engleski

Environmental Impact Due to Heavy Metals Leaching from Landfilled Sewage Sludge Ash by Means of LCA

Sustainable sewage sludge (SS) handling may be defined as a socially acceptable, cost-effective method that meets requirements of efficient recycling of resources while ensuring that harmful substances are not transferred to humans or the environment. At the EU level about 22 % of generated sludge is incinerated (Lynn et al., 2015) and this is constantly increasing. However, even in the case of selecting a solution that involves sludge incineration, significant quantities of sewage sludge ash (SSA), which also require disposal, are being generated. Although this residual product is characterised by a resource potential, it still mostly ends up in landfills for non-hazardous waste. Taking into account the content of certain hazardous and potentially toxic elements and compounds in SSA as well as the need for mandatory implementation of certain protective measures to reduce the impact on all environmental components, it is necessary to consider their impact on human health and the environment in general. The primary consideration is heavy metals content, i.e. their leaching since it is the part that is bio-available. This paper therefore gives a kind of sensitivity analysis: how changes in the leaching concentrations of the heavy metals from landfilled SSA impact the environment, i.e. its basic components (air, water and soil). Based on the literature review and results obtained by testing the SSA obtained by laboratory incineration of SS from four different WWTPs from Croatia, the ranges (min and max) in the leaching concentrations of 10 selected heavy metals at the global level were defined. Nevertheless, the same methodology can be applied also in the case of a far larger base of input data (test parameters) available. The amount of 1 kg of SSA landfilled on non-hazardous waste landfill is defined as a functional unit of the LCA-based model and the total amount of leaching of each metal contributes to air, water and soil emissions. GaBi Professional computer software was used to create the LCA model, and emissions and other environmental impacts were derived from databases integrated into the software. Two indicators describing environmental impacts were studied: human toxicity potential (HTP) and terrestric ecotoxicity potential (TETP). The results are expressed in [kg DCBeq.], as is usual within the world practice for these indicators. Environmental impacts (expressed as HTP and TETP) due to heavy metal emissions in the air are up to several ranks of magnitude higher than impacts due to their emissions to water and soil. Dominant impacts expressed as HTP are due to air emissions, primarily of selenium, and to a lesser extent of cadmium, mercury and nickel, while mercury was found as dominant parameter when TETP was selected as impact category. A more detailed analysis shows that when analyzing leaching concentrations relevant for Croatian laboratory obtained SSA dominant parameters are somewhat different: molybdenum due to water emissions and selenium due to air and water emissions. The analyzes carried out have examined how the change in the leaching concentrations of selected heavy metals, within the real range, affects the potential impacts on the environment due to ash disposal on non-hazardous waste landfills. Although selenium and mercury are identified as key parameters for analysis, as well as cadmium and nickel to a lesser extent, there is also an emphasis on the need for detailed analysis of different SSA due to the high variability in the contents of the trace elements. In the SSAs obtained from Croatian sludge, with selenium and mercury, molybdenum leaching proved to have dominant impact.

sewage sludge, environmental impact, LCA, heavy metals, leaching

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