Design Slow Sand Filtration Unit - Assignment Example

Design slow sand filtration unit Glossary Adsorption The process through which a contaminant attaches itself onto the surface of a solid. Bacteria These are single-celled micro organisms that are typically a few micrometers long. Bio layer It is also known as the schmutzdecke. It is a biological layer formed on the sand-water interface of the slow sand filters and is colonized by microorganisms such as bacteria, algae, protozoa and diatoms. Contamination is the process of polluting water through human and/or natural causes Concrete It is a strong construction material made of a mixture of cement, sand and gravel. Disinfection It is any process that deactivates, removes contaminants or kills pathogens that may be present in water. It is usually the last step in the household water treatment process and comes after sedimentation and filtration. Effective size This refers to the size opening read from the grain size distribution graph that will only pass 10% of the sand. Hygiene Various practices, for example hand washing, which help ensure both cleanliness and good health. Filtration It is the process of allowing water to flow (pass) through the layers of a porous material, such as sand, with the aim of removing pathogens and any suspended solids. It is carried out as the second step in the household water disinfection and treatment process and comes after sedimentation and before the disinfection process. Flow rate It is the time taken to fill a certain container of water, most often a one litre container. It is measured once the slow sand filter is fully filled with water. Nutrient This refers to any that is used by microorganisms for both survival and growth. Although this term is applied to refer to phosphorous and nitrogen in contaminated water, it can also be used to describe other chemicals. Pathogen This term refers to any disease causing living organism. The pathogens that are most commonly found in water are protozoa, bacteria, viruses and helminthes. Pores These are the small spaces between the grains of sand that allow water to pass through them. Sanitation It is maintaining clean and hygienic conditions that ensure prevention of diseases through services like garbage collection and proper waste disposal. Sedimentation It is the process used in settling out solids and particles that are suspended in water under the influence of gravity. Suspended solids These are small solid particles that fall in water and thereby causing turbidity. They are removed through the sedimentation or filtration process. Turbidity Turbidity is caused by the suspended solids that float in water such as sand, clay and silt. It is used to refer to the quantity of light that is reflected off these particles suspended in the water and which make the water look dirty or cloudy. It is measured in Nephelometric Turbidity Units (NTU). Uniformity coefficient It is a measure of how poorly or well sorted the sand is. It is the size of opening that is read from the grain size distribution graph that will allow pass of 60% of the sand divided by the size opening that will allow passing of 10% of the sand. Water quality Refers to the physical, chemical and microbiological characteristics of water. The level of the quality of the water to be used varies depending on the purpose for which it is intended. Wet harrowing This is a technique used in cleaning slow sand water filters whereby the sand surface is agitated gently in order to stir up the bio layer. The cloudy water is then drained off and then afterwards, the filter is allowed to operate for a number of days in order to re-establish the bio layer. Abstract Also known as a bio sand filter, slow sand filters have been used as a technique of water filtration since the nineteenth century. The most widely used version of slow sand filters used is constructed using concrete and has a height of 95 cm and a width of 36 cm and with a flow rate of 20-40 liters per hour. There are over 80, 000 filters used in 36 countries. There are various modifications which can be made to slow sand filters and this adaptations offer an affordable option to the household applications of using slow sand filtration technology to treating and disinfecting drinking water in the both the rural and urban communities especially those in developing countries (Gottinger et al. 1). According to CAWST, an NGO in Canada, and the University of Victoria, Canada, a slow sand filter field survey carried out in Haiti on the effectiveness of slow sand filters in removing e-coli and other contaminants as well as decreasing turbidity was estimated for 107 filters which had been in active use for an average period of 1 to 5 years (Blaker and Duke, 1). CAWST (Centre for Affordable Water and Sanitation Technology) assessed both the sustained use and user satisfaction through interviews in all the 107 households individually. The durability of the slow sand filter, its maintenance requirements and its affordability were appraised by the users by recording various observations during the survey (O’Reilly, 28). Introduction In recent years, slow sand filtration has been assumed to be an old fashioned and inefficient method of treating water for both domestic and industrial use. This method has therefore been suppressed by other modern innovations and filtration techniques. However, under suitable circumstances, using slow sand filtration techniques may not only be simple and cheap but also the most efficient method of treating water. Slow sand filtration comes as an efficient method of any particulate suspended matter hence it is appropriate for treating (purifying) water that contains solids in suspension (Jonathan and Douglas, 7). Apart from Slow sand filtration, there are other methods of water filtration (Bourke, 19). These are; Rapid sand filtration Pressure