Recycling of treated effluents enhances removal of total nitrogen in vertical flow constructed wetlands

It is widely documented that vertical flow constructed wetlands are efficient for producing well nitrified effluent. However, because the treated effluent is oxygen saturated and has a low availability of carbon, the removal of total nitrogen is limited. We hypothesize that by recycling the fully nitrified effluent to the pretreatment unit and the sedimentation tank (where conditions are favorable for denitrification) the removal of total nitrogen from the wastewater can be enhanced. In order to assess the effects of recycling volumes, we monitored the nitrogen removal in an experimental vertical flow constructed wetland, consisting of a 2 m 3 sedimentation tank, two vertical flow beds of 10 and 5 m 2, and the necessary pumping equipment and controllers to manage the loading and recycling volumes. The system was loaded with approximately 0.5 m(3) d(-1), and the recycling volumes evaluated were equal to the loading rate (100% recycling), as well as twice (200%) and three times (300%) the loading. The system produced a fully nitrified effluent independent of recycling rate. However, recycling increased removal of total-N in the system from ca. 1% removal when there was no recycling to 52%, 66%, and 68% removal with 100, 200, and 300% recycling, respectively. The optimal recycling rate seems to be 100-200%. At lower recycling rates, the removal of total-N was poor, and at higher recycling rates, the system became hydraulically overloaded, compromising both the denitrification capacity of the sedimentation tank and the nitrification capacity of the vertical bed.