Abstract
Bacteria are frequently studied due to their involvement in pollutants transformation processes during wastewater treatment. In this study, the treatment efficiency, bacteria densities and their vertical profile were investigated in pilot-scale vertical flow constructed wetlands (VFCW) planted with different plant species under a tropical climate in west Africa. Five beds were planted with local plant species, i.e., Andropogon gayanus, Chrysopogon zizanioides, Echinochloa pyramidalis, Pennisetum purpureum and Tripsacum laxum, while one bed remained unplanted. These species have been rarely used in CWs while some (e.g., T. laxum) are tested for the first time. After a 7-month trial, bacteria densities were measured in substrate samples separated into six layers along the bed depth. Plants presence enhanced the bacterial density and VFCW efficiency; the removal rates of organic matter (90.9–95.9%; COD and 95.2–98.5%; BOD5), nitrogen (74.3–84%; TN and 76–84%; NH4-N) and phosphorus (77.4–96.9%; PO4-P) were higher by 5.9–24.1% compared to the control bed, providing an overall excellent treatment performance for a single-stage VFCW system. Small numbers of anaerobic bacteria were obtained in the VFCWs, explaining the low-to-zero NO3-N removal, except for the VFCWs with T. laxum and P. purpureum. Aerobic bacteria decreased from the upper to bottom layers from 17.4 to 0.1 × 106 CFU/g in the planted beds, while anaerobic bacteria increased from 0.1 to 2.1 × 106 CFU/g. Anaerobic bacteria were more abundant in the unplanted than in the planted beds. The total bacteria count was dominated by aerobic bacteria, and decreased from the surface towards the bottom. Overall, the VFCW with P. purpureum demonstrated the highest efficiency, indicating that this design is an effective and sustainable nature-based solution for wastewater treatment in a tropical climate.
Highlights
Constructed wetlands (CWs) are engineered systems, designed and constructed to use the natural functions of wetland vegetation, soil, gravel, and the associated microbial populations to treat and purify wastewater [1,2,3]
Six pilot-scale vertical flow constructed wetlands (VFCW) beds planted with different local plant species were built and monitored for the treatment of domestic wastewater in west Africa
This study demonstrated the high removal efficiency of the tested single-stage VFCW design
Summary
Constructed wetlands (CWs) are engineered systems, designed and constructed to use the natural functions of wetland vegetation, soil, gravel, and the associated microbial populations to treat and purify wastewater [1,2,3]. This sustainable technology typically requires small amounts of energy and represents a nature-based solution for water and wastewater management [4]. CWs generally have low investment and, especially, operating costs, are easy to operate, and can be integrated in the landscape [3,5] Their high treatment performance has allowed their application for municipal and for various industrial effluents [6,7,8,9]. The use of subsurface CWs reduces the environmental risks, such as the release of odors and gases and the proliferation of mosquitoes [5]
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