Agricultural fields have been frequently identified as major contributors of nitrate leaching into surface and ground waters. Tile drains can act as direct pathways, transferring leached nitrate to surface water. Bioreactor filters are useful for the removal of nitrate from drainage water; however, these filters require an external carbon supply to sustain denitrification. In this study, four organic carbon sources including wood, barley straw, rice husks, and date palm leaves were used to enhance denitrification and the effects of water velocity and influent nitrate concentration on the nitrate removal were evaluated. Cumulative nitrate removal was highest for the date palm leaf treatment and was lowest for the wood treatment. The effects were in decreasing order for date palm leaves, barley straw, rice husks, and wood, respectively. Performance of the biofilters was improved with increasing influent nitrate concentration and decreasing water velocity, allowing for high nitrate removal rates to be achieved. The results showed that all the treatments reduced the effluent nitrate concentration below the USEPA maximum contaminant level for drinking water (45 mg L -1 ) at the end of the study. With increasing the water velocity, available C and denitrifying enzyme activity decreased. Decreasing the available C and denitrifying enzyme activity with increasing of the water velocity demonstrated that water velocity plays an important role in the denitrification performance of the system. Key Words: Denitrification, Nitrate removal, Carbon substrate, Drainage water, Denitrification rate.
