FAILURES AND PROBLEMS OF URBAN DISINFECTION. C.A. Lazcano SEDAPAL Lima, Peru ABSTRACT

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1 Original: Spanish FAILURES AND PROBLEMS OF URBAN DISINFECTION C.A. Lazcano SEDAPAL Lima, Peru ABSTRACT This report analyzes the problem of disinfection in an urban context and recommends actions to improve the living conditions of the population. The problems and failures of disinfection begin at the catchment with the quality of the source, which often needs prechlorination to reduce faecal contamination; postchlorination eliminates the bacteria remaining after treatment. Poor handling of chlorinators, maintenance and the personnel involved are additional factors that lead to failures. The problems and failures in water distribution are related to the handling of valves, prolonged storage, and pipes with slime, bulges and biofilm, which house bacteria and free-living organisms that can carry pathogens. Recommendations include: standardization of chlorination processes, continuous postchlorination, continuity in water distribution insofar as this is possible, pipe maintenance and the replacement of pipes found to be in poor condition. 1. Objectives 1.1 To carry out an objective analisis of disinfection problems and failures at an urban level. 1.2 To recommend actions aimed at improving the living conditions of the population with regard to waterborne diseases. 2. Introduction One of the major problems in developing countries is the high rate of gastrointestinal diseases, compounded by the poor living conditions of the population, deficient in hygiene and sanitation, with scanty economic resources and low educational standards. The problem is more significant in fringe urban areas because of the

2 absence of water pipes, the consumption of contaminated food and the presence of mechanical vectors due to deficient environmental sanitation. Water is the principal vehicle for the transmission of gastrointestinal diseases, mainly typhoid and paratyphoid fever, salmonella, dysentery, cholera and parasitosis. The best way to prevent these infections is by water disinfection, and the most economic means is the use of chlorine. Drinking water companies are obliged to disinfect the water they supply in a permanent and controlled manner, to guarantee the bacteriological quality of the water and thus contribute to the health of the population. Emphasis is placed on the fact that all enteropathogenic bacteria, without exception, are sensitive to low doses of chlorine. However, in the case of protozoa cysts such as Giardia or Cryptosporidium, the treatment must include coagulation-flocculation and filtration processes, which remove more than 99% of these parasitic forms. 3. Chlorination problems 3.1 Supply source Water treatment plants are usually designed and established to use water supply sources with little contamination. However, the quality of the source deteriorates as man performs different kinds of activities upstream from the inlet: rural or semirural populations settle there and throw their wastewater into the source; discharge from mines also reaches the source; as well as the waste from agroindustrial activities and livestock farming, etc. These chemical and biological substances are self-purified by biological oxidation. However, when the rate of contamination is greater than the capacity for self-purification, the dissolved oxygen diminishes and there is an increase in the concentration of living organisms that reach the water treatment plant, including pathogens. The treatment plant therefore has to upgrade its processes constantly, leading to an increase in production costs. To diminish the enteropathogenic pollutants, it is necessary to reduce them to the levels permitted under local laws. This is achieved with an effective prechlorination process, followed by a period of minimum retention or settling before conventional treatment. However, the chlorine dose applied at this stage should be determined according to several factors: the presence of organic matter, the chlorine demand, precursors that form chlorination by-products, the ph of the water, the presence of phenols that combine with chlorine to form bad-smelling compounds, etc. Helminth eggs and parasite cysts are eliminated by sedimentation, since they are highly resistant to chlorine. Each country should have laws to protect water sources, especially if the water is to be used for human consumption. Maximum pollution limits should be established to preserve both the ecosystem and the health of the population. 2

3 3.2 Dose of chlorine applied Direct application of chlorine in treatment plants and wells is one of the most commonly used chlorination methods. As a consequence, the minimum dose needed to obtain free chlorine residuals in the network (and thus ensure the bacteriological quality of the water) is often not applied. Marginal chlorination is another common way of introducing chlorine, but this method fails to take into account the presence of organic matter, so it does not guarantee efficient disinfection. To ensure effective disinfection, it is necessary to apply a chlorine dose above the breakpoint with free chlorine residuals, and this is achieved by carrying out chlorine demand tests, bioassays of bacterial response to dosage, or organic nitrogen analyses. These tests are performed in the laboratory and make it possible to optimize the chlorine used, improving the pre- and post-chlorination processes with the use of standard doses. In pre-chlorination, the presence of precursors of trihalomethanes and other by-products of chlorination must also be evaluated, as well as their formation after application. The chlorine application in pre-chlorination with a dose of 2 mg/l above the estimated demand eliminates the total and thermotolerant coliforms (faecal) in an order of magnitude of 3-4 logarithms. In addition it removes most of the algae that are the primary producers in the trophic chain, thus preparing the water for its subsequent treatment with the addition of chemical coagulants. The doses applied in post-chlorination must also be above the breakpoint and must be standardized to ensure a free chlorine residual of no less than 0.5 mg/l at any point of the system, especially if the systems are very old and there is a risk of crosscontamination with sewers. 3.3 Chlorinators The chlorine normally used for the disinfection of water is acquired in the form of liquified gas in cylinders of 907 kg, or 68 kg for small plants or wells. The cylinders are installed with pipe and valve systems, connected to vacuum feed chlorinators and operated by water pumps. The chlorine gas enters the chlorinator through a rotameter, which measures the incoming flow. Flow regulators, that control the flow rate with which the chlorinator works, are generally operated manually. The problem arises when there are changes in the time flow, because the operator does not always increase or decrease the quantity of chlorine to be applied at the moment of the change; at such times the amount of chlorine introduced is either too much or too little, as the case may be. Moreover, when the chlorinators have a maximum capacity far above the plant capacity, with low performances the regulation of the chlorine flow is not accurate. Sometimes chlorine is applied directly, either because there is no electric power or because the chlorination systems have not been maintained. It is very difficult to control the quantity of chlorine being applied in these cases. 3

