The feasibility of treating heavy metals present in Wastewater Sludge (WWS) as found in Eswatini and the sustainable disposal thereof.

Abstract

Sewerage content found in wastewater treatment sludge is known to be rich in nutrients and organic matter, rendering it an alternative solution for commercial fertilizers. However, wastewater treatment sludge may contain toxic heavy metals that could be detrimental to the environment and can pose a threat to human life if entered the food chain. Several studies globally have reported success in immobilizing heavy metals, but the long-term sustainability cannot be ascertained. This research aims to determine the heavy metals present in the Eswatini wastewater treatment sludge, in order to enhance is potential for sustainable use. Triplicate wastewater sludge sampled from Matsapha and Nhlambeni Wastewater Treatment Plants were first tested for electrical conductivity (salinity), moisture content, pH, nutrients and heavy metals and organic matter. Both Matsapha and Nhlambeni Wastewater Treatment Plant samples were treated with di-hydrogen sulphate (H2PO4) and sorption analysis was conducted to determine phosphorus adsorbed by each sample. The sorption analysis samples were further tested for seepage of heavy metals using a Toxicity Characteristic Leaching Procedure (TCLP). Spinach and grass were planted on both samples to determine plant uptake of heavy metals and leachability was determined through a column study on the mixture of soil and sludge samples using recommendations from the South African legislation. Analysis for acid digested samples and leached water were conducted through ICP-OES. Results indicated significant immobilization of heavy metals present in the wastewater sludge samples, which was confirm by the TCLP test. However, leached samples from column study could not confirm the reality of immobilized heavy metals over long term since the soil itself contained some heavy metals, which its availability for plant uptake is not known. This research will inform, mainly the African local municipalities and parastatals that attempt to treat industrial wastewater sludge (aimed at reducing the solubility of heavy metals) for possible use in agriculture.