Slow Sand Filtration is one of the oldest known ways of water purification, with the first sand filter reported in 1804 by John Gibb in Scotland. In 1855, John Snow proved contaminated water could transmit disease, and by 1886, slow sand filtration was proven to remove bacteria. According to the World Health Organization, "No other single process can effect such an improvement in the physical, chemical and bacteriological quality of surface waters." (Huisman & Wood, 1974) Slow sand filters rely on biological activity and are therefore also known as bio-sand filters. Microorganisms present in the effluent water travel through the sand and collect to form a biological zone through various physical and biological mechanisms. This biological zone removes much of the disease-causing organisms. The top 1-3 cm of the biological zone is called the schmutzdecke, meaning "dirty layer" in German.
Slow sand filters can either be continuously or intermittently operated. Until 1990, continuously operated slow sand filters (COSSF) were considered the only option due to the need for a constant flow of water to provide oxygen and food to the microorganisms within the biological zone. Without the constant presence of food and water, the microorganisms will die. Dr. David Manz, of the University of Calgary, redesigned the traditional sand filter by raising the drain pipe above the top level of the biological zone, which maintains a constant supply of water above the sand without requiring a constant flow of water. If the water is maintained no more than 5 cm above this level, oxygen can still permeate through the water to reach the biological layer. Research has suggested that 2-3 cm of standing water is the ideal efficient level. (Palmateer, et al., 1999) Intermittently operated slow sand filters (IOSSF) can be created as small units, which has been shown to be very effective in providing clean water for families in low-income countries. (Buzunis, 1995)
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Bio-sand filters use both physical and biological mechanisms to filter harmful bacteria from raw water sources. The sand acts as a mechanical filter to remove solid particles from the raw water, and is able to capture particles smaller than the smallest pore size between the individual sand grains. This phenomenon is experienced in both slow sand filtration and rapid sand filtration (which has no biological component), therefore the removal of these smaller particles is not simply due to the biological filtration. (Huisman 1974) The entrapment of the very fine particles is due to both transport and attachment mechanisms experienced as the water flows through the filter. Transport mechanisms such as interception, straining, diffusion, and sedimentation allow the particles to approach areas in the filter bed where they can become attached to the grains of sand and become entrapped within the filter. The attachment of particles to bed grains occurs due to physicochemical and molecular forces. (Huisman & Wood, 1974)
In addition to physical filtration, a large portion of the filtration in biosand filters occurs in the biological zone. Most of the solid particles from raw water are removed within the top 0.5 to 2 cm of sand. The biological zone consists of the schmutzdecke and a layer of biological activity within the sand below the schmutzdecke. The schmutzdecke is made up of decomposing organic materials, iron, manganese, and silica. This combination serves as a fine filter to physically remove colloidal particles as well as serving as the first layer of biological filtration, degrading soluble organics. This biofilm usually takes 2-3 weeks to develop naturally, and research has shown that microbial reductions improve as the biofilm "ripens". (Elliot, Stauber, Koksal, Digiano, & Sobsey, 2008) The schmutzdecke should remain undisturbed throughout the filtration process, which calls for a diffuser plate to evenly distribute the flow of water into the filter. The two main actions that purify the water within the biologically active zone are chemical and microbiological oxidation. The purification of the water occurs due to a variety of factors. The combination of these processes leads to the death of many pathogens, resulting in a decrease in both the number of indicator bacteria and pathogens. The biological removal process is most prevalent at the top of the biological zone and decreases with depth as food becomes less scarce. (Huisman & Wood, 1974) According to the World Health Organization, the four dominant processes are as follows: (Huisman & Wood, 1974)
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Competition for Food - Bacteria require organic matter during the oxidization processes of metabolism. The biological zone does not contain enough food for all the bacteria, so the competition for nutrition leaves many bacteria to starve and die.
Hostile Environment - Intestinal bacteria is unable to multiply in most sand filters where the average temperature is much less than the 37Ëš C body temperature they are accustomed to.
Predation - Predatory organisms in the upper layer often feed on other cells.
Poison - Microorganisms within the biosand filter are known to produce substances which act as chemical and biological poisons to intestinal bacteria.