filtration Mechanical filtration The advantages of using slow sand method of filtration over other techniques have been proved practically over a long period of time. Moreover, certain industrialized cities, rural areas and also small communities recommend the use of sand filtration techniques as the method to use when purifying water and water supplies (Bourke, 19). The greatest advantage of using slow sand filtration over other filtration techniques is that it utilizes local skills and materials which are readily available in the third world and the developing countries and that it is more efficient in removing bacterial contamination (Jonathan and Douglas, 17). The systems also have very little maintenance and operational requirements which can be carried out by local personnel after the appropriate training. Although a slow sand filter is easy to operate and maintain, a highly skilled technician should be present during construction and design in order to ensure that it functions correctly. The main objective of this report is to design a slow sand filtration unit using activated carbon, fine sand and gravel. The filtration unit will be capable of effectively removing pathogenic organisms, organic matter, as well as color and mild turbidity and therefore providing safe and clean water. Slow sand filters filtration mechanisms The basic process of treating drinking water in order to increase its quality to the highest quality possible is as shown in the figure below. During the sedimentation process, larger particles are removed and these are often > 50 % of the pathogens. The filtration process is aimed at removing particles that are smaller in size and 90 % of the pathogens. Disinfection is carried out with the main purpose of deactivating and/or killing any pathogens that may have survived the sedimentation and filtration process (Hendricks, 400). There are several mechanisms that operate in slow sand filters in order for the removal of bacteria, viruses, turbidity and organic matter to take place (O’Reilly, 27). The filtration mechanism of slow sand filters can be divided into two processes- physical and mechanical, and biological processes. (a) Physical and mechanical processes In this process, straining takes place first whereby particles that are very large and cannot fit through the pores in-between the sand grains get logged and are hence removed from the water. For a particle to be physically removed from the water, it must first come into contact with the sand grain and then get attached to it. This process is known as adsorption (Hendricks, 466). Due to the slow flow rates, the large solid particles tend to settle above the sand bed through the process summarized below; Interception – The water flows in such a way that the particles move closest possible to a sand grain in order for it to get attached Diffusion – The particles are brought close to the grains through random Brownian motion Sedimentation – The particles are moved downwards onto the top surfaces of the grains through gravitational forces. Hydrodynamic – The particles in a velocity gradient, that is, where water flows around a grain, may at times develop a rotation that provides lateral forces which move particles out from the water stream and make them come into contact with the sand grains. (b) Biological action Due to the small flow rates in slow sand filters, most solid particles get removed in the top 0.5 – 2 cm of the sand. The top sand layer develops a biologically active area called the schutzdecke (dirty layer) and this is the region where most of the biological activities occur with some of the activities continuing down up to a depth of 0.5 m (Hendricks, 399). It should be noted that the efficiency of slow sand filters depends on the size of the sand particles, the way the sand is distributed, the ratio of the surface area of the filter to both the depth and rate of flow of water through the filter. Sand fractions with the finest grades and granulated rockwool have been proven to be the most efficient in the control of the most widespread nursery diseases such as Pythium, Phytophthora and Fusarium oxysporum. Advantages of using slow sand filters As mentioned earlier, slow sand filtration has several advantages over other methods of water filtration and disinfection. These are; The filtration process consumes low energy Construction and installation of the slow sand filters do not require very skilled personnel and they can therefore be installed by laymen. The cost of building and running the slow sand systems is significantly lower as compared to using other filtration and disinfection methods Since they require minimal operator training and periodic maintenance, they end up been considered as the most appropriate technique to be used in water filtration for poor and/or isolated areas (Jonathan and Douglas, 379). The World health organization (WHO), United Nations (UN), Oxfam, and the United Nations Environmental Protection Agency recognizes slow sand filters as superior technology for treating water. The World Health Organization terms slow sand filtration as not only the simplest and cheapest method of filtration but also the most efficient method that can be used (Blaker and Duke, 2). On the other hand, there are some - although few- disadvantages of using slow sand filtration technique. These are; Although slow sand filtration produces disinfected water of very high quality, there are some contaminants that cannot be removed using slow sand filtration (Hendricks, 38). Examples of these contaminants are sulphates, nitrates, hardness, and both fine and stable colloidal matter. Another disadvantage of using slow sand filters is that the process of scraping and may be laborious and time consuming and may at times require the use of water pumps (Hendricks, 38). Predesign Slow sand filtration is designed and specially