PHYTOREMEDIATION EFFECTS OF Azolla pinnata L. IN CONSTRUCTED WETLANDS FOR WASTEWATER TREATMENT IN AKURE, NIGERIA

Abstract

Two wetlands (CW) with dimensions 60 cm x 30 cm x 35 cm were designed, constructed and fabricated for treating domestic wastewater obtained from Akure, Nigeria. One of the wetlands was planted with Azolla pinnata L. (CW1) to ascertain the efficiency of removing metal contents in the wastewater while the other contained wastewater only (CW2). Initial and final characterizations of the wastewater were carried out as well as weekly measurements of sewage from the two CWs using 15 physicochemical water parameters. These include Dissolved Oxygen (DO), Biological Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), Total Suspended Solid (TSS), Total Phosphosus (TP), Total Nitrogen (TN), Ammoniacal Nitrogen (NH3N), Turbidity, pH, Electrical Conductivity (EC), Iron (Fe), Magnesium (Mg), Manganese (Mn) and heavy metals such as Lead (Pb) and Zinc (Zn) using standard laboratory procedures. The experiment was conducted in the wet season (June to July) and dry season (January to February) that characterized the Nigerian climate, and results were subjected to statistical analysis using packages such as the Analysis of Variance (ANOVA), Statistical Package for Social Sciences (SPSS) version 19 at 95 % significance level. Findings from the studies showed considerable reductions in all parameters and heavy metals in CW1 compared with CW2 in the two seasons considered. Appreciable reduction was observed in Zn in all the wetlands within the two seasons. Zn concentration levels reduced from 11.27+1.21 mg/L at initial characterization to 4+0.03 mg/L after 28 days of experimentation in CW1 (dry season). A similar behavior was observed in CW2 but the latter had a much slower reduction rate in Zn concentration from 11.27+1.21 mg/L to 7.5+0.67 mg/L at the same time. In the wet season, a similar pattern of reduction in Zn concentration was observed as concentration levels reduced from 6.34+0.67 mg/L to 1.9+0.9 mg/L while in CW2, it reduced from 6.34+0.67 mg/L to 4.11+0.39 mg/L. Removal efficiencies of 70.03% and 64.51 % were recorded for CW1 while 35.17% and 33.45 % were recorded for CW2 in the wet and dry seasons respectively. There was no significant (p = 0.05) difference in the removal efficiencies of Fe in both seasons as 99.55 %, 59.09 %, 88.89 % and 53.56 % were recorded in CW1 and CW2 respectively. The combined efficiency of constructed wetlands and biological plants such as A. pinnata as excellent hyper-accumulator of heavy metals from domestic wastewaters has been shown from this study.