Bioaugmented constructed wetlands for denitrification of saline wastewater: A boost for both microorganisms and plants

Authors: WANG, XINYI - Chinese Academy Of Agricultural Sciences; ZHU, HUI - Chinese Academy Of Agricultural Sciences; YAN, BAIXING - Chinese Academy Of Agricultural Sciences; SHUTES, BRIAN - University Of London; Banuelos, Gary; WEN, HUIYANG - Chinese Academy Of Agricultural Sciences

Submitted to: Environmental International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2020
Publication Date: 5/20/2020
Citation: Wang, X., Zhu, H., Yan, B., Shutes, B., Banuelos, G.S., Wen, H. 2020. Bioaugmented constructed wetlands for denitrification of saline wastewater: A boost for both microorganisms and plants. Environmental International. 138. https://doi.org/10.1016/j.envint.2020.105628.
DOI: https://doi.org/10.1016/j.envint.2020.105628

Interpretive Summary: High volumes of nitrogen rich saline wastewater have been produced from agricultural practices in saline areas, including various industrial sectors (e.g., winery and pharmacy, etc.), and other secondary sources like wastewaters produced from membrane and electro dialysis operations. High water salinity and nitrogen levels have a negative impact on biological activities and survival of aquatic plants, animals and microorganisms exposed to such waters. As a water remediation technology, constructed wetlands (CWs) have been widely applied in wastewater treatment due to their lower costs, high efficiency, and energy saving operations with no production of secondary pollutants. The presence of soluble salts in saline wastewater can, however, inhibit the microbial activity in CWs and consequently limit their purification potential. In addition, the inhibition of salt stress on plant and microbial functions can lead to the reduction of the nitrogen removal capacity of constructed wetlands (CWs) under saline conditions. In this study, we evaluated the effectiveness of CWs inoculated with a salt-tolerant microbial inoculum (designated as Bio-CW) for removing nitrogen from saline wastewater at different salinity levels. After 5 days, results showed that the denitrification capacity of CWs was improved under lower saline conditions by adding the salt-tolerant microbial inoculum. Moreover, the removal percentages of different forms of nitrogen were higher than that in CWs without inoculum (un-Bio-CW) at the lower salinity levels. The bacteria populations and species associated with nitrogen removal increased in Bio-CWs, irrespective of salinity level. In conclusion, the addition of the salt-tolerant microbial inoculum can enhance the nitrogen removal efficiency of CWs under specific saline conditions by protecting the function and health of both microbial communities and plants in CWs used for cleaning nitrogen-lade saline waters.

Technical Abstract: The discharge of nitrogen-laden saline water from saline agricultural regions can promote the eutrophication of coastal waters, as a biological waste water treatment, constructed wetlands (CWs) have been used as a water remediation technology for cleaning such waters. The presence of soluble salts in saline wastewater can, however, inhibit the microbial activity in CWs and consequently limit the purification capacity of CWs. In this regard, we hypothesized that the inoculation of CWs with a salt-tolerant denitrifying bacterium might enhance the nitrogen removal efficiency of the CW. In this study, mesocosms were established with the plant species -Canna indica-under greenhouse conditions. The mesocosms contained three different salinity levels (< 1 (non-saline), 15 and 30 mS/cm), different forms of nitrogen, and were inoculated with a bacterium (Alishewanella); designated as Bio-CW. After 5 days, nitrogen (total N and NH4+-N) removal was almost 100% in the non-saline mesocosms but the removal efficiency for total N, NH4+-N and NO3—N, were suppressed by medium and high salinity in mesocosms containing no inoculum (un-Bio-CW). In contrast, removal of both NH4+-N and NO3—N was 20% higher in Bio-CW mesocosms at both salinity levels. The evaluation of structure and population numbers in different microbial communities indicated that the number of microbial species associated with nitrogen removal and plant growth decreased with increasing salinity levels in the un-Bio-CWs. In the Bio-CW mesocosms, the population numbers of Proteobacteria, Pseudomonas and Thauera increased, which explains why the addition of the inoculum helped maintain the denitrification capacity of the Bio-CW mesocosms. In addition, NH4+-N and NO3 removal percentages and total plant N were almost 27% and 18% higher, respectively, in Bio-CW mesocosms compared to un-Bio-CW. In conclusion, the addition of the salt-tolerant microbial inoculum not only possesses denitrification capacity itself, but its addition to CWs can effectively boost the function, structure and health of both microbial communities and plants in CWs used for cleaning nitrogen-laden saline waters.