain bacteria and debris from source water as it passes through the filter media using a woven bag or a cartridge with a wrapped filament filter.
These systems are efficient against Giardia cysts, but they aren’t powerful enough to get rid of germs, viruses, or chemicals. As a result, they’re best suited to higher-quality source waters with little turbidity.
Bag and cartridge technology is continuously evolving and is specifically designed for small-scale treatment facilities. These systems are very simple to operate and maintain, requiring little operator skill. The cost depends on how frequently the filters must be replaced.
Cartridges, like many other filters, are readily contaminated by high-particulate water, hence low-turbidity water is desirable. Pretreating water can also be done with “roughing filters,” which employ sand, mesh screens, cartridges, and other materials to physically remove bigger pollutants.
Filter materials must be replaced on a regular basis, and more frequently if the supply water has a high level of particles.
Microbes may grow on filters with frequent usage of bag and cartridge systems, although this problem can be mitigated by using a disinfectant. If water testing finds that source water virus eradication is required, disinfectants may be required.
Ceramic filters have been used to treat water for hundreds of years. While most ceramic filters are still sold for centralized water treatment systems, they are now being made for point-of-use applications. They are made domestically in poor nations, often as a self-financed micro-enterprise. These filters are often fashioned like a flowerpot or a bowl and are treated with small colloidal silver particles to disinfect the filter and prevent bacterial development. The filter is housed in a spigot-equipped 20- to 30-liter plastic or ceramic container.
Laboratory testing has demonstrated that these devices can eliminate or inactivate practically all bacteria and protozoan parasites if designed and manufactured correctly. Its antiviral efficacy is uncertain.
Because filter cleaning and maintenance are so important, it’s best paired with an instructional campaign regarding safe storage, filter cleaning, and other best practices, just like other low-cost point-of-use systems.
Ceramic filters have the advantages of being simple to use, having a long life (if not broken), and being relatively inexpensive. Because there is no chlorine residual, there is a risk of recontamination of stored water, as well as a low flow rate of one to two liters per hour.
Slow sand systems, particularly in underdeveloped nations, have recently been adapted for point-of-use systems. They are commonly referred to as “biosand” filters in this context.
A biosand filter is often a sand-filled container that is less than a meter tall and 30 cm wide and deep. Keep the water level above the top of the sand to preserve the biologically active layer, which takes a week or two to fully emerge. This bioactive layer, like slow sand filters, helps to filter, adsorb, kill, or inactivate microorganisms. When water is added, a permeable plate is normally placed above the sand to minimize disruption of the bioactive layer. Users just fill the top of the equipment with water and collect the treated water from the exit.
In laboratory and field testing, biosand filters successfully eradicated nearly all protozoa and most bacteria. It is unknown how effective they are against viruses.
Concrete, a widely available and relatively inexpensive material, can be used to construct the equipment. The maintenance is usually as simple as stirring the sand’s upper surface once a month or so and manually collecting the suspended debris. Because there are few or no parts to replace, the cost of maintenance is negligible.
To learn more, visit the treatment section on our website.