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Water treatment

for HGFS and from 3 to 5

for HGFS and from 3 to 5 for HGF. These results seems to validate the hypothesis stated earlier (section suggesting that poor hydraulic performance of MHGF line could be one of the reasons behind its lower removal efficiencies with respect to other CGF lines having similar gravel bed lengths than MHGF line but with units working in series. Those HGF studies performed with long, narrow (15 to 20 cm diameter (φ)) filtering pipes (e.g. Wegelin, 1986; Collins et al, 1994; Ahsan, 1995) seem to be irrelevant with respect to the hydraulic behaviour of full scale units. Hydraulic behaviour of these bench or pilot scale filtering cells with low φ/length ratios should be better than full-scale units with rather high φ/length ratios. Therefore, contaminant removal and cleaning efficiencies at real scale may be lower than expected after studies with inappropriate pilot facilities. Operation and maintenance aspects of CGF lines Headloss developments in the CGF lines are shown in table 3.32. The initial mean values were obtained from measurements made after monthly PCA. The maximum mean values were obtained from measurements made before monthly PCA. Headloss developments in the HGFS and HGF lines are very high in the first section, which seems to be in line with the hydraulic performance of these systems. It was noted that recuperation of initial headloss values was very good implying that filter cleaning is rather effective. In spite of gravel bed changes made before these additional studies, headloss in the 2 nd compartment of UGFS line is limited, suggesting that this unit does not contribute as much as the other two in suspended solids removal. This requires further review as it may offer an opportunity for optimisation of the system, but it should then also be established what the effect on treatment efficiency for example for faecal coliforms and other parameters is brought about by this second unit. Table 3.32. Typical headlosses at the beginning and end of filter runs in CGF units (Based on Ochoa, 1996; Galvis et al, 1996) Water headlosses (cm) Type of UGFS HGFS HGF values UGFS 1 UGFS 2 UGFS 3 HGFS 1 HGFS 2 HGFS 3 HGF 1 HGF 2 Initial value 3 1 1 3 0 0 1 0 Final value 35 7 26 11 1 1 11 0 Drainage velocities and water volumes used for filter cleaning. Mean drainage velocities in CGF were as follows: 9.1 m 3 m -2 h -1 for UGFS, 24.5 m 3 m -2 h -1 for HGFS and 16.3 m 3 m -2 h -1 for HGF, with drain valves located in the range 0.9 m to 1.4 m below the bottom of CGF units. The high solids load at the end of filtration run in UGFS seems to be hindering high drainage velocities. Table 3.33 presents mean water volumes required for filter cleaning expressed per square meter of filter surface area. The monthly total for three short weekly hydraulic cleanings and one extensive cleaning is lowest for the UGFS with 2.2 m 3 .m -2 , whereas the HGFS requires 3.4 m 3 .m -2 and the HGF 2.7 m 3 .m -2 . The higher value for the HGFS in series may be the result of an inconvenience in the design, which made use of the previous MHGF structure, resulting in some influence on the drainage volume of the sections in between the units. 127

Table 3.33. Water volumes required for optimised partial CGF cleaning activities. Type of cleaning Water volume (m 3 m -2 ) UGFS MHGF HGF Weekly 0.3 0.6 0.6 Monthly 1.3 1.6 0.9 Total volume per month ( * ) 2.2 3.4 2.7 (*) Total of three weekly and one month cleaning 3.3 General Discussion Points of discussion about CGF Identified in the literature review (Chapter 2) are reconsidered in this section, based on the results presented. This general discussion is focussed on identifying criteria for selecting MSF main treatment stages based on the experience of processing Cauca River water at pilot scale. Discussion of water treatment concepts and performance objectives in MSF plants will be made in chapter six after reviewing experiences at full scale in chapter four. 3.3.1 Dynamic gravel filtration (DyGF) DyGF was initially proposed by the author and further developed during this study. The potential of DyGF to protect subsequent treatment stages from high solids loads, and contribute in improving the overall water quality in MSF plants, was verified in this pilot scale study. The “protection capacity” of DyGF, as proposed in this thesis, originates from the combined effect of removal of contaminants by the filter media and reduction of flow during filtration runs. Initial filtration rates in the range 1 to 3.7 mh -1 were found to be appropriate from both protection capacity and O&M requirement viewpoints. Lower filtration rates, around 1 to 2 mh -1 , will produce higher mean removal efficiencies and longer partial cleaning cycles (PCC) but, shorter total cleaning cycles (TCC), slower reaction to SS peaks, lower water productivity, and higher capital investments than higher filtration rates. These lower velocities seem to be the option only in those cases in which overall water quality improvement is the priority. However in the Andean Region context, DyGF being the first stage in MSF treatment plants, the author recommends higher filtration rates, around 2 to 3 mh -1 . This gives priority to longer TCC, quicker reaction to SS peaks, and lower capital investment. These higher filtration rates require frequent partial cleaning activities (PCA), at least twice a week when processing Cauca River Water. Besides, filtration rates in the range 2 to 3 mh -1 still leaves some room for higher velocities (up to around 4 mh -1 ) during PCA or TCA in one of several DyGF units running in parallel. Maximum filtration rates. DyGF units with extremely high filtration rates (≥ 4.8 mh -1 in this study) will have low removal efficiencies but more importantly, they will require frequent TCA due to the high proportion (≥ 50% in this study) of sludge penetrating into the lower gravel layers. Therefore, in the absence of specific studies with other water sources at local level, the author recommends maximum filtration rate ≤ 4 mh -1 . TCA are labour demanding and may nullify the sustainability of the treatment system. Furthermore, the low investment cost in the DyGF stage, between 7 and 10% of the cost of a MSF plant (Galvis et al, 1998), does not seem to justify the application of higher filtration rates. 128

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