Effect of Plant Species on the Performance and Bacteria Density Profile in Vertical Flow Constructed Wetlands for Domestic Wastewater Treatment in a Tropical Climate

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.

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Bacterial Biomass Effect on Performance Improving of Vertical Flow Constructed Wetlands for Domestic Wastewater Treatment

10.21203/rs.3.rs-507260/v1 ◽

2021 ◽

Author(s):

Franck Michael ZAHUI ◽

Jean-Marie Pétémanagnan Ouattara ◽

Mahamadou Kamagaté ◽

Lacina Coulibaly

Keyword(s):

Constructed Wetlands ◽

Anaerobic Bacteria ◽

Bacterial Biomass ◽

Domestic Wastewater ◽

Pennisetum Purpureum ◽

Aerobic Bacteria ◽

Vertical Flow ◽

Distribution Profile ◽

Andropogon Gayanus ◽

Total Bacteria

Abstract Bacteria are frequently studied in constructed wetlands (CWs) due to their effective involvement in pollutants purification processes. In this study, aerobic, anaerobic and total bacteria densities and their vertical distribution profile within pilot-scale vertical flow CWs planted with different plant species were investigated. Five beds were planted in monoculture with Andropogon gayanus, Chrysopogon zizanioides, Echinochloa pyramidalis, Pennisetum purpureum and Tripsacum laxum, and one unplanted bed was used as control. At the end of the treatment trial, bacteria were collected by taking cores of sediment samples at the corners and the center of each bed following six layers in the vertical profile. In fact, the presence of plants on CWs improved the bacterial density and removal efficiencies in the system, with yields from 5.9 to 24.1% regardless the pollutant. However, few anaerobic bacteria were obtained in the different wetlands, and unable to reduce NO3−, excluding for beds planted with T. laxum and P. purpureum. Besides, the number of aerobic bacteria determined decreased (i.e., 17.4 106 to 0.1 106 CFU.g− 1), while that of anaerobic bacteria increased (i.e., 0.1 106 to 2.1 106 CFU.g− 1) from the upper to the bottom layers in the planted beds. Otherwise, anaerobic bacteria were more abundant in the control than in planted beds. Then, total bacteria were dominated by aerobic bacteria, and decreased from surface toward the bottom. As P purpureum promotes the best performance, CWs with this type of plant could be a cost-effective alternative method of treating wastewater.

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