Excess nitrogen in aquatic ecosystems can cause many problems such as eutrophication,,which have negatively affect on biodiversity,climate and human health (Stevens et al. 2019). Therefore, the remove of nitrogen from waste water it is necessary. Traditional activated sludge technology has been used for nitrogen removal in multiple wastewater treatment plants (WWTPs) due to its high efficiency (EPA, 1993). However, these conventional technologies require high construction cost and consume more energy (Liu et al. 2016).
In recent years, constructed wetland (CW) is an ecological technology. Due to its various advantages over traditional wastewater treatment technologies, it has been rapidly developed in wastewater treatment in scattered areas in industrialized counties and low-income countries (Liu et al. 2016 ; Zhang et al. 2019). These include low construction and operating cost, easier maintenance and good integration into the landscape and promotion of biodiversity (Álvarez et al. 2017; Paing et al. 2015). However, the removal of nitrogen in constructed wetlands (CWs) exhibits large fluctuations and is often unsatisfactory ( Wang et al. 2017). The total nitrogen removal in vertical flow or in horizontal flow constructed wetlands is usually not completely removal, but instead converts it to various nitrogen compounds (Pelissari et al. 2017).
In fact, the vertical flow constructed wetlands is an unsaturated systems, which often fed with several pulses intermittently throughout the day, resulting in a high oxygen transmission capacity, that is beneficial to the nitrifying bacteria in the bed (Platzer et al. 1999). However, horizontal flow constructed wetlands are mainly operated under anoxic/anaerobic conditions, which makes it a suitable environment for denitrification process (Vymazal et al. 2007). Generally, the nitrogen removal limitation in constructed wetlands can be explained by the competition for oxygen by autotrophic and heterotrophic microorganisms heterotrophic (Saeed et al. 2012), and the limit of organic carbon available for the process of denitrification (Lavrova et al. 2010). Vertical flow constructed wetlands have more attention than horizontal flow constructed wetlands due to its less demand for land ( Meng et al. 2014). On the other hand, traditional vertical flow constructed wetlands cannot remove total nitrogen (TN) satisfactorily due to the lack of appropriate hypoxic conditions for denitrification (Pelissari et al. 2017)
In order to completely remove nitrogen, various types of enhanced vertical flow constructed wetlands have been studied, such as artificial aeration, tidal flow and integrated vertical flow constructed wetlands (Pelissari et al. 2018; Hu et al. 2016). There are other operational conditions to enhance the removal efficiency of total nitrogen in vertical flow constructed wetlands, such as recirculation of effluent to improve nitrification efficiency ( Wu et al. 2016) and stepwise feed to enhance carbon source supply to promote denitrification ( Li et al. 2017). However, these modifications have increased operating costs and maintenance complexity. The latter strategy uses a partially saturated vertical flow constructed wetland configuration to create anoxic/anaerobic conditions at the bottom of the bed and aerobic conditions at the top of the bed to promote adequate condition for simultaneous nitrification and denitrification (Torrijos et al. 2017; Kim et al. 2014). This model is more efficient in total nitrogen removal than traditional unsaturated vertical flow constructed wetlands with sequential nitrification and denitrification (Silveira et al. 2015; Dong et al. 2007; Kim et al. 2015a).
In general, all efforts aimed at maximizing the nitrogen removal in constructed wetlands are directly related to the activities of enhancing microbial communities, which are responsible for the conversion of various nutrients in the filter media and rhizosphere biomass (Mayo et al. 2005). Many studies have evaluated the dynamics of bacterial communities in constructed wetlands. Foladori et al. (2015) showed that the number of viable bacteria in the surface layer is 3.7 times that of the deep layer. Adrados et al. (2014) characterized the prokaryotic microbial communities of vertical flow constructed wetlands, horizontal constructed wetlands and biological filter sand, and reported higher bacterial activity than archaea in all research systems. Other studies have shown that the diversity of bacterial communities in constructed wetlands may affect the quality of the final effluent (Calheiros et al. 2009). Button et al. (2015) indicated that microbial metabolic functions identified in different constructed wetlands types are related to the design of each system, spatial position within the bed, and especially with levels of pretreatment.
There is no rigorous research on the knowledge of nitrogen-transforming bacteria in vertical constructed wetlands. Therefore, it is clearly necessary to further understand the kinetics of nitrogen conversion bacteria in unsaturated vertical flow constructed wetlands, especially in partially saturated vertical flow constructed wetlands, in order to increase the total nitrogen removal in a single vertical flow constructed wetland. The purpose of this study is to: (1) compare the conversion and removal efficiency of nitrogen in saturated and unsaturated vertical flow constructed wetlands; (2) identify nitrifying and denitrifying bacteria covered with gravel in unsaturated and saturated zones.