10 months ago


Water treatment

Surface area of CGF and

Surface area of CGF and SSF units. All MSF treatment stages require frequent cleaning procedures, including surface raking (DyGF), shovelling (CGF) or scraping (SSF). Consequently, proper maintenance activities are of crucial importance to obtain the potential benefits of MSF technology in a sustainable way. In designing this technology it is very important to ensure that the operator can do a good job and this implies that treatment units should be rather small in size. This may somewhat increase the initial construction cost but gives major returns in facilitating operation and maintenance tasks. 3.3.3 Multistage filtration after the pilot system study at Puerto Mallarino The research results clearly show the potential of combining two-stages of gravel filtration (dynamic gravel filtration and different options of coarse gravel filtration) with SSF to treat water from a grossly polluted river. This is very important as MSF brings the possibility of providing safe drinking water supply closer to many people relying on similar or less contaminated surface water sources in the Andean Region, and possibly in other regions of the world. The microbial and physicochemical improvements brought about by MSF without the addition of chemical coagulants were considerably better than thought possible on the basis of the literature review. These improvements could be the result 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. All coarse gravel filtration methods (upflow, downflow and horizontal flow) tested in the second filtration stage performed very well. It was clearly shown that they not only have a great capacity to remove SS and turbidity, which has been the main traditional reason for their development, but equally, or even more importantly is their potential to reduce microbial contamination, natural organic matter, iron and manganese. Having overcome in great part the combination of high and fluctuating levels of microbial, physical, and chemical contamination, a low dose of chemical disinfectant seems to be possible wherever it could be accepted and sustained at local level. Based on the results at Puerto Mallarino, and considering that good engineering practice would indicate selecting the alternative with the lowest life-cycle cost, a preliminary selection can be made between CGF alternatives included as the second treatment stage in the MSF pilot system. Taking into account depth/length of the gravel bed, mean removal efficiencies, partial cleaning requirements, and total cleaning cycle lengths, the prime choice is the UGFL for the less, and UGFS for the more polluted water sources. The “ conventional” HGF however, having a larger sludge storage capacity and similar removal efficiencies, may be an alternative for surface water high in suspended solids, even though it could be more expensive. HGFS tested during this research work might become a better option than HGF, but it requires further development and evaluation. Performance of UGFS can possibly be even further improved and/or cost can be reduced, as the headloss development in its second unit is considerably less than in the other two. This suggests that the second unit is less important for SS removal. It may however be important for the improvement of other water quality parameters, as considered in chapter 6. 133

4. MULTISTAGE FILTRATION EXPERIENCES WITH FULL SCALE SYSTEMS 4.1 An Overview of the Institutions in the WS&S Sector in Colombia Until the decade of the 1930s the water supply and basic sanitation (WS&S) sector in Colombia was developing at local level, with the participation of private companies, but with little involvement of the central government. By the end of this decade the regulation of the sector became necessary but difficult due to economic constraints and the limited integration of the national territory. In the 1940s the concept of WS&S as a public service was established and the state promoted its development through the department and municipal levels. During the 1950s two different systems coexisted. The system of the main municipalities keeping financial and managerial responsibilities at the local level, and the national system attached to the central government, with intermediate and smaller municipalities played a passive role. INSFOPAL, Instituto Nacional de Fomento Municipal (National Institute for municipal development, including the water sector), was created, with offices at department level to interact with the municipal level. During the 1960s the PSBR, Plan of Saneamiento Básico Rural (national programme for rural WS&S), was created, similar to INSFOPAL, but orientated to small towns (less than 2,500 people) and rural areas. PSBR plan was assigned later to INS, Instituto Nacional de Salud (National Health Institute). Sometimes, INS made agreements with other organisations at departmental level to delegate its programme of work. This was the case with Cauca Valley, in which PSBR was delegated to the Departmental Public Health Secretariat. This Secretariat also made agreements with other organisations, such as the Coffee Growers Organisation (CGO) of the Cauca Valley Department and the Cali Municipal secretariat of Public Health, to develop the PSBR programme. By 1986, catalysed by international trends towards a reduction of the role of the central government and the transfer of responsibilities to the local level, a process was initiated to abolish the role of both INSFOPAL and INS, because their efficiency was considered to be too low. FINDETER, the national financial agency for territorial development was created. The decentralisation process and the reforms in the sector were formalised by law 77 in 1987. This law passed the responsibilities for WS&S to the municipalities for both urban and rural settlements. An office was created in the Ministry of Public Works and a Division of Water in DNP, Departamento Nacional de Planeación (National Planing Agency), to provide support and guidance to the municipalities. Finally, during the 1990s, the WS&S sector was assigned to the Ministry of Economic Development, but the government confirmed the role of the Ministry of Health in water surveillance and control. During the 1980s the WS&S sector started to move from focusing solely on coverage and quantity to include an emphasis on water quality improvement in both existing and new systems (DNP, 1991). During the 1990s the WS&S sector has reflected all the political changes taking place in Colombia under the decentralisation process. The new democratically elected mayors and their city councils have been learning how to deal with their new role in WS&S and the financial resources being transferred from the central government to the local level. The 134