4 days ago


Water treatment

mgl -1 . Changes in the

mgl -1 . Changes in the sediment transport in the river may also affect the capacity of the wells. One possible disadvantage of this system is that changes may occur underground, which can be difficult to remedy by maintenance activities. Infiltration Galleries. In Infiltration Galleries or Riverbed filtration, water is abstracted using perforated pipes through the natural riverbed material, or if the permeability is too low, through an artificial bed of coarse sand and gravel. Several riverbed filtration systems are reported (Smet et al, 1989) with different water retaining methods such as longitudinal and lateral drain systems, modular sub-sand abstraction, and river dam filtration system. Flow velocity through the filtering bed ranges from 0.25 to 1.5 mh -1 , depending on turbidity levels and effluent requirements. Removal efficiencies up to 98% have been reported for riverbed dam filtration from rivers with turbidity levels in the range of 48 to 200 NTU (Salazar, 1980), however, in a field evaluation the efficiencies were found to be around 20% (Cinara and IRCWD, 1988). This was may be due to the difficulties to implement the periodic cleaning or reposition of the clogged filtering material, particularly during the rainy season, when the rivers present high flows and high solids transport capacity. Because clogging of the infiltration area can make necessary the reconstruction of the riverbed filter or the infiltration area, pretreatment filtration alternatives completely separated from the surface water source are receiving more attention (Cleasby, 1991). Plain sedimentation. Exposing the water to relatively quiescent conditions will allow removing suspended matter by the action of gravity, and the natural particle aggregation, without the use of coagulants. This process is called Plain sedimentation. Ideally, the clarification efficiency of a settling basin, for a particular suspension of discrete particles, depends only on S 0 , the surface charge (relation between the flow and the settling surface area). In practice, however, disturbing factors such as turbulence and short-circuiting, reduce the effective settling velocity. Plain sedimentation may have short retention times, of the order of minutes or a few hours. In this situation grit chambers and sedimentation tanks can efficiently remove heavy and coarse solids such as sand and silt particles, but is far less effective for colloidal particles. For small WS systems, Wegelin (1996) recommends rectangular sedimentation tanks, and considers that inorganic matter larger than about 20 µm could usually be removed. Meanwhile, Shultz and Okun (1992) had limited this value to about 10 µm, when their specific gravity is 2.65. Then, short-term plain sedimentation seems to be useful only as a preliminary treatment step. In any case, the technical and economical feasibility of achieving this treatment goal may be estimated using sedimentation columns (Huisman, 1989; Droste, 1997). Improved flow conditions in the settling zone (laminar and stable flow) and lower values of S 0 (greater surface area for a given flow) can be obtained in a given conventional sedimentation tank introducing parallel plates set at a short distance, 5 to 10 cm. To ensure self-cleaning capacity, these plates are tilted or inclined at a steep angle of 50 to 60 0 to the horizontal (Huisman, 1989). Tilted plate settlers may reduce the required area of a conventional settler (without plates) by some 65%. Tilted plate settlers are widely used in chemical water treatment, but their application with non-coagulated water is very limited. Besides, in small systems, if area is not a critical issue, this option may have comparable capital costs to the conventional settling, 40

ut higher running costs, since more frequent attention and cleaning would be required due to its lower sludge storage capacity (Castilla and Smet, 1989; Huisman, 1989; Cleasby, 1991). Plain sedimentation may have long retention times, of the order of days or weeks. In this case other factors are important, including wind, thermal, and photosynthetic effects (Cleasby, 1991). This is usually an expensive solution to be adopted exclusively for water supply purposes in small systems (Pardón, 1989). A classical goal of storage basins is to provide supplies during periods of low rainfall in multipurpose projects, and off-channel storage can provide a source during short-term pollution events. Storage basins can be used as preliminary treatment. For extremely turbid waters, above 1,000 NTU, storage provides the best pretreatment (Shultz and Okun, 1992). In England the water depths in the pumped storage reservoirs are typically about 10 to 20 m and the theoretical retention time ranges from about 10 to 50 days (WRC, 1977, quoted by Cleasby, 1991). In London, in long-term storage prior to SSF, turbidity reductions from values around 30 NTU to values below 4 NTU have been reported (WRC, 1977). As shown in figure 2.8, in the period from 1961 to 1970 the average E. coli faecal coliform counts were reduced by 96%, from 6680 to 240 UFC/100 ml (Windle-Taylor, 1974). However, the periodic blooms of algae made it necessary to introduce microstrainers or rapid filters before the SSF units (Ridley, 1967). Management techniques have been developed to minimise algae blooms and other detrimental water quality effects in the reservoirs. These techniques include pumping devices to control the thermal stratification. The potential of long term storage to protect SSF in small systems directly or in combination with other treatment steps needs to be evaluated under local conditions, introducing the possibility of a multipurpose reservoir. Coarse media filtration. Porous media such as gravel and sand are old water clarification processes with documented applications in several European countries, since the 1800s (Baker, 1981). The development and promotion of this technology was interrupted with the arrival of chemical and mechanised water treatment technologies. Since the 1970s, the use of SSF technology in small WS systems has gained increasing attention because of the potential of coarse media filtration (CMF) to improve the quality of deteriorating surface waters. During the 1980s, it became clear that CMF was a good option to condition the water before it reached the SSF units, based on studies conducted in Africa, Asia, Europe, and Latin America. These technologies studies and new ones are still being developed at the end of the 1990s. 2.8 Coarse Media Filtration as Pretreatment Step for Slow Sand Filtration Short-term plain sedimentation may be the first conditioning stage of surface waters that transport relatively large and heavy particles, such as grit or sand. However, rivers usually transport a wide range of particles, including those with sizes