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RESUMEN

The fluoride content of Tanzanian and Kenyan magadi has been estimated to be in the range 0.1-17.9 mg F(-) g(-1), which is comparable with that reported elsewhere, but indicating a considerable variation in levels. The median fluoride content of crystalline magadi harvested from the alkaline lakes was 2.1 mg g(-1), which was higher than the median of 1.4 mg g(-1) for scooped magadi harvested from the surface soil. The highest median fluoride contents of 3.2 and 2.9 mg g(-1) were found in magadi originating from Lake Magadi, Kenya, and Lake Natron, Tanzania, respectively. It was found that the fluoride content varied significantly even for magadi originating from the individual lake, e.g. the fluoride content in magadi from Lake Magadi was between 0.1 and 8.7 mg g(-1). In a lump of magadi originating from Lake Magadi, it was found that the fluoride content in 20 smaller part samples was subject to considerable variation indicating that the fluoride-bearing minerals were unevenly distributed in the lump. Results show that the fluoride is mainly present in grains <1.0 mm that made up 25% of the magadi sample. When daily eating the popular meal makande as in Tanzania, the exposure to fluoride through magadi in 70% of cases was estimated to be <4 mg per adult day(-1), as recommended by the WHO. Thus, the health hazard from magadi-fluoride is estimated to be significant in cases where the magadi is heavily contaminated.

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RESUMEN

Changes in the bacterial quality of drinking water were examined in the supply system of tropical Dar es Salaam, Tanzania. The water treatment fulfilled WHO standard guidelines. However, the water quality allowed for significant regrowth of bacteria, demonstrating that the WHO guidelines are insufficient to check whether or not the drinking water has an acceptable regrowth potential. An experimental pipe system was examined on site. A growth model was established, based on a zero order bacterial detachment kinetics for the biofilms colonizing the inner surface of the pipes, with a release rate factor R, and a first order growth kinetics in the water flowing in the pipes with a growth rate factor µ. The rate factors were estimated to be R=1.9×10(8) m(-2) h(-1) and µ=2.5 h(-1). It was concluded that growth in the biofilm played a dominant role in the bio-stabilization processes, while the initial microbial quality of the water works water and the growth in the water phase were of minor importance. The bio-stabilization process is understood herein as the reduction in the concentration of nutrients and assimilable organic carbon in the water, and thus as the reduction of the growth potential of the water. Bacterial growth and recontamination were examined during storage at the user site. The results showed that heterotrophic bacteria grew, while faecal coliforms decayed during storage. It was concluded that the users handled the water hygienically.

RESUMEN

The decay of free chlorine (Cl2) and combined chlorine (mostly monochloramine: NH2Cl) and the inactivation of bacteria was examined in Dar es Salaam, Tanzania. Batch experiments, pilot-scale pipe experiments and full-scale pipe experiments were carried out to establish the kinetics for both decay and inactivation, and to compare the two disinfectants for use under tropical conditions. The decay of both disinfectants closely followed first order kinetics, with respect to the concentration of both disinfectant and disinfectant-consuming substances. Bacterial densities exhibited a kinetic pattern consisting of first order inactivation with respect to the density of the bacteria and the concentration of the disinfectant, and first order growth with respect to the bacterial density. The disinfection kinetic model takes the decaying concentration of the disinfectant into account. The decay rate constant for free chlorine was 114 lg(-1)h(-1), while the decay rate constant for combined chlorine was 1.84 lg(-1)h(-1) (1.6% of the decay rate for free chlorine). The average concentration of disinfectant consuming substances in the water phase was 2.6 mg Cl2/l for free chlorine and 5.6 mg NH2Cl/l for combined chlorine. The decay rate constant and the concentration of disinfectant consuming substances when water was pumped through pipes, depended on whether or not chlorination was continuous. Combined chlorine especially could clean the pipes of disinfectant consuming substances. The inactivation rate constant λ, was estimated at 3.06×10(4) lg(-1)h(-1). Based on the inactivation rate constant, and a growth rate constant determined in a previous study, the critical concentration of free chlorine was found to be 0.08 mg Cl2/l. The critical concentration is a value below which growth rates dominate over inactivation.