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Water treatment

Constant and declining

Constant and declining filtration rates. Surface velocities up to 0.2 ms -1 did not play any role in improving SS removal efficiencies, neither contributing to reduce PCA frequencies by scouring, nor transporting away previously removed solids on top of the gravel bed of DyGF units. Therefore, overflow can be eliminated as a major hydraulic element in DyGF operation, but retaining 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 built with this innovation will work at a constant filtration rate until headloss becomes greater than the height of the weir. Afterwards the units will work at declining filtration rate. Observations at local level are recommended to adjust the height of this weir in such a way that effectively responds as a “safety valve” by allowing overflow in the presence of peaks of SS, keeping the possibility of applying the protection capacity concept. Gravel bed. In spite of the risk of filter media intermixing during TCA, a stratified gravel bed seems to be a better option in DyGF for having faster reactions during abrupt water quality changes and reducing sludge penetration into gravel bed and TCA frequency. The installation of strong plastic mesh between gravel bed layers is recommended to facilitate excavation activities without filter media intermixing. Furthermore, sieving tools should be available at MSF plants to select gravel bed layers after TCA activities of CGF stages. Cleaning frequency. Based on the experience with Cauca River water, DyGF requires frequent PCA (surface raking and fast bottom drainage) around twice a week and TCA (gravel bed excavation, cleaning and replacement) around once every three months. 3.3.2 Coarse gravel and slow sand filtration After the 1 st comprehensive comparative study of CGF options working in series with SSF units the following points are highlighted. Before doing so it should be noted that CGF lines included in the MSF pilot system were set to work at constant filtration rate and did not benefit from the declining filtration rate effect of DyGF. Still CGF lines did benefit from important reductions in mean and peak SS values accomplished by the DyGF stage. Maximum SS concentration as design criteria. Based on the protection capacity shown by DyGF during this study, it is not advisable to use maximum SS concentration in raw water to dimension the 2 nd CGF stage as recommended by Wegelin and Mbwette(1989). Consequently, having DyGF as the 1 st filtration stage, smaller and cheaper CGF units can be projected to deal with sharp peaks of solids, which are common in small Andean rivers. SS concentration before SSF. The potential of all CGF lines to remove SS was verified during this study. During the last two test periods (phase I) all CGF lines produced mean SS levels < 5 mgl -1 but only those SSF units (after UGFS, HGF, and DGFS) with influent mean and SD values < 2 mgl -1 had mean run lengths > 30 days. Consequently SS concentration < 2 mgl -1 seems to be an acceptable guideline for water to be treated by SSF units. Due to the possible limitations of local organisations to perform SS measurements with clear water samples, turbidity levels below 10 NTU should be a more practical criterion. Additionally, filterability tests should also be done to enrich the practical meaning of either SS or turbidity measurements. 129

Microbiological water quality improvement. The great capability of CGF working in series with SSF to reduce microbial contaminants was clearly established beyond limits previously reported in the specialised literature. Mean removal efficiencies up to 5.6 log units were observed with CGF stages contributing as much as the SSF stage. During the 1 st three test periods (CGF rates ≤ 0.6 mh -1 ), and after a month of “maturation” for indicator bacteria, all samples taken from the effluents of the UGFS + SSF 1 line had coliform levels inside available guidelines for small water supply systems before terminal disinfection as a safety barrier. During test periods II and III all CGF lines working in series with SSF units were practically within the guidelines but only 2% of the samples taken from the effluent of the UGFL + SFF 2 line. Therefore, for CGF rates up to 0.6 mh -1 and SSF rates up to 0.15 mh -1 , all CGF + SSF lines produced effluents with low microbial risk (mean ≤ 3 and maximum ≤ 25 CFU/100 ml). These results open the possibility of terminal disinfection as a safety barrier after MSF processing water even from highly polluted water sources. Turbidity removal. The individual removal efficiencies for turbidity in the CGF stages were all in the range of 43 (UGFL, period II) to 80% (UGFS, period I), which is a normal range taking into account values reported in the literature. In spite of these removal efficiencies CGF lines did not produce effluents consistently < 1 NTU. However, After period I, with the highest raw water turbidity values (mean 109 and SD 109 NTU), SSF units produced turbidity values ≤ 5 NTU in 85 to 99% of the samples. During test period III (mean raw water turbidity of 55 and SD of 52 NTU) 27 to 65 % and 65 to 84% of samples collected in CGF effluents had turbidity ≤ 5 and ≤ 10 NTU respectively. During the same period 48 to 61% and 98 to 99% SSF effluents had turbidity ≤ 1 and ≤ 5 NTU respectively. Therefore, despite the low microbiologically sanitary risk reached with the MSF pilot plant system, around half of the time turbidity values were above the desirable level (

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