Woodchip Biofilters for Treatment of Particulate Phosphorus in Agricultural Runoff

Woodchip filters have received attention in recent years for their ability to sustain denitrification activity across multiyear time frames. However, in most freshwater aquatic ecosystems, phosphorus (P) rather than nitrogen (N) is the nutrient considered most responsible for eutrophication. Previous studies have indicated that woodchip filters have limited ability to remove dissolved P, but P export in agricultural runoff is often dominated by particulate P (PP). Woodchip media, because of its high porosity, permeability, surface roughness and plate-like structure of the particles, could be effective for physical filtration of particulate phosphorus. In this study, woodchip filter systems were tested for treatment of PP in agriculturally impacted surface waters at five sites in southern Ontario. A woodchip filter system installed near Bradford, ON was used to treat highly turbid root vegetable wash water from a local farm and focused on the treatment of total suspended solids (TSS) and PP. The full-scale treatment system consisted of a sedimentation tank (12.3 m3) followed by the woodchip filter (16.1 m3) and had two stages of testing. In the initial stage, the filter media consisted of woodchips with a layer of sawdust, and in the second stage, the media contained woodchips only. The full-sale treatment system was sampled from November 2014 to March 2016 and proved effective for TSS and PP removal during both treatment stages, averaging overall removal of 99% and 91%, respectively, in the first stage, and 96% and 77%, respectively, in the second stage. During the operation of the full-scale treatment system, the sludge within the sedimentation tank was regularly monitored and was removed on two occasions. Also during this time, sludge accumulation within the top layer of woodchips required replacement of the top layer on one occasion, September 2015. A woodchip filter was installed near Barrie, ON to treat particulate P in an agricultural drainage ditch adjacent to fields where row crops are grown. In this case the filter consisted of 20 m3 of woodchips trenched in to the bottom of the stream (stream-bed filter). Stream flow was induced through the filter by placement of a gravel riffle at its downstream end. This filter was monitored from December 2014 to March 2016 and proved effective for P removal in the stream water, which had low to moderate turbidity, averaging total P removal of 58%, the majority of which was PP. Nitrate removal in the filter was