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Performance Comparison of the Biosand Filter in Laboratory Studies and a Longitudinal Field Study in Bonao, Dominican Republic

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More than one billion people in the developing world lack access to improved sources of drinking water. This leads to billions of cases of diarrhea and 1.8 million deaths annually. The ideal solution to the problem of unsafe water is to provide universal access to inexpensive, safe, reliable piped water. However, in many nations that reality is decades away. Point of use (POU) drinking water treatment allows people without access to safe water sources to improve the quality of their water by treating it in the home, thereby taking control of the safety of their drinking water. One of the most promising emerging POU technologies is the biosand filter (BSF), a household-scale, intermittently operated slow sand filter (SSF).

As in conventional SSFs, biological ripening or maturation occurs in BSFs with time. During ripening, head loss and microbial reductions increase. Studies on BSF performance in the laboratory suggest initial reductions of E. coli as low as 65% that improve over time to 99%. Field studies suggest 93–98% reduction of E. coli by ripened filters. However, BSF performance has yet to be monitored longitudinally in the field for E. coli reductions from initial installation to ripening, as has been done in the laboratory.

The objective of this research was to compare the rate and effects of ripening in plastic and concrete BSFs in the laboratory at the University of North Carolina and contrast those results with the rate and effects of ripening of concrete BSFs in a longitudinal field study in Bonao, Dominican Republic.

Two 6-8 week laboratory experiments were performed: one on a plastic BSF and one on a concrete BSF. In both, filters were dosed daily with 20 L lake water seeded with 1% pasteurized settled sewage and E. coli. BSFs were sampled at weekly intervals for reduction of E. coli, turbidity and changes in flow rate. In February 2006, 81 households received concrete BSFs as part of a randomized controlled trial in Bonao, Dominican Republic. BSFs were monitored at two week intervals for reduction of E. coli and turbidity. Flow rate was measured four times throughout the six month study period.

Laboratory Study

The time-weighted geometric mean E. coli reductions in plastic and concrete BSFs were 97% and 93% respectively. Geometric mean E. coli reductions were significantly higher in the plastic BSF than in the concrete BSF. Turbidity reductions in the BSFs were similar, with both BSFs achieving average turbidities = 1.1 NTU following the ripening period. pH of the BSF treated water from the concrete BSF was higher than in the plastic BSF, with mean values of 8.7 and 7.9 respectively. BSF ripening occurred in both concrete and plastic BSF operation over 6-8 weeks of operation.

Longitudinal field study

The average percent reduction in E. coli by BSF during the six-month study was lower in the field than in the laboratory studies (80% compared to 93-97%). Despite a lower percent reduction, the geometric mean of MPN in the product water was very low (5/100 mL) because the level of E. coli in the feed water was lower in the field study. Another indicator of good product water quality was low turbidity (0.9 NTU). However, the effectiveness of BSF in removal of turbidity cannot be judged because the feed turbidity was also very low (1.8 NTU). There was far less BSF ripening in the field than in laboratory studies. This was inferred by relatively little decline in BSF flow rate (12% compared to 70%) over the six months of BSF operation again suggestive of a generally cleaner feed water. Even in the absence of significant ripening, some increase in E. coli removal occurred in the field study over time (49 to 87%).

In laboratory studiesE. coli reductions were higher and increased more over time in the plastic compared to the concrete BSF. Possible explanations for differences include the higher pH and pore velocity in the concrete BSF. A higher pH could affect the composition of the biolayer and possibly reduce filtration efficiency. Colloid filtration theory predicts that higher pore velocity will reduce physical and chemical filtration efficiency. The concrete BSF in the field performed differently than the either the plastic or concrete BSF in the laboratory. The flow rate reduction measured during the field study was far less than observed in the laboratory suggesting less ripening. E. coli reductions were also far lower in the field but the lower levels of E. coli in the feed water still resulted in product water that, on average, met the WHO Criterion for “low risk” microbial water quality. The failure of laboratory studies on the BSF to predict BSF performance in this field study is likely the result of differences in feed water characteristics (a generally cleaner water in the DR field study), filtration frequencies and dosing volumes (both highly variable in the field).

This is the first longitudinal field study of BSF performance upon initial field installation. On a percent reduction basis, we document less effective performance in the field by concrete BSFs compared to plastic and concrete BSFs in the laboratory. However, percent reduction is not necessarily the best performance indicator to use for bacterial removal processes in the BSF. That is, without a fundamental mechanism to explain removal, there is no reason to expect the same percent removal when feed concentrations of bacteria differ. In fact, the product water concentration of bacteria may remain relatively constant such that percent removal varies more or less directly with the feed concentration. The observed differences in performance between plastic and concrete BSF is also worthy of more study given that the plastic design is being introduced in the DR and other countries.
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Keywords: E. coli; biosand filter; point of use; slow sand filtration

Document Type: Research Article

Publication date: 01 January 2009

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