ABSTRACT
The efficiencies of organic pollutants and heavy metal removal from the wastewater of Ahmadu Bello University, (ABU), Zaria by ABU waste stabilization pond and horizontal flow sub-surface constructed wetland were evaluated and compared at seven (7) days hydraulic retention time for a period of ten (10) weeks. A constructed wetland with dimensions 14m x 3.5m x 0.45m, lined with geomembrane of polyethylene to prevent percolation of wastewater into the groundwater and filled with gravel 10-15mm size to the depth of 0.2m at the bottom, followed by sand layer 0.3-0.5mm size to the depth of 0.20m depth and 0.05m local soil was used. Influent wastewater was provided from elevated storage tank of 3000 litres to the constructed wetland through pipe network. The effluents from the wetland and waste stabilization pond of ABU were sampled for analysis. The constructed wetland was able to remove considerable amounts of the heavy metals in the wastewater of ABU. These heavy metals otherwise would have been discharged into Kubani River at the downstream of the University dam. The efficiencies obtained during the study by the constructed wetland for Pb, Zn, Mn, Mg, Ag, Co, Cr, Cd, Fe and Cu were 86.8%, 71.83%, 97.69%, 80%, 68.81%, 90%, 86.51%, 89.34%, 68.18% and 90% respectively. On the other hand, the maximum removal efficiencies measured for the ABU waste stabilization pond for Pb, Zn, Mn, Mg, Ag, Co, Cr, Cd, Fe and Cu were 10.53%, 7.64%, 6.91%, 8.07%, 8.05%, 5.8%, 4.12%, 4.61%, 11.23% and 9.91% respectively. During the same period, the constructed wetland removed TSS, FC, BOD, COD, PO4-, NO3- and NH4+ by 89.2%, 94.3%, 93%, 80.5%, 67.5%, 80.5% and 32.92% respectively compared with the ABU waste stabilization pond by 78.6%, 81.1%, 78.3%, 69.8%, vii 38.12%, 72.6% and 17.02% respectively. The result of assessment of removal efficiencies of certain organic pollutants from ABU waste stabilization pond is in conformity with the result of the previous work done by Oke et al (2006). The DO concentration increased from 2.70 - 4.70mg/L during ten (10) weeks of study. This indicates that constructed wetlands may be eco-friendly in terms of dissolved oxygen content. The average pH and temperature of effluent from constructed wetland were 7.16 and 28.03 oC respectively and they fall within the permissible discharge limits of Food and Agricultural Organisation (FAO) and World Health Organisation (WHO) Standards. A low or zero energy input was required by the constructed wetland just like waste stabilization pond , making its operation and maintenance costs much lower compared to conventional treatment systems. The results of physicochemical and biological measurements of the treated effluent by the constructed wetland fall within the permissible discharge limits of Food and Agricultural Organisation (FAO) and World Health Organisation (WHO) effluent standards, making the treated effluent amenable for wastewater reuse in agriculture. The Microsoft Office Software EXCEL XP and SPSS programs were used to derive reliable statistical efficiency models which are closely similar in goodness of fit. The statistical analysis revealed satisfactory agreement between model predictions and measurements proving the adequacy of the models. The relationship between waste stabilization pond and constructed wetland effluents are highly and positively correlated. There was statistically significant difference at p < 0.01 level in removal efficiencies of heavy metals for WSP and CW F (9, 181.348) = 21.632, p=0.01. In addition to reaching statistical significant, the actual difference in mean values between WSP and CW was significant too (i.e. Mean for WSP and CW = 1.667420 and 0.745740 respectively). There was also statistically significant difference at the p < viii 0.01 level in removal efficiencies of physico-chemical pollutants for WSP and CW F (9, 181.348) = 21.632, p=0.01. The actual difference in mean values between WSP and CW was also significant (i.e. Mean for WSP and CW = 29.6500 and 20.0946 respectively). This study observed that the constructed wetland significantly removed heavy metals and physico-chemical pollutants more than waste stabilization pond.