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Water treatment

5.8 General Discussion

5.8 General Discussion The following observations can be made concerning the construction and cost models described in this chapter: • The proposed method to obtain these models has been found useful to obtain preliminary estimates of cost values. The models have been developed for treatment capacities in the range of 2 to 25 ls -1 . Nevertheless, the assumptions and criteria indicated in Section 5.5 have to be kept in mind while interpreting the data, as the cost models as presented are only applicable for the region where the unit costs have been established. • The models can be adjusted for a situation or particular region. This requires a systematic evaluation of experiences with the technology in the region concerned. • The level of reliability of the cost estimate depends to a large extent on the quality of information concerning the unit cost. • For the MSF technology the cost increases with the risk level associated with the water source. A higher risk requires a larger number of filtration steps and so larger construction quantities. • Under similar operating conditions (plant capacity, construction materials, unit and labour cost and raw water quality), the option of MSF including UGFL is the most economical, followed by the options including UGFS (2) and UGFS (3). However, when these MSF alternatives operate with different filtering rates, cost models could become a practical tool to facilitate economical analysis. The cost model obtained for MSF systems in the Andean Cauca Valley provides the following specific observations: • The component with least effect on cost is the DyGF, contributing only between 6 and 8 percent (table 5.7). The SSF is the largest with 42 to 56 percent. This implies that the optimisation of SSF could provide a considerable cost reduction. • The factors grouped under the heading “ others” represent between 15 and 25 percent (table 5.7.), confirming the 80/20 percent cost distribution. • The coefficient “ b” is in the order of 0.80 for the components UGFL, UGFS (2), UGFS (3), and SSF; for DyGF this value is in the order of 0.75 (figure 5.1A). Components, which have a higher impact on the costs, have a lower economy of scale. Nevertheless, for the combined systems (figure 5.1B) these coefficients range between 0.78 and 0.79, indicating some economy of scale. • The cost of MSF systems operating at filtration rates other than indicated in table 5.1, can also be estimated, because the models are based on the area of filtration. • Example illustrated in figure 5.3 shows that conventional RF technology has a greater economy of scale (b = 0.36) than MSF technology (b in the range 0.78 to 0.79). Consequently, MSF technology should have shorter design periods than conventional RF technology. • Figure 5.3 indicates that MSF in the Cauca Valley could have lower initial construction than conventional RF technology for treatment capacities < 8 (for DyGF + UGFS·+ SSF) to 21 ls -1 (for DyGF + SSF). Considering both construction and OM&A cost in medium to long term basis, MSF become more economically competitive than conventional RF technology at least up to 25 ls -1 . This means that for rural communities and small municipalities MSF is also economically competitive. 189

6 SELECTION OF MULTISTAGE FILTRATION ALTERNATIVES Well-protected sources should be selected to provide safe drinking water supply (Okun, 1996). However, if water treatment is needed, development programmes aiming to improve drinking water quality must select and transfer technologies that match local conditions. In this chapter an example of a selection guide is proposed for several multistage filtration (MSF) alternatives based on experiences in the Andean Cauca Valley with pilot and full-scale plants processing surface (river/stream) water. Based on results and discussion presented in Chapter 3 and 4, only upflow gravel filtration in layers (UGFL) and series (UGFS) are included as filtration alternatives for the second (CGF) stage of MSF plants. In summary, UGFL, having the shortest filter bed depth of all CGF alternatives, has economic advantages for treating less polluted surface water sources. Downflow gravel filtration in series (DGFS) showed similar (for suspended solids and faecal coliform bacteria) or lower (for turbidity and colour) mean removal efficiencies than UGFS. DGFS and UGFS have similar initial (capital costs), but UGFS seems to have advantages from the maintenance point of view. Horizontal gravel filtration (HGF) showed lower removal efficiencies than UGFS when both CGF options had similar filter bed lengths. Besides, after comparative studies in full scale MSF system used for groundwater recharge in Switzerland Wegelin et al (1991) found that UGF units could be hydraulically cleaned more efficiently than HGF. However, “ conventional” HGF, with long filter bed lengths without hydraulic cleaning facilities, seems to have advantages for non-permanent (or emergency situations) due to its large silt storage capacity and simplicity of construction. The selection and transference of technology involves a large number of factors. The approach presented by the University of Oklahoma (Reid, 1982) distinguishes four information sets of factors that provide the framework for technology selection: social and technical aspects, indigenous resources, raw water quality, and demographic data. This approach was limited to evaluating the feasible treatment methods for a single community. Another concept presented in the literature takes technical, socio-economic, environmental, and managerial issues as main factors (Hofkes and Visscher, 1990). Together with previous factors, cultural aspects and the political, legal, and institutional framework were found important during some technology transference experiences in Colombia (Galvis, et al, 1997; Garcia et al, 1997; Quiroga et al, 1997; Visscher et al, 1997; Galvis and Vargas, 1998). However, as the example of a selection guide to be presented only compares different MSF alternatives, several of the factors identified before can be disregarded. No comparison is made with other treatment alternatives such as rapid filtration (RF) of chemically coagulated water. RF options do not match the management capacity of the majority of the rural communities and the small and medium-sized municipalities, and in most cases they cannot guarantee the availability of chemical supplies required to keep the system functioning. The MSF technology on the other hand, is now well received in the Andean Cauca Valley by water-related institutions and community based organisations because its administration, operation and maintenance are much simpler than the requirements of other treatment 190