Ciotola, Enrica (2025) Ultrasound-assisted upcycling of anaerobic digestate: resource recovery and trace organic contaminants reduction. [Tesi di dottorato]

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Abstract

The increasing production of sewage sludge and the growing environmental concerns associated with its management have intensified the need for sustainable strategies to promote its circular valorization. In this context, anaerobic digestion (AD) represents a key technology, enabling energy recovery through biogas production. Moreover, the residual sludge produced from the AD process, anaerobically digested sludge (ADS), is rich in micro- and macronutrients; therefore, its reuse on soil is encouraged by the European Union. However, the process is limited by the slow hydrolysis rate of organic matter and by the persistence of trace organic contaminants (TrOCs), including pharmaceuticals, flame retardants, and polycyclic aromatic hydrocarbons (PAHs), which may hinder safe sludge reuse. In recent decades, low-frequency ultrasound (US) has emerged as a promising green technology capable of enhancing organic matter solubilization, improving methane yields, and potentially contributing to contaminant removal. Nevertheless, several knowledge gaps remain regarding the optimal operating conditions for US application, which currently limit its integration into full sludge treatment lines. In particular, ADS treatment has received limited attention in the literature, despite its potential to promote the disintegration of recalcitrant organic fractions and improve the overall energy balance of the AD process. Furthermore, most existing studies investigate the effect of US parameters individually, often neglecting interactions between variables. This PhD research explored, for the first time, the application of low-frequency ultrasound for the valorization of contaminated ADS through an integrated and multidisciplinary approach. To this aim, analytical methodologies were developed for the extraction and determination of TrOCs in sludge. A robust Soxhlet extraction method was optimized for the simultaneous recovery and quantification of 32 different TrOCs via LC-MS/MS, including those listed in the latest revision of the Urban Wastewater Treatment Directive, achieving high extraction efficiencies and reproducibility. Additionally, an ultrasound-assisted extraction (USE) method coupled with gas chromatography–mass spectrometry (GC–MS) was employed for the determination of the 16 PAHs listed by the U.S. Environmental Protection Agency (EPA). Laboratory-scale US tests were conducted at 20 kHz to investigate the influence of amplitude, treatment time, and total solids (TS) content through a multivariable experimental design. Statistical modelling yielded high coefficients of determination (R² > 0.95), confirming the robustness and predictive capability of the models. Treatment time emerged as the most influential factor, followed by amplitude and TS. Ultrasound treatment resulted in a marked increase in soluble chemical oxygen demand (sCOD), primarily due to the solubilization of proteins and carbohydrates, thus confirming substantial disruption of the solid matrix and the release of biodegradable substrates. An increase in volatile fatty acids (VFAs) was also observed, indicating the formation of readily available substrates for subsequent digestion. The comparison between static and flow-through configurations revealed a strong dependence of US performance on mixing conditions and energy distribution within the cavitation field. Biochemical methane potential (BMP) assays on sonicated digestate demonstrated an increase in specific methane production of up to 60% compared to the untreated control, depending on the applied energy input, confirming the beneficial effect of ultrasound on biodegradability. Regarding TrOCs, US promoted their desorption from the solid to the liquid phase and induced partial sonochemical degradation. In PAH-contaminated digestate at environmentally relevant concentrations, sonicated samples exposed to higher energy inputs exhibited enhanced PAH biodegradation compared to untreated controls. Overall, this work demonstrates that low-frequency ultrasound can serve as an effective treatment for improving digestate biodegradability and facilitating the mobilization of trace organic contaminants. The study demonstrated that the multivariable approach enables a faster and, above all, more reliable optimization of the process. The results highlight the need to extend the investigation to pilot and full-scale, given the strong dependence of process efficiency on geometric parameters and energy transfer dynamics. Moreover, the study emphasizes the importance of operating under real concentration conditions to ensure the practical applicability and scalability of the proposed strategy within circular sludge management frameworks.

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