Bio-stabilization of drinking water in supply systems as observed in the tropics

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=1.9×108 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.