10 months ago


Water treatment

Models for assessing

Models for assessing construction quantities. In this section models for the estimation of the construction quantities for MSF systems are presented, based on the following assumptions: • Models have been developed for plants with capacities between 2 and 25 ls -1 , taking into account the size of the existing rural communities and small municipalities in Andean Cauca Valley and Colombia. To establish the models four typical designs have been established for plant capacities of 2, 8, 15 and 25 ls -1 , using the design criteria indicated in table 5.1; • Four MSF alternatives have been reviewed on the basis of previous experiences with both pilot and full-scale plants in the Andean Cauca Valley. They are: DyGF + SSF DyGF + UGFL + SSF DyGF + UGFS (2) + SSF DyGF + UGFS (3) + SSF; • Valves have been included on the basis of the required specifications in size and numbers, for each treatment unit (Annex 6). The valves, accessories and manifolds are specified on the basis of a wash-flow of 15 mh -1 and 20 mh -1 for DyGF and upflow CGF units respectively. • For the structural design, the filtration units are considered to be constructed in the ground, with a carrying capacity of at least 1.8 Kgcm -2 ; • Eighty (80) percent cost-distribution has been used for the major items such as: concrete, gravel, sand, etc. (see table 5.2) and 20 percent for other (minor) costs. In some areas the cost distribution may be different, particularly as a result of the cost of the gravel and sand. In other cases the land may imply a considerable cost factor that may change this distribution somewhat, requiring adjustments in the models. Table 5.1. Design criteria for filtration stages of MSF plants used as basis for the models. MSF component Design criteria DyGF UGFL UGFS (2) UGFS (3) SSF Filtration rate (mh -1 ) 2 0.6 0.6 0.6 0.15 Depth of filter bed (m) 0.60 1.2 1.2 (1) 1.2 (1) 1.05 (2) Filter height (m) 0.80 1.4 1.4 (1) 1.4 (1) 1.90 Height of supernatant 0.05 0.1 0.1 (1) 0.1 (1) 0.75 Free board (m) 0.15 0.1 0.1 0.1 0.10 Maximum surface area (m 2 ) 10 25 25 25 100 Number of units in parallel 2-6 2-6 2-6 2-6 4-12 Number of units in series 1 1 2 3 1 (1) For each filter unit (2) Including support media layer of 0.20 to 0.25m, assuming commercially available corrugated PVC manifold drainage pipes 5.5.2 Operation, maintenance, and administration costs Based on field visits and informal interviews with local MSF plants caretakers, members of community-based organisations, and staff members of water sector institutions and Cinara, preliminary estimations of time required for OM&A were made. In spite of the limited amount of data collected in a methodical way, around 10 years of practical experience in the Andean Cauca Valley with MSF technology has been used to estimate values, meanwhile the actual available information is being improved. The experience with MSF technology in the Cauca Valley in Colombia indicates that staff costs represented approximately 85 percent of the operational costs. The other 15 percent covered items such as electricity, wash water, and gardening. The cost of pumping is not included as many MSF systems are adopting gravity water supply. The pumping cost is assumed to be similar for all alternatives under review and therefore can be left out of the comparison. 181

5.5.3 Cost comparison with conventional RF technology In most situations in the Andean Region, it is expected that MSF technology would be the only socially, culturally and technically viable option to treat polluted surface (rivers/streams) water sources for small communities. However, for the Andean Cauca Valley it was considered useful to have cost comparison analysis between MSF alternatives and conventional RF treatment plants. The procedure used to do this comparison is known as minimum cost analysis. This procedure is based on calculating the net present value (NPV) of costs associated with each treatment alternative. In other words, NPV is the equivalent, in actual pesos or dollars of all the benefits, minus the present and future costs associated with each alternative (Infante, 1993). Based on this criterion, a development project could be accepted as an economically viable alternative when NPV value is greater than zero. Assuming that both alternatives produce similar benefits, the economical choice would be that having minimum cost value due to payments or expenditures during a selected life cycle of the project. Consequently, the minimum cost technique uses only the equation 5.9 to calculate NPV of costs. NPV C C C C = ∑ (5.9) n−1 i 1 n−1 = C i 0 + + .... + 1 n−1 i= 0 (1 + r) (1 + r) (1 + r) In which r is discount rate; i is the time in which cost (C i ) become effective; C i cost at time i; and n (years) is the life cycle of the project. To decide economically between different MSF alternatives is rather simple. However, when this involves other treatment technologies with different economies of scale it is more complex, because different costs need to be taken into account in time, depending on plant capacities. The minimum cost technique considers these difficulties by taking into account the economy of scale factor in the design periods of treatment facilities in the development projects (Duque, 1992). According to Duque (1992) MSF alternatives could have optimum design periods in the range of 4 to 10 years, meanwhile conventional RF plants could have design periods in the range of 8 to 15 years, because the latter has greater economy of scale. The concept of “project horizon” is introduced to avoid comparing different design periods with different benefits. With the project horizon concept a minimum common multiple of design periods is used. In this way benefit periods become similar, making it also easier to combine both construction and OM&A costs. NPV of all economic costs are then accomplished for the horizon period of the project, including the option of both technological alternatives. 5.6 Results 5.6.1. Initial investment cost Formulation of the models. Using the data from table 5.1, we can transform the plant capacity into the filtration area (A). The construction quantities for each component of the 182