Effect of increased heavy metal toxicity on the quality of vermicomposted biosolids: (a case study of sludge from bugolobi and lubigi wastewater treatment plants)

Abstract

Sewage sludge management is one of the current global environmental issues. In many developing countries like Uganda, the problem is amplified due to uncontrolled and untreated industrial effluents. This tends to increase heavy metal toxicity of wastewater and consequently that of sewage sludge thereby posing danger to human health and the environment. This research explored means of stabilising such sewage sludge of increased heavy metal toxicity through vermicomposting to allow for safe disposal. Sewage sludge samples were picked from Bugolobi and Lubigi sewage treatment plants and tested for quality and bio-safety. An optimal earthworm stocking density for sewage sludge vermicomposting was determined using 0.0, 1.0, 2.0, 2.5, and 3.5kgworms/square meter of exposed surface area of vermicompost bins. Using an optimal earthworm stocking density, vermicomposting of sewage sludge at higher heavy metal concentration was carried out and the resulting biosolids assessed with respect to their Total Nitrogen, Phosphorus, Organic Carbon, Carbon-Nitrogen ratio, faecal coliforms, and electro-conductivity. The results from the tests were compared with standards for organic fertilizers set by Uganda National Bureau of Standards. The results indicated the presence of faecal coliforms, Lead levels, and Electro-conductivity beyond maximum permissible limits. The characteristic of sewage sludge was observed to improve with age while sewage sludge from Lubigi treatment plant which predominantly receives domestic waste water exhibited relatively better characteristics in comparison to sludge from Bugolobi treatment plant. Earthworm stocking density had significant influence on the level of faecal coliforms and electro-conductivity of vermicompost. The research ascertained that a stocking density of 2.5kgworms/m2 was optimal for sewage sludge vermicomposting. Heavy metal toxicity was observed to cause earthworm mortality beyond 0.0021 moles of Lead in sewage sludge. Increased bio-accumulation of heavy metals Lead and Zinc in earthworms was observed at higher heavy metal concentrations. Vermicomposting at higher heavy metal toxicity levels yielded better quality biosolids in terms of bio-safety and quality except for increment in concentration of the spiked metals and electro-conductivity. Further research, therefore, on speciation of spiked heavy metal concentrations after vermicomposting is recommended in order to conclusively ascertain the absence of any potential threat of such vermicompost to the environment. Keywords: - Vermicomposting, Heavy metal, Toxicity, Biosolid, Sludge, Stocking density