Abstract
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.
Highlights
Since the implementation of the European Council Urban Wastewater TreatmentDirective 91/271/EEC [1], the annual production of sewage sludge (SS) from wastewater treatment plants (WWTPs) has been rapidly increasing [2,3,4]
We considered a stage that is generally neglected in the evaluation of the performance of sewage treatment plants: namely the period spent by treated sludge in thickening beds and storage facilities
This work showed that several quantitative and qualitative changes occurred in the humic fraction of SS organic matter, during thickening and storage, under both aerobic and anaerobic conditions
Summary
Since the implementation of the European Council Urban Wastewater TreatmentDirective 91/271/EEC [1], the annual production of sewage sludge (SS) from wastewater treatment plants (WWTPs) has been rapidly increasing [2,3,4]. The use of SS as fertilizer is a suitable recycling strategy, which increases the sustainability of agricultural production by reducing the need for synthetic inorganic fertilizers [6,7] This practice may lead to soil contamination and health problems [8,9] due to the accumulation in soils of persistent organic contaminants (POPs) [10], toxic metals (e.g., Cd and Hg [11]), and the presence of pathogenic microorganisms [12]. Suitable treatments and regulation of municipal and industrial waste disposal have greatly reduced risks from toxic metals and pathogens, but other types of contaminants are becoming increasingly diffuse in SS These emerging contaminants, among which plasticizers [14], pharmaceuticals [15], chlorinated compounds, and linear alkylbenzene sulfonates (LAS) [16,17], are not always satisfactorily eliminated during SS treatments
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