Many people in Cambodia consume groundwater with arsenic concentrations above the WHO guideline. In this study, an iron-mixed porous pellet adsorbent was put into a lightweight bio-sand filter to treat arsenic. The filter was intermittently charged daily with 30 L influent water until the effluent arsenic concentration exceeded 10 μg/L. The results indicated that the Morrill Dispersion Index was less than 2.0, implying that the filter had preferential plug flow. Head loss accumulation led to flow rate reduction over a period of 30 days. Arsenic removal efficiency was between 97 and 99% for the influent concentration, being in the range 355 to 587 μg/L. No significant leaching of iron or organic carbon was observed. The high dissolved oxygen concentration is likely to have contributed to the aerobic conditions in the filter bed. The filter removed arsenic more efficiently than was achieved in some previous studies and might be suitable to provide household-scale, arsenic-safe drinking water.