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Water treatment

The efficiency levels

The efficiency levels summarised before should not be directly used for design purposes as some specific characteristics of the raw water such as the high iron content may contribute to the high efficiency. In this respect it is important to stress that in areas where limited experience exists with MSF it is very important to be somewhat conservative in the design to avoid possible failures through overloading which may put in jeopardy the whole idea of the provision of good quality drinking water. Headlosses in HGF units. “ Conventional” HGF without hydraulic cleaning facilities showed a headloss close to 15 cm after 2.1 years of continuous operation processing Cauca River water. Based on this experience the author recommends filling filter material to 20 cm above the effluent’s weir level. This value is higher than the value of 10 cm suggested by Pardon (1989) but lower than the range of 30 to 40 cm advised by Wegelin and Mbwette (1989) and Wegelin (1996). Headlosses in vertical-flow CGF. Vertical-flow CGF alternatives had higher total headlosses than horizontal flow CGF options. The highest headloss value during monthly PCA was reached by UGFL (46 cm) followed by UGFS 1 (26.4 cm) and DGFS 1 (25.7 cm). A headloss development pattern by the end of filter runs (before monthly PCA) of vertical flow CGF options was identified during this research work. Initially (at the end of the first filter runs), headloss takes place mainly at the bottom (in UGF) or top (in DGF) gravel bed layers. Gradually (with more filter runs) it becomes evenly distributed through the filter bed. Finally it concentrates mainly at an intermediate gravel layer around 30 cm thick (total gravel bed depth used was 1.5 m per CGF unit). During the comparative study at Puerto Mallarino, this headloss pattern developed fastest at UGFL, and slowest in UGFS, with DGFS having an intermediate development rate. Based on this experience the following recommendations are made: • To have energy available for maximum headlosses (before TCA) it is recommended to design CGF units with the following available hydraulic heads: 45 cm for UGFL, 30 cm for UGFS 1 and DGFS 1 , and 20 cm for UGFS 2, 3 and DGFS 2, 3. • To have longer periods between TCC (total cleaning cycle) in CGF, it is necessary to reduce sludge penetration into intermediary gravel bed layers. Consequently, it is advisable to increase from once a month to at least once a week the PCA used during the 1 st phase of this study. In the case of UGFL and DGFS this frequency should include both surface shovelling and bottom drainage PCA. In the case of UGFS weekly frequency could include only bottom drainage followed by surface cleaning twice a month. • To have good dislodging impact with PCA total gravel bed depth inside each vertical flow CGF unit should be lower than 1.2 m. Gravel bed depth in the range of 0.9 to 1.2 m seems to be desirable to keep a good balance between removal efficiencies and gravel bed-cleaning requirements. • To improve the impact of bottom drainage in dislodging the units initial drainage velocities could be increased from the range 10 to 25 mh -1 used in the present study, to the range 20 to 30 mh -1 . Changes in flow pattern inside gravel bed by sequential openings and closures of the fast opening valve at the beginning of hydraulic cleaning procedure are preferred, instead of using higher drainage velocities. Headlosses in SSF units. An exponential headloss development pattern takes place along SSF runs, according to headloss values observed in this pilot study and data reported in the 131

specialised literature. Based on this evidence it is recommended that traditional supernatant water layer heads in the range 0.9 to 1.5 m (see table 2.4) could be reduced to values in the range 0.6 to 0.8 m, depending on local conditions. This recommendation is considered relevant in those situations in which running cost (mainly labour) are less critical than initial capital investment. Upflow vs. Downflow CGF options with filter units in series. On the basis of statistical analyses UGFS and DGFS provide similar (significance 1%) SS and faecal coliform 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 slower headloss development pattern and longer TCC than DGFS, UGFS would be the preferred choice between these two CGF options. Vertical vs. horizontal flow options. During the 1 st phase of this study UGFL showed one of the two lowest mean removal efficiencies of all tested CGF lines, some times being statistically the same or even better than MHGF. The shortest filter bed length of UGFL between all tested CGF lines 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. The less good removal efficiencies of MHGF, having similar filter bed lengths to UGFS and DGFS, were initially assumed, and later (during the 2 nd phase) supported with experimental results, as being related to poor hydraulic performance of HGF. Based on results from the 2 nd phase, UGFS seems to be a more suitable CGF option than HGF and HGFS, as it presents better mean removal efficiencies and lower cleaning water requirements with similar PCA. Therefore, at least for MSF applications with the Cauca River water quality, HGF is less competitive than UGFS and probably this conclusion can be extended to other situations because of its poor hydraulic performance. The research also showed that “ conventional” HGF could be improved by establishing a better hydraulic behaviour. The HGFS designed during the present study gave good results even though the applied design had some limitations, as it needed to make use of the existing MHGF infrastructure. So it is felt that further improvements can be made in the HGFS. Important advantages of “conventional” HGF remain its large silt storage capacity and simplicity of construction of the main filter box, which may be beneficial for temporary structures (e.g. refugee camps). Then the system can be designed for a limited period of says two years as a temporary structure without drainage or other cleaning facilities. Even if this period is too short then still the gravel may be removed, washed and replaced or, depending on socio-economic considerations, a new system can be built. HGF may also still have some application for continuous higher turbidity levels but this requires further review as the advantage of a large silt storage capacity may be outweighed by its higher cost as compared to the other alternatives, particularly if gravel is costly at local level. 132

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