suited for use by both individual persons and small communities as its cost of operation is low and that its less passive. Therefore, slow sand filtration should be used as the most appropriate technology for treating water in homes and also for small communities with a population of one thousand to two thousand people – when large scale slow sand filters are used. However, communities with a large population should make use of other methods of water filtration, such as rapid rate filtration, since using slow sand filtration would involve the use of large amounts of both sand and land which relates to the labor cost of processing the large amounts of land been greater than when alternative methods such as rapid rate filtration are used. However, the point of the above cross over depends on various circumstances. For example, Gimbel and Graham (123) have listed a number of cities with a very large population yet slow sand filtration is still the water filtration technique that is mostly used. Examples of this cities are The city of Salem, OR, which has a population of 107, 000 people, West Hartford, CT, which also uses slow sand filtration to serve a population of 300, 000 among other cities. The process of slow sand filtration is carried out with the main objective of getting rid of biological particles such as oocysts, cysts, bacteria, algae, viruses, nematode eggs, parasite eggs as well as amorphous organic debris at levels of between 2 – log and 4 – log levels once the filter bed has become biologically mature (Gimbel and Graham, 156). However, mineral particles may pass through the filter bed which is effective in the removal of biological particles. This means that the removal of turbidity may not be a very useful indicator of the performance of the slow sand filters (Logsdon, 121). Nevertheless, the slow sand filter’s turbidity ought to comply with the set regulatory requirements. Design information Mechanical components The basic materials to be used during in the design of the slow sand filtration system are: Fine sand Activated carbon Gravel Concrete Pipes Each of the above materials plays a very vital role during the filtration process. Before designing a slow sand filter, the amount of water to be filtered should be determined. This determines the size and capacity of the filter. In this design, a filter box that is 51.2cm long will be used and it is in this size where the filter layers will be distributed (Logsdon, 66). In the slow sand filter design, sand with a size of 0.15 to 0.35 mm will be used. This is the effective size of sand capable of removing a large percentage of impurities such as coliforms, Giardia cysts and cryptospridum. These impurities operate with maximum efficiency at a flow rate ranging between 0.1 to 0.3 m/h (Gimbel and Graham, 168). The following is a sketch of a typical slow sand filter. Material selection criteria This section will discuss the basic steps taken when selecting the materials to be used during the process of constructing the slow sand filter. Sand and gravel The best sand and gravel that is to be used when constructing a slow sand filter is that which has is AWWA 100 or NSF/ANSI 61 approved. Incase this is not available, one should determine the origin of both the sand and the gravel as well as what is and what is not in the sand. Care should be taken in order to ascertain that the sand and/or gravel are free from any industrial chemicals. The industrial materials may be things like mercury, lead, insecticides, and petroleum derivatives among others. In the event of the sand and/or gravel been contaminated, they should be washed clean of any of the organic materials and other contaminants such as mud and clay (Abel, 34). The sand should be washed thoroughly until the water that is draining from it becomes clear. Sand that has already been cleaned and sterilized may also be purchased from home improvement stores but it will only work effectively if the sand’s size is fairly uniform. Alternatively, builders sand may also be used. However, it will only work satisfactorily if the grains are not too coarse and when the size of the sand grains is uniform. Size of sand and gravel to be used in the slow sand filters It is very important to note that fine sand filters better than coarse sand; and that, fine sand offers better and more resistance to water flow than coarse sand. The range of sand sizes that is the most appropriate and the most commonly used is that which has an effective size of between 0.35 mm and 0.15 mm and which has a uniformity coefficient that is less than 2. However, the sand in the filter layer should all be of the same effective size. The top 30 – 40 cm layer of sand should contain sand with a small effective size. On the other hand, the gravel on the bottom of the filter box should be small enough to prevent the fine sand above them from seeping into them and at the same time large enough in such a way that they cannot pass through the holes in the drain pipes. The drain pipes should be covered by the gravel up to a height of about 6 inches. In general, the smaller the effective size of the sand used the more effective the slow sand filter will be. If sand with a very small effective size is used, water will not flow through the slow sand filter fast enough. On the other hand, if sand with a very large effective size is used the bio layer will not form as effectively as required and hence the slow sand filter will fail to purify the water (Abel, 34). Turbidity in slow sand filters Apparently, slow sand filters are capable of coping with turbidities of between 100 – 200 NTU for a few days and 50 NTU for a longer duration. However, the best results are produced when there is an average turbidity of < 10 NTU (Huisman and Wood, 1974). Other writers, such as Montiel et al. (1988), however suggest a lower turbidity limit of Read More