For continuously operated slow sand filters, research performed by the American Society of Civil Engineers (ASCE) has shown that the majority of biological filtration occurs in the top 40 cm of the filter. (BioSand Filters, 2004) The depth of the biological zone of intermittently operated slow sand filters depends on the amount of standing water atop the schmutzdecke. Shallower depths allow for greater oxygen penetration leading to a deeper biologically active zone. Research has shown that 12.5 cm of standing water produces a 10 cm deep biological zone. (Buzunis, 1995). In order for the microorganisms which fuel the biological processes to survive, a number of variables there must be a constant supply of food and oxygen. Biologically productive raw water allows for constant food supply. (Palmateer, et al., 1999) The water level must be kept at a level that allows for oxygen permeation into the biological zone, which can be achieved by locating the outlet level above the sand level. In addition, there must be sufficient contact time between the water and the filtration media to ensure adequate time for the various processes to occur. Research at the University of California, Davis investigated both the effect of pause time on the efficiency of removal of viruses, bacteria, and turbidity as well as parameters that effect flow rate (e.g. hydraulic loading and sand size) which in turn affects residence time. They found that water quality improves with longer residence times and the bacteria and virus removal is best with filters with finer sand and lower head. The ideal combination was 0.17 mm sand with 10 cm head. (Jenkins, Tiwari, Darby, Nyakash, Saenyi, & Langenback, 2009)
The rate at which the raw water flows through the filter is very important for achieving desired results. Darcy's Law (Q=K(Ah/L) shows that flow rate (Q) is proportional to the cross-sectional area of the sand (A) and the pressure head (h) of the water atop the sand. Using this law, small test filters can be scaled up to larger real-life models. Flow rate is also affected by the length of the column, the properties of the water and the characteristics of the sand. (BioSand Filters, 2004) Flow rates for slow sand filters should ideally be between 0.1 - 0.4 m/hour. According to research done by Elliot et al intermittently operated slow sand filters operate at near-plug low conditions, meaning that all molecules of the water travel at the same speed, making residence time for each molecule approximately the same. (Elliot, Stauber, Koksal, Digiano, & Sobsey, 2008). Slower flow rates are ideal for biological removal rates for intermittently operated filters, and these slower rates can be achieved with the ideal combination of sand fineness (0.17 mm) and head (10 cm). Slower flow rates allow for longer contact time, leading to increased pathogen removal, and can allow the biofilm to develop better. (Huisman & Wood, 1974) Additionally slower flow rates help ensure that both the pathogens and the food supply for bacteria are not pushed to a deeper depth as would happen with higher flow rates, which would require increased bed depths. Turbidity and color removal is also more efficient at lower flow rates in continually operated slow sand filters (Muhammad, Ellis, Parr, & Smith, 1996)
Biosand Filter Components
Biosand filters can be made in a variety of ways. Concrete filters (made from metal molds) are the most common type, but alternatives such as pre-made plastic filters or metal or plastic drums can be used as well. Dr. Manz' research at the University of Calgary led to the creation of a concrete box or cylinder with a pipe imbedded in its walls. Course gravel is at the bottom of the filter to prevent finer sad from clogging the pipe outlet. This gravel is topped with levels of fine sand. A diffuser plate is found at the top of the filter, to facilitate even, gentle water flow to protect the schmutzdecke from being upset when the filter is used. Concrete biosand filters can be constructed relatively cheaply, with the primary cost being the cement. They are very durable and last a long time. These filters have seen great success in the developing world, mostly through introduction by NGOS. Small-scale businesses have been developed which can build and sell filters in the community. Unfortunately, due to the need for the metal mold, they cannot be easily mass-produced, as each mold can only make one filter ever two days. The concrete molds are also susceptible to cracking during transportation and when moved within the house. Due to their weight, they can be quite expensive and unwieldy to transport. (BioSand Filters, 2004) Alternatively, plastic barrels can be used to create plastic filters, and old plastic drums or old fuel barrels can be used to make drum filters. Anecdotal evidence from the Machako District in Kenya's Eastern Province suggests that concrete filters are preferred by those who use the filters due to their durability and the effect of "sweating" through the concrete which allows the water to cool. (Mol, 2001)
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Filtration media can also vary, but sand is most commonly used. The bottom-most layer in a biosand filter is usually 5 cm of 6-15mm gravel, followed by 5 cm of 1-6mm coarse sand. These two layers serve to prevent the finer sand from clogging the PVC inlet pipe located on the bottom of the filter. Mosquito mesh can also be used to serve this purpose. (BioSand Filters, 2004) When sand is used as the fine filtration media, the most important factors are sand grain size and sand bed depth. The sand should have an effective size of 0.15-0.35mm and a uniformity coefficient of 1.5-3 (less than 2 is preferable). (Huisman & Wood, 1974) Effective size is the particle size where 10% of the particles (by weight) are smaller, whereas the uniformity coefficient shows how well graded a sample is