Effect of plant physiological characteristics on the removal of conventional and emerging pollutants from aquaculture wastewater by constructed wetlands
Graphical abstract
Introduction
Aquaculture has been a growing industry due to its socio-economic benefits, sustainable development and potential to alleviate the world hunger (Ling et al., 2004). However, aquaculture wastewater can cause severe environmental problems because of its high discharge amounts, high chemical oxygen demand (COD), and high nitrogen and phosphorus contents (Ling et al., 2004). Meanwhile, antibiotics are widely used in aquaculture to prevent infectious diseases among aquaculture species. Although antibiotics exist at low concentrations in aquatic environments, their persistence can promote the development of bacteria with antibiotic-resistant genes (ARGs) as a result of selective pressure (Fernandes et al., 2015, Michael et al., 2013, Cabello, 2006). This consequence can reduce the therapeutic potential of antibiotics against human (Heuer et al., 2009) and animal pathogens (Michael et al., 2013), and change bacterial flora in sediments and water columns (Cabello, 2006). Therefore, the removal of conventional and emerging pollutants from high-density aquaculture wastewater should receive more attention.
As a treatment technology for aquaculture wastewater, constructed wetlands (CWs) have the advantage of low investment requirements, implementation costs, and energy consumption (Meng et al., 2014, García et al., 2010). Previous studies demonstrated that CWs were capable of removing a majority of environmental pollutants including nitrogen, phosphorous, COD (Wang et al., 2013, Li et al., 2013, Hu et al., 2012, Lin et al., 2002), emerging contaminants, such as antibiotics (Berglund et al., 2014, Chen et al., 2015, Hijosa-Valsero et al., 2011, Liu et al., 2013) and ARGs (Chen et al., 2015, Liu et al., 2013). These advantages are attributed to the interactions among soil/sediment, plants and microorganisms (Carvalho et al., 2012), where multiple physical, chemical and biological processes occur simultaneously, like adsorption, photolysis, volatilization, plant uptake and accumulation, plant exudation and microbial degradation (Garcia-Rodríguez et al., 2014, Vymazal et al., 2010). Therefore, the performance of pollutant removal of CWs is dependent on design parameters, which include plant species, flow type, substrate, hydraulic loading rates, hydraulic retention time (HRT), and applied pollutant loadings (Hijosa-Valsero et al., 2011, Wu et al., 2015, Wu et al., 2014, Weerakoon et al., 2013, Hijosa-Valsero et al., 2010).
The presence of macrophytes distinguishes CWs from unplanted soil filters or lagoons (Vymazal, 2011). Many studies have identified plants and microorganisms associated with plant roots contributed the CWs performance (Bouali et al., 2014). The belowground parts of plants in CWs can provide adsorption sites for bacterial, release oxygen and root exudates to the rhizosphere, which is beneficial for the growth of microorganism (Hijosa-Valsero et al., 2011, Vymazal, 2011b, Vymazal, 2011a). Therefore, CWs cultivated with different plant species have different influence on the removal efficiencies of pollutants. In the past few years, studies have mainly focused on the effects of single plant species on the treatment performance of CWs, but in recent years, more attention has been paid to the effects of mixed planting using plants with different physiological characteristics. In our previous study, we found that CWs planted with Iris pseudacorus had higher purification efficiency for nitrogen in aquaculture wastewater than Phragmites australis (Huang et al., 2016). Other studies have reported the advantages of Phragmites australis in the removal of antibiotics and ARGs (Carvalho et al., 2012, Liu et al., 2013, Choi et al., 2016). Given the emerging pollutant characteristics of aquaculture wastewater, this study aimed to explore the effects of different plant physiological characteristics (plant species and planting patterns) of four pilot-scale CWs on the removal efficiencies for conventional and emerging pollutants, such as antibiotics and ARGs. In addition, the microbial community structures of the four pilot-scale CWs were characterized through quantitative polymerase chain reaction (qPCR) assays and high-throughput sequencing.
Section snippets
Experimental design and operation methods
These wetland microcosms were located at an aquaculture farm of yellow catfish in Changzhou (31°35′N, 119°52′E), Jiangsu Province, China. The size of four horizontal subsurface-flow constructed wetlands (HSSFCW) was 150 cm long, 40 cm wide, and 80 cm deep (effective depth of 60 cm), and the materials were polyvinyl chloride. The two ends of the microcosm were separated by perforated plates, forming a 15 cm water distribution area and a 15 cm water collection area. Both areas were filled with
Operational performance of the four CWs
The conventional wastewater parameters (DO, COD, TN, NH4+–N, NO3––N, NO2––N, and TP) of the influent and effluent samples were summarized in Table 1. In the influent, DO, COD, TN, NH4+–N, NO3––N, NO2––N and TP were detected at the average concentrations of 2.45, 66.6, 3.60, 2.35, 0.51, 0.134 and 0.23 mg/L, respectively. In the effluents (S1, S2, M1, M2), these parameters were declined to 0.97–1.14, 30.7–36.5, 0.59–0.77, 0.39–0.52, 0.05–0.08, 0.001–0.003 and 0.04–0.07 mg/L, respectively. The
Operational performance
Regarding to the removal rates of COD, the difference of removal rate between S1, S2, M1 and M2 wasn’t significant (P > 0.05). S2 planted with Phragmites australis achieved the highest removal rate, which is consistent with the result of Wang (Wang et al., 2012). The belowground biomass of Phragmites australis is well-developed, which is positively correlated to radial oxygen loss (ROL) and stimulates substrate respiration rate and phosphatase activity (Wang et al., 2009, Wießner et al., 2002,
Conclusion
The different removal performances of CWs planted with two plant species and two planting patterns might be attributed to the plant physiological characteristics and their rhizosphere microenvironments. Mixed planting improved the removal of TN and NH4+–N in the aquaculture wastewater through the cooperation between the nutrient uptake of Iris pseudacorus and the enhanced nitrification efficiency of Phragmites australis. The vigorous root exudate of Iris pseudacorus and the oxygen release of
Acknowledgments
This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07204002, 2017ZX07204004, 2012ZX07101006) and Fundamental Research Funds for the Central Universities (0400219375).
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