Accumulation of biomass and deposition of suspended solids at the surface of a sand filter can lead to clogging of the filter media. A laboratory intermittent sand filter column, which included three sand strata, was operated for a period of 806 d before failure occurred through surface clogging. Upon dismantling the column, the cause and effects of the surface clogging were investigated. The main mechanism responsible for sand clogging appeared to be biomass buildup. Maximum loss on ignition of filter media samples was 2.35%, and it occurred in the upper 0.01 m of the sand. There was a reduction in field-saturated hydraulic conductivity in the top 0.01 m of the upper sand stratum from a value of 1.9 x 10(-3) +/- 1.7 x 10(-4) m s(-1) (for virgin sand with an effective size, d(10), of 0.45 mm) to 3.5 x 10(-5) 7.5 x 10(-6) m s(-1). The soil-water characteristic curve, which relates the volumetric water content (theta(v)) to the soil suction, also reflected the changes in the filter media due to clogging. The water-holding capacity greatly increased as biomass accumulated in the filter media. Scanning electron microscopy (SEM) confirmed the existence of a clogging organic layer on the surface of the top sand layer.