Household scale slow sand filtration in the Dominican Republic

Abstract

Slow sand filtration is a method of water treatment that has been used for hundreds of years. In the past two decades, there has been resurgence in interest in slow sand filtration, particularly as a low-cost, household-scale method of water treatment. During January 2004, the author traveled to the northwestern Dominican Republic to evaluate the performance of BioSand filters installed over the past two years. BioSand filter performance was evaluated based on flow rate, turbidity removal and total coliform removal in communities surrounding the cities of Mao, Puerto Plata and Dajabon. Filter owners were interviewed about general filter use, water storage methods, filter maintenance practices, and water use. Data analysis revealed that even though the majority of filters were removing large portions of both total coliform and E. coli contamination, no filters met the WHO water quality guideline of less than one CFU/100 ml. Analysis also revealed that at low turbidities, turbidity removal and total coliform removal are not correlated. Examination of flow rate and bacterial removal near Puerto Plata revealed that filters with fast flow rates and intermittent chlorination were observed to have the lowest total coliform removal rates. Analysis of storage data revealed that failure to use safe water storage containers leads to recontamination of filtered water. During Spring of 2004, a laboratory was conducted to examine longer-term thermotolerant coliform and turbidity removal. The study compared removal rates between two BioSand filters, one of which was paired with a geotextile prefilter used in the construction of the Peruvian Table Filter. The study revealed that thermotolerant coliform removal rates by the BioSand filter without

(cont.) the geotextile stabilized after an initial period of lower bacterial removal efficiency. Thermotolerant coliform removal in the BioSand filter with the geotextile prefilter dropped throughout the experiment, suggesting that pairing a BioSand filter with a prefilter is detrimental to filter performance. Combining the results of the survey analysis and data gathered in the Dominican Republic with the results of the laboratory analysis of Spring 2004 suggests that BioSand filter users in the Dominican Republic should continue to use their filters. If possible, BioSand filter use should be combined with post-filtration chlorination to kill the remaining bacteria. The BioSand filter is a valuable and effective household-scale water treatment method for the Dominican Republic.