In order to address the United Nations Millennium Development Goal (MDG) target #7 for water and sanitation, the World Health Organization (WHO) has identified point-of-use (POU) water treatment technologies as an option for providing safe water to households. The BioSand filter (BSF) is a commonly used POU system that has been implemented in Cambodia and over 20 countries worldwide. While the health benefits of using a BSF in terms of reduction of diarrheal disease have been fairly well documented, little research has focused on the ability of this technology to treat for other contaminants that could pose health concerns. To address these concerns, a study was developed to evaluate this technology in rural Cambodia in terms of microbiological and chemical quality of the treated water. The study revealed that simultaneous nitrification and denitrification is occurring inside the BioSand filters. Nitrite concentrations in treated water consistently exceeded WHO guidelines. Seventeen of 20 filters on average did not meet the 3.0 mg l−1 NO2 guideline and the combined nitrate-nitrite guideline ratio of 1. Denitrification seemed to predominate when BSFs were fed surface water. In addition, nitrate-ammonification occurred in some filters fed surface water, causing increases in ammonia in treated water.