Since that time, slow sand filters have continued to provide potable water to consumers throughout the world. In fact, some experts claim that slow sand filtration currently is experiencing a resurgence in North America - especially in smaller communities - primarily because it is a cost-effective and reliable method of purification. Specialists say it may also be a suitable treatment choice for rural homeowners who depend on private water sources for household use.
How It WorksIn a slow sand filter, a combination of physical straining and biological treatment effectively purifies the raw water. The process itself is relatively slow- filtration rates ranging from 0.015 gpm/ft.2 to 0.16 gpm/ft.2 are common. Because of their rather lethargic filtration rates, slow sand filtration systems often must take up a large amount of space to produce substantial amounts of filtered water. (Smaller systems do not require such extensive physical space.) Extensive pilot testing during the design stage is critical to ensure that the filter performs up to par.
In the system, untreated water percolates through a bed of uniformly graded porous sand that overlies a gravel bed (see Figure 1 for diagram). The water enters over the surface of the filter and is drained from the bottom. In a mature filter, a rich, sticky, mat-like biological layer called a Schmutzedecke forms in the top layers of the sand, where particles tend to settle because of the slow rate of filtration. The Schmutzedecke is composed of biologically active microorganisms, including bacteria, algae, and other single- and multiple-cell organisms. The microorganisms break down and feed off of organic matter in the water that is passing through the Schmutzedecke, and inorganic particles are trapped and strained by this layer, as well. The Schmutzedecke assumes the dominant role in slow sand filtration because it allows the process to remove particles smaller than the sand could trap on its own. To ensure that the biological community in this layer remains effective, the filters should operate at a constant rate.
Eventually, flow becomes reduced because the filtered material and debris begin to block up the Schmutzedecke. To increase the flow rate, the filter must be cleaned by scraping and removing the top layer of sand. Until the biological layer replenishes itself, the filtered water should not be used.
AdvantagesWhile there are many other monitoring and operational tasks that need to be performed (some daily), the scraping of the top layer probably is the most time-consuming maintenance-related task that the slow sand filter requires. However, even if one does not clean this top layer on a regular basis, the quantity of filtered water will be reduced, but the quality of the water will not suffer.
This limited maintenance to-do list is just one of the major advantages that slow sand filters offer. However, cost is probably the biggest benefit of slow sand filtration method. Materials used to build the system may be locally found, making the cost of construction relatively inexpensive. Also, since close, constant supervision is not necessary, the cost of operation also is reasonably low.
Another benefit of the filtration technique is that it there is no known negative impacts of using this technology on the environment. In fact, because it is a low-energy consuming process, slow sand filtration can actually help protect the environment, as compared to other water disinfection techniques.
Other advantages of slow sand filtration:
- No pre-treatment chemicals, are required
- The system has great adaptability in components and applications
- Problems handling sludge are minimal
LimitationsNothing is perfect in this world, and slow sand filters are no exception. The systems have several limitations that one should consider before investing in them. As mentioned previously, slow sand filters need a great deal of land, as well as filtration materials, to produce significant amounts of treated water. Therefore, if a large amount of water is needed, substantial space needs to be set aside for the system, which may not be feasible in some environments. A lengthy testing period - preferably a year - also must be reserved to ensure adequate performance throughout the four seasons.
There are some limitations in regard to condition of the raw water that these filters can treat, as well. For example, the raw water turbidity must generally be low because high turbidity levels tend to plug up the sand quickly. Also, the filters treat cold water less effectively because chilly temperatures tend to discourage the biological growth that needs to occur in the system. Water with too much algae is unfavorable in a slow sand filter, as is water with too low a nutrient count or water mixed with very fine clay.
Furthermore, while slow sand filters are effective in reducing the levels of many undesirables, they are not capable of removing all of them. Color removal is only fair to poor, and according to the National Drinking Water Clearinghouse, "slow sand filters do not completely remove all organic chemicals, dissolved inorganic substances, such as heavy metals, or trihalomethane (THM) precursors - chemical compounds that may form THMs when mixed with chlorine." Please see Table 1 for data concerning slow sand filters' removal capacity of other contaminants.
Is Slow Sand for You?Slow sand filtration has many advantages, especially for small communities and rural homeowners, but like every purification technique, it has its limitations. Do your homework first to find out if this technique is right for you.
Table 1. Typical Treatment Performance of Conventional Slow Sand FiltersWater Quality Paramenter vs. Removal Capacity
Turbidity <1.0 NTUU
Coliforms 1-3 log units
Enteric Viruses 2-4 log units
Giardia Cysts 2-4+log units
Crptosporidium Oocysts >4 log units
Dissolved Organic Carbon <15-25%
Biodegradable Dissolved Organic Carbon <50%
Trihalomethane Precursors <20-30%
Zinc, Copper, Cadmium, Lead >95-99%
Iron, Manganese >67%
Table 1 is adapted from M. R. Collins' "Assessing Slow Sand Filtration and Proven Modifications," presented at In Small Systems Water Treatment Technologies: State of the Art Workshop, a NEWWA Joint Regional Operations Conference and Exhibition, Marlborough, Mass., 1998.
The diagram and table accompanying this article are courtesy of the National Drinking Water Clearinghouse, a public service organization at West Virginia University that is funded by the U.S. Department of Agriculture's Rural Utilities Service. Its mission is to help rural and small communities learn more about how to maintain the safe, clean drinking water supplies. For more information about the group, visit www.ndwc.wvu.edu.