Organic load and hydraulic regime influence over the bacterial community responsible for the nitrogen cycling in bed media of vertical subsurface flow constructed wetland
Introduction
Vertical flow constructed wetlands (VFCW) are an ecotechnology widely employed as treatment solution to different influent wastewater compositions. These units are designed to promote advanced treatment performance, in terms of carbonaceous organic matter and suspended solids removal as well in nitrogen transformations, markedly the nitrification (Kadlec and Wallace, 2009).
VFCW sizing is dependent on the hydraulic loading rates (HLR) and the organic loading rates (OLR) applied on its superficial area (Sezerino et al., 2012, Hoffmann et al., 2011, Platzer, 1999). Besides HLR and OLR, the hydraulic regime is also a fundamental parameter to maximise treatment performance in constructed wetlands (CW). Intermittence of feeding and rest periods allows the control of biomass in the bed media (Molle et al., 2008).
These design and operational variables are directly related with the nitrogen transformation processes that happen in CW (Saeed and Sun, 2012). Besides that, they also affect the microbial community inside the bed media, since the microbial diversity, density and stability are directly related with the treatment performance of such units (Faulwetter et al., 2009).
Nitrogen removal in CW can occur through well-known mechanisms as nitrification followed by denitrification, microbial and vegetal uptake, and adsorption on bed media. Not enough, this phenomenon can also happen due to recently discovered mechanisms reliant on microorganisms only (Saed and Sun, 2012). Several studies report the existence of others microbial metabolic pathways to nitrogen transformations than nitrification and denitrification, such as the anaerobic ammonium oxidation (ANAMMOX) (Wang and Li, 2011, Dong and Sun, 2007), completely autotrophic nitrogen removal over nitrite (CANON) (Sun and Austin, 2007, Hu et al., 2014), heterotrophic nitrification and aerobic denitrification (Austin et al., 2006)
Nowadays, the microbial interactions and their role in treatment performance are being increasingly main subjects of many studies conducted on VFCW. Hu et al. (2016) elucidated through the identification of functional gene expressions, the microbial nitrogen removal pathways in those systems. The relation of viable and dead bacteria in different depths along the VFCW profile was addressed by Foladori et al. (2015). Guan et al. (2015) evaluated the influence of substrate composition on the microbial community. The impact of different macrophyte species on the bacteria community was evaluated by Zhang et al. (2010). Salomo et al. (2009) evaluated the metabolic diversity of the microbial community in different VFCW layers. Tietz et al. (2007a) assessed the ammonia oxidizing bacteria (AOB) community in a VFCW.
Even though there is a high number of studies that investigate microbial dynamics in CW, there is not a clear understanding of how intermittent feeding and alternating operational periods affect the structure of the bacterial community engaged in nitrogen transformation in VFCW. In that way, a detailed knowledge about the structure and physiology of the microbial community involved in nitrogen transformations within this operational outline is essential. Firstly, because this hydraulic regime is widely employed in real systems in Brazil (Trein et al., 2015) and, secondly, due to the possible optimization of nitrogen removal pathways, and therefore, unit’s performance, once these mechanisms are better understood. Furthermore, the relations of this group of microorganisms with the total microbial community and the design and operation variables can also enhance nitrogen removal efficiencies.
Given that, the aim of this study was to evaluate the effect of the OLR and the hydraulic regime over the nitrifying and denitrifying bacterial community present in the bed media of vertical flow constructed wetland (VFCW) employed as wastewater treatment solution.
Section snippets
Experimental units
This study was conducted in bench scale using two microcosms (microcosm 1 and microcosm 2), which represent a vertical profile of bed media from VFCW. The microcosms had superficial area equal to 0.0176 m2, depth of 0.30 m and coarse sand (d10 = 0.3 mm and uniformity coefficient = 6.2) as bed media. Both microcosms were operated under an average temperature of 20 °C. The treatment units were unplanted, Nevertheless, Tietz et al. (2007b) showed no statistically significant difference in bacterial
Treatment performances of microcosms
The average influent and treated effluent characteristics achieved during the 360 days of monitoring are exposed in Table 2. Microcosm 1 (operated with an average HLR of 72 mm d−1 and a constant ORL of 41 g COD m−2 d−1), presented an average COD removal rate equal to 89%, for the 4 periods of operation. Microcosm 2, which has been operated with higher OLR (constant HLR of 170.5 mm d−1 and average OLR of 104 g COD m−2 d−1), showed close COD removal efficiency (83%) to microcosm 1. The OLR increase is
Conclusions
Taking in consideration the microcosms 360 days of monitoring, which simulated VFCW with various applied loading rates of NH4-N, OLR, and HLR (4.5 g NH4-N m−2 d−1, 41 g COD m−2 d−1, and mean HLR of 73.5 mm d−1 for microcosm 1; average of 10.2 NH4-N m−2 d−1, 104 g COD m−2 d−1, and HLR of 170.5 mm d−1 for microcosm 2), it can be concluded that:
Rest periods of 30 days did not interfere in relative abundance of the Bacteria domain. However, it did affect relative abundance of nitrifying bacterial community.
Acknowledgement
The authors would like to thank the National Council of Scientific and Technological Development (CNPq) for funding this research.
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