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Water treatment

al., 1985). In Europe,

al., 1985). In Europe, the SSF technology continues to be an important component of several major water treatment systems, but now with a different perspective motivated by the level of industrialisation, the type and level of contamination and strict water quality regulation. In general, it is now used as one of the last treatment steps to “ polish” the water quality in terms of disinfection as well as in the removal of organic carbon. The first application of SSF in the USA is reported in Poughkeepsie, New York, in 1872. The short filter runs of the SSF units were caused by turbid surface water in different regions of the country, and stimulated the development of RF (Bellamy et al, 1985). In 1940, 2,275 RF plants and approximately 100 SSF systems existed (Fox et al, 1994). Based on a survey of 47 of these SSF systems, Slezak and Sims (1984) and Sims and Slezak (1991) provide the following overview. Seventy-six percent of these plants were serving populations smaller than 10,000 people; 21 percent were between 10,000 and 100,000, and 3 percent were above 100,000 people. The majority, 54 percent, used small rivers, while 41 percent used lakes or dams, and 5 percent groundwater. The average turbidity of these sources was 2 NTU with peak values of 15 NTU. Some 88 percent of the systems produced effluents with turbidity below 1 NTU. The faecal coliform levels in 80 percent of the water sources were below 100 CFU per 100 ml and more than 70 percent of the systems produced effluents below 1 CFU per 100 ml. After the publication of the SWTR, It was expected that SSF would find a new place in the USA, and that a considerable number of SSF units would be constructed to meet the requirements of filtration of surface waters (Fox et al, 1994). By 1994 the amount of SSF treatment plants had grown to 225, an increase of more than 100 since 1940 (Brink and Parks, 1996). In Latin America and the Caribbean, SSF was used in the treatment of water for larger cities such as Buenos Aires and Kingston. The majority of the cities in these regions, however, used RF technology. The introduction of SSF to the region was in most cases carried out without adjusting SSF to the local conditions, and as a result its impact has been very limited. Most of the SSF plants were constructed in countries such as Brazil (Hespanhol, 1969) and Peru (Canepa, 1982; Pardón, 1989; Lloyd and Helmer, 1991) and presented major difficulties in design, operation and maintenance. Similar situations have been encountered in Africa, in countries such as Cameroon, Kenya and Zambia; and in Asia, in countries like India, Pakistan and Thailand. In Andhra Pradesh, in India, for example, over 1,100 SSF system exist but most of them have deficiencies in their design and functioning (Visscher, 1993). Despite these difficulties, renewed interest in SSF became apparent at the end of 1970s. This interest was stimulated by the UN agencies in the context of the Water Decade, during the 1980s. Research and demonstration projects carried out by international organisations based in Europe (Particularly England, the Netherlands and Switzerland) with national and regional collaborating centres based in Africa, Asia, Latin America and the Caribbean, contributed to understand the possibilities and limitations of the technology. Interest also raised from the new role of SSF identified by EPA under the SDWA and the SWTR in the USA. There is also a better environment, and political commitment to strategies and technologies oriented to improve drinking water quality in the Americas after the cholera epidemic that affected the region since early 1991 (PAHO, 1998). 20

2.5 The Slow Sand Filtration Process There are some typical operational differences between SSF and RF units. Filtration rates are around 50 to 150 times lower for SSF. Flow retention periods are about 30 to 90 times longer for SSF. Filter run lengths are about 30 to 90 times longer for SSF, and the surface of the SSF units are usually scraped at the end of the filter runs, whereas RF units are cleaned by backwashing. These differences originate from the most relevant and distinctive feature of SSF, its biological life. The water treatment in SSF is the result of a combination of physicochemical and biological mechanisms that interact in a complex way. Inorganic and organic matter enter the SSF units in the raw water. Photosynthesis gives rise to another fraction of organic matter. Soluble matter in the sand bed is utilised by bacteria and other microorganisms. Zooplankton grazing occurs and respiration of the entire biomass is continuous (Woodward and Ta, 1988). The principal physical mechanisms contributing to particle removal are surface straining, interception, transport, and attachment and detachment mechanisms (Yao et al, 1971; Amirtharajah, 1988). Some of the concepts mainly developed for RF have been extrapolated to SSF in the process of trying to understand the physical dimension of its particle removal mechanisms. However, there are limitations in this approach since in RF the particles have been previously destabilised by chemical coagulants and the biological activity is not so relevant. By the straining or sieving mechanism particles that are too large to pass through the pores of the filter media are removed. It takes place mainly at the surface of the filter bed, where the head loss is concentrated, and is independent of the filtration rate. According to Amirtharajah (1988), the size of pore openings is in the range of 0.07 to 0.1 d c (collector or grain diameter). Hence with sand grains of 0.20 mm, this means complete removal of particles with d p (particle diameter) >20 µm. As long as the filter captures this size or bigger particles straining is enhanced, allowing the capture of smaller particles as the filter skin develops. Haarhoff and Cleasby (1991) consider this development at least partly responsible for the initial improvement in performance at the beginning of each filter run, usually referred to as the ripening period. In this respect, the continuous flow bench scale experimental filtering cells made by Lloyd (1974; 1996) seem to be relevant. He found that the presence on the surface of sand of the peritrich protozoa Vorticella convallaria, a ciliary suspension feeder, improved significantly the bacterial removal in comparison with an uninoculated control sand sample. This improvement was greater when the population of protozoa was bigger and when the elapsed time between the samples was longer, showing clearly their biological role in progressively improving the filter performance during the ripening period. The removal of smaller particles that enter the pores of the filter requires transport mechanisms to reach the surface of the sand (figure 2.4) and attachment forces to adhere to the sand grains. As particles accumulate inside the bed, the fluid velocities within the pores will increase, resulting in increasing drag forces on the deposited particles. Eventually, if these forces are big enough, the deposited particles will be detached and transported deeper 21

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