Nutrient Removal and Phosphate Recovery Using Agricultural Residue Media in Denitrifying Bioreactors

Agricultural residues have been proposed as alternative organic carbon sources to woodchips for nitrate removal in denitrifying bioreactors. However, the phosphate removal potentials of agricultural residues have not been carefully evaluated. The objectives of this study were to determine the long-term nitrate and phosphate removal capacities of agricultural residue media in bioreactors and to evaluate the phosphate desorption potentials. Laboratory denitrifying bioreactors using corn cobs, corn stalks, barley straw, and woodchips were operated for 390 days. The reactor average nitrate load reduction rates were 57.1, 21.9, 19.4, and 10.1 g N/m³/day, and the average phosphate load reduction rates were 0.35, 0.16, 0.14, and 0.05 g P/m³/day for corn cobs, corn stalks, barley straw, and woodchips, respectively. Barley straw quickly lost nitrate and phosphate removal capacities over time. Corn cobs consistently removed the highest amounts of nitrate and phosphate and showed the least decline in removal performance throughout the experiment. The phosphate desorption potentials after the bioreactor experiment were in the order of corn stalks > corn cobs > barley straw > woodchips. The results of this study suggest that corn cobs can be used as bioreactor filling materials to effectively remove nitrate and recover phosphate from contaminated waters.