9 months ago


Water treatment

The selection of MSF

The selection of MSF alternatives is becoming more flexible, with several possible combinations of gravel filtration options with SSF units. This study presents a methodology for selecting between these options, based on pollution levels in the water source, expected efficiencies at each filtration stage, and fulfillment of established water treatment objectives. • Raw water sources, water quality guidelines and MSF technology Raw water sources included in this study showed a wide range of contamination levels. The identified tendency is to have higher levels of contamination at or close to the flat area of the Cauca Valley. However, even the less polluted sources monitored during this research require filtration before terminal disinfection to fulfil drinking water quality guidelines. This seems to be the prevalent situation in the lower areas of the Andean region, although systematic information about water quality in most surface water sources, particularly those used by small water supply (WS) systems, is not available. The microbial and physicochemical improvements brought about by MSF during this study were considerably better than thought possible on the basis of the literature review. These improvements could be the results of several factors including the impact of DyGF in the subsequent treatment stages, good environmental conditions (such as temperature and nutrients), long term monitoring after an initial maturation period, and frequent careful maintenance procedures. However, MSF is not a panacea. For example, even the most robust MSF pilot system tested at Puerto Mallarino showed limitations to treat mean and 95 percentile values above 70 and 200 NTU respectively. Besides, It was only during test period III that all MSF alternatives at Puerto Mallarino produced water with levels ≤ 15 PCU, processing raw water with mean and maximum colour values of 35 and 72 PCU respectively. The practical application of water surveillance activities, initially introduced in the Andean Region during the 1980s, is highly needed to produce more systematic information about the risk associated with water sources used for human consumption, which could improve selection and design criteria of remedial actions, included MSF plants. • Removal efficiencies in MSF plants and water treatment concepts In harmony with the multistage water treatment concept, the potential of combining one or two main gravel filtration stages with SSF has been thoroughly established. In this study, MSF pilot alternatives, processing a heavily polluted surface water source, showed mean faecal coliform, turbidity and colour cumulative removal efficiencies in the ranges of 4 to 5.6 log units, 95 to 98%, and 80 to 94% respectively. Full scale plants processing surface water sources with different levels of pollution showed mean faecal coliforms, turbidity and colour cumulative removal efficiencies in the ranges of 2.6 to 4.7 log units, 79 to 96%, and 48 to 86% respectively. These removal efficiencies, together with those of other parameters, such as chemical oxygen demand, iron and manganese, indicate that after this study MSF technology can be successfully applied to a broader variety of water qualities, as compared to those thought possible before this study. 211

At least 98% of the effluent samples showed levels inside available guidelines for small WS systems before terminal disinfection. Therefore, MSF technology seems to be a technically competent alternative for contributing to enhance the application of low dose terminal disinfection in the Andean region. All MSF alternatives included in this study showed a tendency to adapt their removal efficiencies to influent contaminant level and to share the contaminant removals between the different treatment stages. The practical application of this conclusion, in harmony with the integrated water treatment concept, requires an understanding of strengths and weaknesses of each treatment stage. The research results show that the high capacity of DyGF units to reduce solid levels, with low initial capital and running costs is the great strength of this filtration stage. This stage is protecting subsequent treatment stages, which are playing a major role in reducing microbial and chemical contaminants but are more demanding in capital investments and running costs. Two different MSF alternatives having the same cumulative removal efficiencies may have very different running costs and vulnerabilities, depending on which stage its playing the main role in reducing the risks associated with a particular influent contaminant. Evidence to support this conclusion was provided, for example by data from Puerto Mallarino, treatment lines 1 (Including UGFS, upflow gravel filtration in series) and 2 (including UGFL, upflow gravel filtration in layers). Both had similar turbidity removal efficiencies, but the UGFS, being more robust than UGFL, protected better its SSF 1 . SSF 1 had longer filtration runs and better quality effluents. Therefore, both MSF alternatives (DyGF + UGFL + SSF 2 ) and DyGF + UGFS + SSF 1 ) fulfilled the multistage water treatment concept, but treatment line 2 (DyGF + UGFL + SSF 2 ) did not properly follow the integrated water treatment concept. • Dynamic gravel filtration (DyGF) as first filtration stage The potential of DyGF to protect subsequent treatment stages from solid loads and contribute in improving the overall water quality in MSF plants was clearly established during this study with both pilot and full scale MSF plants. 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 due to gradual or abrupt clogging 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 views in the DyGF stage. Lower velocities (1 to 2 mh -1 ) seem to be the option only in those cases in which overall water quality improvement is the priority. 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 total cleaning activities due to high proportion (≥ 50% in this study) of sludge penetrating into the lower gravel layers. Therefore, in the absence of other studies about this topic, a maximum filtration rate ≤ 4 mh -1 is recommended. Research results showed that cross surface velocities up to 0.2 ms -1 did not play any role in improving SS removal efficiencies neither contributing to reduce cleaning frequencies by 212