Abstract. Bio-sand filters (BSFs) are point-of-use (POU) potable water filtration systems commonly used in low-income communities at household level. The principle of operation is similar to that of a slow sand filter and the major difference is that they are operated intermittently at the POU. It is one of the emerging low-cost technologies which makes use of readily and locally available construction materials but is poor in the removal of nitrates. In order to enhance the removal of nitrates through denitrification, a modified BSF with ethanol as an external carbon source at C  /  N ratios of 1.1 and 1.8 was investigated. In the absence of an external carbon source, the nitrate removal efficiency was 32 %, whilst removal efficiencies at C   /   N ratios of 1.1 and 1.8 were 44 and 53 % respectively. The inflow rate reduced significantly from an initial flow rate of 0.04 to 0.01 m3 h−1. The reduction in the inflow rate was mainly due to the growth of the biological layer on the filter media. The study showed that the use of an external carbon source like ethanol in bio-sand filtration enhances the removal of nitrates in potable water.

<p><strong>Abstract.</strong> Bio-sand filters (BSF) are point of use (POU) potable water filtration systems commonly used in low-income communities at household level. The principle of operation is similar to that of a slow sand filter and the major difference is that they are operated intermittently at the point of use. It is one of the emerging low cost technologies which makes use of readily and locally available construction materials but is poor in the removal of nitrates. In order to enhance the removal of nitrates through de-nitrification, a modified bio-sand filter with ethanol as an external carbon source at C / N ratios of 1.1 and 1.8 was investigated. In the absence of an external carbon source, the nitrate removal efficiency was 32 % whilst removal efficiencies at C / N ratios of 1.1 and 1.8 were 44 % and 53 % respectively. The inflow rate reduced significantly from an initial flow rate of 0.04m<sup>3</sup>/hr to 0.01m<sup>3</sup>/hr. The reduction in the inflow rate was mainly due to the growth of the biological layer on the filter media. The study showed that the use of an external carbon source like ethanol in biosand filtration enhances the removal of nitrates in potable water.</p>