4 The following measures will make it possible to solve these problems: standardization of chlorination in plants and wells; automation of chlorine application to bring it in line with time flow changes and chlorine demand; and the periodic control of the free chlorine residual at the plant outlet and other strategic points of the systems. 3.4 Personnel The members of personnel responsible for running the chlorination system in treatment plants are key factors in the chlorination results, because of the difficulties involved in ensuring proper control of the dose of chlorine. These difficulties are mainly due to the following situations: Operation of the system is usually manual. Capacity of the chlorinating equipment is far above the required maximum demand, which makes it difficult to effect precise dosing and reading. Application does not correspond properly to the volume and quality of the water to be chlorinated. There are no dosification control records in many cases. Personnel are not properly trained to manage the chlorination system. Automation of the processes should be considered as a solution to the chlorination failures. However, as a short-term measure, chlorination can be made more accurate by training the personnel to handle, control, operate and maintain the equipment, and by raising their awareness of the importance of their work. 3.5 Distribution systems The chlorine residual in the distribution systems should eliminate any bacterial pollutants still present in the water after treatment. Such pollutants can be the result of cross-contamination or of old pipes in a bad state of conservation. Cross-contamination is mainly due to damaged pipes through which contaminants from the soil or from sewerage pipes reach the water. Normally the flow conditions are under considerable positive pressure. However, siphon-type suction can occur in the distribution system if the water pressure drops and there are defective connections or fractures in the pipes. Polluting organisms can thus be absorbed into the water pipe. Problems of this nature occur more frequently when the drinking water pipe is laid too close to the sewage collector. It is very difficult to predict the quantity of chlorine that will be needed to neutralize this type of pollution. When the polluting bacteriological charge is very high, infectious foci can occur in the vicinity of the problem. In cities with very old pipe connections, and even in recently installed piping, the inner walls have layers of bacterial slime, bulges and biofilm. The microscopic 4

5 organisms, which include, among others, bacteria, fungi and actinomycetes, grow freely in the water to form a slime on the pipe walls. These organisms become more resistant to the chlorine residual present in the water; the natural chemistry of the water is altered by the microbial metabolism; and the levels of dissolved oxygen are reduced, producing final products such as nitrates and sulfides. These microbial slimes are the main food supply used by larger organisms of the cyclops type, nematodes, etc., which easily adapt to living on the pipe walls, where they grow and proliferate. These species that thrive inside the distribution systems, are the ones that give rise to most of the user complaints. The bulges that are formed on the pipe walls house a large quantity of aerobic and anaerobic bacteria (10 6 /g /g) including those which produce biocorrosion. Many of the bacteria isolated are opportunist pathogen agents. The presence of these microorganisms in large numbers can affect the health of the population, in particular newborn babies, immunodeficient individuals, convalescents and elderly people. The biofilm is formed by the use of chemical reagents in the water treatment process, especially chlorine. It is mainly formed by stressed bacteria protected by a fine layer of polysaccharides, which as in the case of the layers of slime and bulges formed, also house algae and free-living organisms that may come out from the faucets of the users. This gives rise to complaints to the water-supply company because of the deterioration of the water s organoleptic quality, especially if the water has an unpleasant smell or taste or is turbid. The biofilm also causes problems in the distribution system: (a) it increases the friction resistance of the fluids, producing a great loss of pressure or reduction in the water flow if the pressure is constant; (b) it produces anaerobic conditions with H 2 S which can cause the water to smell and taste unpleasant; and (c) it increases the chlorine resistance of the organisms that form the biofilm and contributes to the regrowth of indicators of faecal pollution and pathogens in the pipes. In addition to the free-living organisms, the slime and biofilm also increase the chlorine demand, since they consume the circulating free chlorine residual. However, the free-living organisms are highly resistant to chlorine (> 15 mg/l) and can carry inside their intestines pathogen bacteria that might be protected from the action of chlorine. There are other problems in the systems that interfere with disinfection or weaken its effect. They can be summarized as follows: A. In the case of direct supply, problems arise when distribution is not continuous; the water quality deteriorates because of dead points in the systems, only a few sectors are affected, turbidity increases and the chlorine residual generally tends to "0"; and the existence of very old pipes, already mentioned. B. When supplying stored water, in addition to the above-mentioned problems, there can be prolonged storage of the water without rechlorination. There could also be failures in the operational control of the storage, for example, if there is no valve-actioning program to see that the quality does not deteriorate: if the 5

