Improvement of slow sand filtration of warm water by using coarse sand.

摘要

Slow sand filtration is a relatively inexpensive and low-maintenance method for producing potable water. It is therefore particularly well-suited for use in developing countries, where millions of children die each year from diarrheal diseases caused by unsafe water supplies. Many slow sand filters have been constructed in developing countries, with warm or tropical climates, according to specifications derived in temperate climates. As a result, slow sand filters in developing countries have operated only for short periods of time (5 to 13 days) before requiring cleaning. The purpose of this research was to increase operation time and improve performance of slow sand filters under tropical conditions.;Pilot-scale slow sand filters were operated in the laboratory, and were supplied warm water (25C) containing concentrations of bacteria and organic carbon representative of a highly-contaminated river which might serve as a tropical community's water supply. Investigations of the filter run time, hydraulic conductivity, and removal of bacteria and organic carbon were used to evaluate the performance of filters using three very different sands as filter media.;Sand having the size prescribed an all design manuals published to date was found to be unsatisfactory for treating warm water contaminated with bacteria and organic carbon. This sand resulted in filter run times of only five days before surfaces of the filter beds had to be scraped due to head loss. Unprocessed sand from a river bed was also inadequate for similar reasons. Uniform, coarse sand having an effective size nearly five times that of conventionally-prescribed sand greatly improved filter removals of bacteria and organic carbon, and yielded filter operation times of up to 45 days before cleaning was required.;From this study, it was concluded that slow sand filters built in warm or tropical climates should have uniform, coarse sand (effective size = 0.92 mm) as a filter medium, rather than the small, uniform sand recommended to date, in order to improve performance and decrease maintenance requirements.