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The purpose of this research was to investigate the phosphorus binding capacity of active filtration through hydrated Ca-rich oil shale ash sediment in a laboratory-scale experiment at different retention times. Three column experiments were established with hydraulic loading rates of 12 L d(-1), 18 L d(-1), and 24 L d(-1), with residence times of 12, 8, and 6 h, respectively. The results showed good removal efficiency (up to 91% at loading of 1.66 gP m(-2) d(-1)) in the experiment with the longest retention time. The overall performance of the filters did not, however, improve with increasing retention time, and the decrease in binding capacity was 41% at longer retention times. Chemical clogging by carbonate precipitates probably reduces the availability of Ca from the dissolution of unstable ash sediment mineral phases, and the phosphate removal decreases rapidly from 91% to 49% at loading of 1.66 gP m(-2) d(-1).