Drinking water treatment by multistage slow sand filtrations
The ability to remove turbidity and coliforms, using a multistage filtration system, was studied in the laboratory. The aim of the study was to show that this technology constitutes an effective solution for water treatment in rural regions and small communities in developing countries. In this study, a system was assembled and tested. The system comprised two slow sand filters in series, which were preceded by two coarse upflow filters. The effectiveness of this technology was evaluated by measuring the turbidity and the number of total and thermotolerant coliforms. The results showed excellent removals of turbidity despite the high level of turbidity in the raw water (from 20 nephelometric turbidity units (NTU) to more than 100 NTU). The turbidity of the effluent from the final slow filtration system was well below the World Health Organization (WHO) standard for drinking water. There was a good removal of the total and thermotolerant coliforms, which was a function of the biological maturity of the filters used. Each individual stage of the multistage system was an important treatment barrier in the overall process of turbidity and coliform removal. The coarse filter was not only important for protecting the slow sand filter from solids loading and thus increasing its run length, but was also a significant contributor to coliform removal. The second slow sand filter in series provided additional capacity to the process by reducing the effluent turbidity to close to 1 NTU and it also provided additional removal of coliforms under challenging operations.