Effects of influent nitrogen loads on nitrogen and COD removal in horizontal subsurface flow constructed wetlands during different growth periods of Phragmites australis

Highlights: • Excessive amounts of organic matter limited the nitrification of ammonium nitrogen (NH₄-N) and hindered the NH₄-N removal. • A dissimilatory nitrate reduction to ammonium may have occurred in HSSF-CWs under low influent nitrogen loads. • An increase of nitrogen loads resulted in an enhancement of NH₄-N, nitrate nitrogen and total nitrogen removal during the same growth period. • Nitrogen removal efficiencies were higher during the mature period than during the rapid growth period. Horizontal subsurface constructed wetlands (HSSF-CWs) planted with Phragmites australis were established to examine the effect of influent nitrogen loads on the removal efficiencies of nitrogen and chemical oxygen demand (COD) during different plant growth periods of plants. Under low influent nitrogen loads, most of the dissolved oxygen was consumed during the oxidation of organic matter in the wetland systems, and a dissimilatory nitrate reduction to ammonium (DNRA) may have occurred in HSSF-CWs when excessive amounts of organic matter were present, which limited the nitrification of ammonium nitrogen (NH₄-N) and hindered the NH₄-N removal. An increase in the influent nitrogen loads resulted in an enhancement of the removal efficiencies of NH₄-N, nitrate nitrogen (NO₃-N) and total nitrogen (TN) during the same growth period, except for NO₃-N under the highest influent nitrogen loads, whereas fluctuations occurred for the COD removal efficiency. Compared with the rapid growth period, the removal efficiency of NH₄-N, NO₃-N and TN increased during the mature period; however, the COD removal efficiency decreased. The change of COD: N (COD:TN in wastewater) ratios with retention times indicated the sufficiency or deficiency of organic matter as an electron donor in the wetland systems. The changes in the pH value and oxidation-reduction potential (ORP) indirectly demonstrated that many factors affected the effluent pH value and ORP, such as retention time, influent loads, plants and wetland substrate, and microorganisms. In this study, the changes of ORP also illustrated that the dissolved oxygen concentrations decreased with increasing retention time in the HSSF-CWs; however, no significant increase in the ORP was observed during the two growth periods.