Subsequent to the increasing diffusion of wastewater treatment, particularly in high- and middle-income countries, the sewage sludge generated should be treated and valorised in an ecological and economic way, thus contributing to the circular economy. In this study, the monitoring of Heavy Metals (HM), Extractable Organic Halogens (EOX) and Linear Alkylbenzene Sulphonate (LAS) in sewage sludge from 10 different wastewater treatment plants located in Friuli Venezia Giulia (Italy) was reported, and their macronutrient content provided. The obtained results showed, for all tested samples, that HM content in sewage sludge was below the maximum permitted limits provided for by Italian and European regulations for agricultural reuse. Comparison with a similar monitoring campaign carried out in 2006 revealed how, while wastewater treatment plants efficiently resolved water pollution, they accumulated heavy metals and other persistent toxic compounds in sludge, thus restricting their potential reuse. Consequently, consistent and regular sludge monitoring should be undertaken to prevent soil and groundwater contamination. These outcomes could be of particular relevance for the future perspective of agricultural reuse of sewage sludge in waste management practices.
Land application of sewage sludge on agricultural soils can be sustainable only if pollutant contents and organic matter quality meet the requirements imposed by minimization of environmental risks. This study investigated the degradation of linear alkylbenzene sulfonates (LAS) and extractable organic halogens (EOX) and the formation of humic substances (HS) during the thickening and storage phases of sewage sludge treatment. Changes in spectroscopic properties (UV-Vis, FT-IR, and excitation-emission matrix (EEM) fluorescence) of HS were also evaluated to assess the occurrence of biological activities during these curing phases of sewage sludge (SS). Humic acids (HA), fulvic acids (FA), EOX, and LAS were extracted from sewage sludge sampled from four municipal wastewater treatment plants of different size and treatment sequence, before and after 90 days of aerobic or anaerobic storage. During storage, the loss of organic C in the SS ranged from almost null to 31%. No significant changes of FA were registered, whereas HA increased in almost all samples, up to 30%. The amount of humic substances synthesized during storage correlated with the percentage of C lost. Spectroscopic changes of FA and HA showed an increase in their aromaticity, with a corresponding decrease in the aliphatic contribution. These changes show the improved agronomical quality of SS. LAS decreased during storage up to 30%, surprisingly more under anaerobic than aerobic conditions, whereas EOX decreased significantly in all samples, even up to 81%. In conclusion, although storage may be normally considered not influencing the quality of SS, their organic matter quality improved and contamination decreased during 90 days of storage, whatever the conditions of oxygen availability applied.
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