Comparative study of nitrogen removal and bio-film clogging for three filter media packing strategies in vertical flow constructed wetlands

This study investigated the discrepancies in nitrogen removal and clogging for vertical flow constructed wetlands (VFCWs) using different packing strategies. Parallel lab-scale VFCWs reactors were created and packed with quartz sand filter media. Three packing strategies were adopted: increasing-sized (I-packing), decreasing-sized (D-packing) and uniform-sized (U-packing) packing. The clogging rate coefficient and biomass accumulation rate were defined to assess clogging. The results demonstrated that the nitrogen removal was highest for the I-packing reactor at 43%. The ammonia and COD removal rates reached 56% and 64%, respectively. I-packing reactor presented an aerobic-to-anoxic transition area, where DO ranged from 4.6 to 0.3 mg/L. Such condition is ideal for the nitrification and denitrification reaction. Clogging is primarily the result of the rapid reduction of the effective porosity due to bio-film growth. Clogging is related to the position of biomass accumulation, while the reduction of the internal void space is not the determining factor for the I-packing reactor. The clogging rate of the I-packing reactor much lower than D-packing and U-packing reactors. Bed resting between operations can be used for clogging mitigation and system recovery, but this approach was not effective for D-packing. The I-packing reactor was proven to be the most efficient for nitrogen removal and showed the strongest clogging prevention capability.