A study on the effects of effective size of sand media in a rapid filtration process with respect to turbidity, filter run and flow rate was carried out at Sungai Langat Water Treatment Works. The findings of the study show no significant difference in the final turbidity achieved with effective sizes (E) range between 0.4 and 0.9 mm. However, filter run increases with higher effective sizes. Filter run is also affected by a change in flow rate a 50% increase in flow rate results in the reduction of filter run by two hours for the effective sizes studied