Sustainable drainage systems are more and more implemented in the urban space, though their operational efficiencies are not always well known. This article presents a focus on the fate of particulate pollutants in a horizontal sand filter for decentralised runoff treatment in a Paris suburb. The filter, equipped up- and downstream for hydrologic monitoring and for pollutant monitoring, shows a satisfactory efficiency for most of the monitored parameters. On one hand it reduces and delays the hydraulic extremes and on the other hand it shows concentration reductions in the order of 60% for TSS and associated pollutants. Statistic treatments of the data showed the accumulation of more conservative elements in the filter like lead and HAP. The nature TSS changes only partially during the filter trajectory and receives slight sand footprint. An evaluation of the stock of particles and micropollutants trapped in the filter since the filters were put into operation, i.e. 10 years ago, shows a very limited intercepted mass. Based on the latter, the average concentration reduction induced by the filter would only be about 6 mg/l for TSS and 1.5 µg/l for zinc. The mass balance shows that the filter depollutes more by sedimentation in the upstream collector, than by trapping elements in its filter medium.