6 valves are opened suddenly, the water comes out under pressure forming a turbulent flow that drags the sediment along and makes the water turbid. Problems can also arise in the pumping booths if there are poor conditions of hygiene; and during manipulation by the users, who can contaminate open reservoirs by using dirty and contaminated containers for their supply. Household connections in a bad condition also contribute to a deterioration in the water quality. C. In the distribution of underground water, problems arise when chlorinators are out of order because they have not been properly maintained, or there is a lack of electricity, or the stock of chlorine cylinders has run out. Other problems are due to poor hygiene and sanitation conditions in the pumping booths either because they are badly located (close to an irrigation ditch, infiltrations from irrigation of adjacent land using wastewater, or in places where there are septic tanks nearby). Another factor that can cause problems is the lack of maintenance (existence of sinkholes, cracks or holes that allow water to percolate, with resulting contamination of the subsoil). 3.6 Standardization of chlorination When chlorine is applied without laboratory tests having previously been carried out to determine bacterial response to doses and chlorine demand, non-uniform criteria are used in the dosage. Water disinfection deficiencies are the result, as well as the risk that polluted water might be produced in some periods. To standardize chlorination processes in treatment plants is a goal that all water supply companies should set themselves. Standardization involves previous studies concerning the process, controls at every step of the process and the drawing up of protocols which must be strictly complied with by the operators. Because of the great variability regarding water quality, and to prevent the chlorine demand from rising, chlorination needs to be complemented with a program for the adequate cleaning and maintenance of the treatment units, including reservoirs, catch basins, settlers, filters, treated water reservoirs, etc. This will prevent the accumulation of organic matter, formation of crusts on the walls and the proliferation of living organisms, mainly algae that are the primary producers, initiators of the trophic chain, that interfere with chlorination. 6

7 4. Conclusions and recommendations 4.1 The problem of disinfection at the urban level is common in developing countries. Supply sources generally become polluted with different types of effluents, mainly domestic discharges. They contain pathogens proper to the area or region and bacteria which are indicators of faecal pollution far above the levels recommended by the laws or standards of each country. This contamination originates in the periurban growth upstream from the intake. 4.2 Prechlorination is necessary when bacterial contamination exceeds the limits set by national standards or laws. The application of a 2 mg/l dose of chlorine above the raw water demand determined by chlorine-demand tests with a contact time of no less than 60 minutes, eliminates faecal coliforms in an order of magnitude of 4 log. as well as all the enteropathogen bacteria. A control of the formation of chlorination by-products needs to be carried out. 4.3 The chlorine dose applied in post-chlorination should be calculated on the basis of the chlorine demand tests and it should be standardized in such a way that free residuals of no less than 0.5 mg/l will be obtained at any point in the system. 4.4 Technical and professional personnel must be properly trained to follow all the chlorination protocols, so that the applied doses will comply with laboratory recommendations. 4.5 Chlorine residual controls must be carried out at strategic points at the outlet of the treatment plants, reservoirs and distribution systems. It is important that these controls be effected by technicians specialized in taking appropriate corrective measures. 4.6 Preventive maintenance of the chlorination equipment must be scheduled for plants and wells in order to prevent unforeseen stops that would interrupt the chlorination process. 4.7 To reduce the use of chlorine in pipes with a high demand, cleaning by means of venting and/or superchlorination must be scheduled to eliminate all the slime and free-living organisms inside. Piping that is very old or in a bad condition must be changed. 4.8 To standardize the chlorination process in treatment plants, it is necessary that the protocols be strictly followed, applying the doses recommended by the laboratories on the basis of the tests for chlorine demand and bacterial response to dosage. A program for the proper cleaning and maintenance of the treatment units (catch basins, settlers, filters, reservoirs, etc.) must also be applied to prevent the accumulation of organic matter and the proliferation of 7

8 living organisms. Depending on the possibilities of each utility company, it is also recommended that automatic chlorine dosing systems be installed, with telemetric measures and in conformity with the chlorine demand and the recommended free residual, to be located at the points of application. 4.9 The chlorination process must be considered essential. Chlorination should not be stopped or reduced because of the assumption that by-products are formed from the chlorination. The long-term effects of the trihalomethanes in inducing some forms of cancer caused by water consumption have not been verified. However, waterborne infections are more frequent and have a high rate of morbidity and mortality in our countries. 5. References G. Bitton. Wastewater microbiology. New York. Editorial Wiley-Liss, E.E. Geldreich, M. J. Allen and R. H. Taylor. Interference to coliform bacteria detection in potable water supplies. En: Charles W Hendricks. Evaluation of the microbiology standards for drinking water. E.P.A., Washington D.C. 1988: N.F. Gray. Calidad del agua potable, problemas y soluciones. Saragoza, (España), Editorial Acribia S.A.; E. Van der Wende and W. G. Characklis. Biofilms in Potable water distribution systems. En: Gordon A. Mc Feters. Drinking water microbiology. Progress and recent developments. U.S.A. 1990:

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10 Figure 1 - Causes of problems and failures in urban chlorination 10

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