Nitrogen removal in an aerobic gravel filtration-sedimentation pond-constructed wetland-overland flow system treating polluted stream waters: Effects of operation parameters.

Nitrogen pollution in streams can be controlled by serially constructing natural wastewater treatment (NWT) systems inside streams. Therefore, a pilot-scale hybrid NWT system consisting of gravel filtration (GF), sedimentation pond (SP), gravel filtration-free water surface constructed wetland (GF-FWSCW), and gravel filtration-overland flow (GF-OF) was installed in order to prevent contamination in a stream (Nigde stream) that feeds and pollutes surface water source (Akkaya lake). The pilot-scale system was installed on a new 60 m-long channel which was located off the stream. Changes in ammonium (NH-N), organic nitrogen (N), total nitrogen (TN), and biochemical oxygen demand (BOD) concentrations was routinely monitored in influent and effluent of all stages of the hybrid NWT system and the bottom sediment of the SP over a one-year operation period. According to the study results, while NH-N plus N was reduced by an average of 75% (from 49,1 mg L to 7,1 mg L), TN was reduced by an average of 85% (from 50,2 mg L to 12,4 mg L). Colder seasons and higher hydraulic loading rates (HLRs) negatively affected nitrogen removal efficiency of the pilot-system. The use of vegetation and filter medium had a positive effect on the average removal efficiencies. The results showed that nitrogen pollution in polluted streams could be greatly reduced by establishing a NWT system in series within them.