Accumulation of particulate matter and microorganisms present in wastewater as biofilm on the surface of filters can lead to clogging of the media. If clogging of filters occurs, they need to be temporarily decommissioned before they can be operated again. The mechanisms causing clogging may only be delineated through destructive sampling of a filter. The aim of this study was to characterise the build-up of biofilm in the upper layer below the surface of polishing filters intermittently loaded with effluent from a novel horizontal flow biofilm reactor used for the treatment of domestic-strength wastewater. Three filter media were used: crushed glass, sand, and a shallow podzolized soil. The parameters used to measure biofilm build-up were: soil water retention, total phosphorus (Tot-P) content and loss on ignition (LOI). The LOI and Tot-P deposition near the filter surface were lowest in the glass filters. Soil water retention curves indicated that biofilm formation mainly occurred in the uppermost 0.03 m depth below the filter surface and gradually decreased with depth. This indicates that measurements of volumetric water content using time domain reflectrometry probes may be used as an in situ proxy for measurements that would normally require the destructive sampling of a filter. (C) 2010 Elsevier B.V. All rights reserved.