Heavy Metal Pollution and Ecological Risk Assessment of Sludge Deposits from Selected Wastewater Treatment Plants in Lagos, Nigeria

Fidelis Nkeshita, Opeyemi Gbadewole, Adebola Adekunle, Funmilayo Alayaki

Abstract


Sewage sludge is usually generated during treatment in wastewater treatment plants (WWTPs). Its disposal could cause the release of heavy metals into the environment, thereby, causing toxic pollution. This study involved the ecological assessment of sewage sludge samples which were collected from four government-owned wastewater treatment plants located at Alausa, Iponri, Abesan and Oke Afa in Lagos State, Southwestern Nigeria and analyzed the Cadmium (Cd), Copper (Cu), Lead (Pb), Nickel (Ni), and Zinc (Zn) contents using standard methods. The results show that all the heavy metals apart from Cd and Ni that were analyzed had concentrations that fell below regulatory limits, however, the ecological assessments used which included Contamination factor (Cf), Geo-accumulation index (Igeo), ecological risk factor (Er) and potential ecological risk index ( recorded varying results. The correlation analysis showed that there was a strong positive correlation between Cu and Pb (0.971), Cu and Ni (0.995), and Pb and Ni (0.945) while there was a marginally negative correlation between Cu and Zn (­-0.491), and Ni and Zn (-0.559). There were marginal correlations between Cd and Cu, Cd and Pb, Cd and Ni, Cd and Zn, as well as Pb and Zn.  The Cf  show a decreasing trend of Cd > Ni > Zn > Pb > Cu with sludge sample results varying from moderately polluted to severely polluted wih Cd accounting as the main culprit apart from Oke Afa WWTP. Also, the Igeo results were in decreasing trend of Cd > Ni > Zn > Pb > Cu with results ranging from strongly polluted to very strongly polluted for cadmium and unpolluted for Cu, Pb and Zinc. The Er results were in decreasing trend of Cd > Ni > Cu > Pb > Zn with Cd far exceeding its limits and showing a very high ecological risk while Cu, Pb, and Zn did not pose any serious risk. values indicate that apart from Oke Afa WWTP, the WWTPs exhibited very high ecological risk with Cd accounting for over 90% contribution and subsequent threat to the environment. It is therefore, recommended that sewage sludge from all the WWTPs be treated before reuse for other purposes such as agriculture.


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