9 months ago


Water treatment

scouring and

scouring and transporting away previously removed solids on top of the gravel bed of DyGF units. Therefore, it is recommended to eliminate overflow, which was a normal hydraulic element in DyGF operation, and include a shallow overflow weir, with its crest around 2 to 5 cm above the top gravel layer, at the end of the filtering box. DyGF units built with this innovation will work at a constant filtration rate until water level reaches the height of the weir, due to the increment of headlosses inside the gravel bed. Afterwards the DyGF units will work at declining filtration rate. Based on the protection capacity shown by DyGF during this study, it is not advisable to use maximum suspended solids (SS) concentration in raw water to size the second gravel filtration stage. Having DyGF as the first filtration stage, smaller and cheaper units can be projected for the second gravel filtration stage to deal with sharp peaks of solids, which are common in small Andean rivers. • Comparative studies on gravel filtration alternatives as second filtration stage The potential of all gravel filtration alternatives to remove SS was verified during this study. However, mean run lengths > 30 days were observed only in those SSF units following those gravel filtration options with mean and SD values < 2 mgl -1 . Consequently SS concentration < 2 mgl -1 seems to be an acceptable guideline for water to be treated by SSF units. On the basis of statistical analyses UGFS (upflow gravel filtration in series) and DGFS (downflow gravel filtration in series) provide similar (significance 1%) SS and faecal coliforms removal efficiencies. UGFS showed better (significance 1%) turbidity and colour removal efficiencies than DGFS. However, these differences in mean removal efficiencies seem to be not practically relevant for the overall performance of MSF plants. But, considering that UGFS showed better possibilities than DGFS for frequent partial cleaning procedures, UGFS is recommended between these two CGF options. UGFL (upflow gravel filtration in layers) showed one of the two lowest mean removal efficiencies of all tested gravel filtration options at Puerto Mallarino, some times being statistically the same or even better than that of the MHGF (modified horizontal gravel filtration unit). The shortest filter bed length of UGFL between all tested gravel filtration alternatives easily explains its somewhat lower efficiencies. However, UGFL being the cheapest tested CGF option, it should be an alternative to be considered in dealing with less polluted water sources than the Cauca River, as shown during the experiences with full-scale systems. The less good removal efficiencies of MHGF, having similar filter bed lengths to UGFS and DGFS, were initially assumed, and later supported with experimental results, as being related to poor hydraulic performance of HGF (horizontal gravel filtration) technology. Based on the research results of the second phase at Puerto Mallarino, when all gravel filtration options had similar cleaning procedures, UGFS seems to be a better option than HGF and HGFS, as it presents better mean removal efficiencies and lower cleaning water requirements. 213

• Construction costs and cost models of MSF technology 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, labour cost and raw water quality), the option of UGFL is the most economical, followed by UGFS-2 (upflow gravel filtration with two units in series) and UGFS-3. However, when these MSF alternatives operate with different filtration rates, cost models could become a practical tool to facilitate economical analysis. 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 component with least effect on cost is the DyGF, contributing only between 6 and 8 percent depending on the characteristics of the other filtration stages in a MSF plant. The SSF is the largest with 42 to 56 percent. This implies that the optimisation of SSF could provide a considerable cost reduction. Based on examples prepared with data valid for the Cali area, MSF alternatives including DyGF, UGF (upflow gravel filtration in layers or in series) and SSF units, show economies of scale values in the range 0.78 to 0.79. Conventional rapid filtration plants have a greater economy of scale (0.36) than MSF technology. Consequently, MSF technology should have shorter design periods than conventional RF technology. Based on cost data for the Cali area in the Cauca Valley, MSF could have lower initial construction costs than conventional RF technology for treatment capacities lower than 8 (for DyGF + UGFS·+ SSF) up to 21 ls -1 (for DyGF + SSF). Considering both construction and OM&A costs in a medium to long term basis, MSF become more economically competitive than conventional RF technology at least up to 25 ls -1 (the highest flow capacity included in the database for analysing MSF costs). This means that for rural communities and small municipalities MSF technology is not only technically but also economically competitive. • Selection of MSF alternatives A procedure to identify and combine filtration stages in a treatment plant has been proposed in this study. It is based on a set of treatment objectives, contamination levels in raw water sources, and removal efficiencies expected at each treatment stage. The selection guides for MSF alternatives included in this thesis may not apply in different geographical or cultural conditions from those described for the Andean Cauca Valley. 214

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