Marino, Emanuele (2025) Hydrogenotrophic denitrification of supply water using granular sludge: process optimisation and impact of emerging contaminants. [Tesi di dottorato]

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Abstract

The progressive scarcity and contamination of freshwater resources represent one of the most pressing environmental challenges. Among the major pollutants compromising drinking water safety, nitrate (NO3-) has emerged as a global concern due to its intensive use in agriculture and its leaching into surface and groundwater. Elevated NO3- concentrations pose severe risks to human health, including methemoglobinemia and the formation of carcinogenic N-nitrosamines, and to aquatic systems through eutrophication. Simultaneously, the occurrence of emerging contaminants (ECs) (e.g., pharmaceuticals, pesticides, endocrine disruptors) has become a growing threat to water quality. ECs in water can accumulate in ecosystems, disrupting aquatic life and promoting antibiotic resistance. They may persist through food chains, exposing humans via drinking water and contaminated food. Long-term risks associated with ECs include endocrine disruption, immune and neurological effects, and possible carcinogenic impacts. In this context, this doctoral thesis explores the feasibility and the optimisation of hydrogenotrophic denitrification (H2Den) using anaerobic granular sludge (AnGS) as an innovative inoculum for NO3- removal from low C/N waters. Besides the environmental and health concerns, ECs present in water streams can interfere with microbial metabolism and potentially inhibit biological nitrogen removal processes such as nitrification, denitrification, and anammox. Thus, the effects of ECs on AnGS H2Den were evaluated. In the first experimental study, the effects of inoculum percentage (v/v) (i.e., 10% vs 20% vs 40%) and the hydrogen supply (i.e. stoichiometric vs 50% excess vs 100% excess) on process performance were evaluated in batch mode. Coupling 10% (v/v) inoculum percentage with 100% excess of H2 supply was the most favourable condition, ensuring a NO3- removal efficiency of up to 96%. Under the most favourable conditions, nitrite (NO2-) accumulation was limited, and negligible nitric oxide (NO) and nitrous oxide (N2O) emissions were observed. Subsequently, based on these results, an innovative dual-recirculation upflow anaerobic sludge blanket (UASB) reactor was designed to enhance H2 dissolution and operated for 420 days, studying the effects of varying hydraulic retention time (HRT), hydrogen supply and reactor configuration. Under optimal conditions (i.e., 200% H2 excess and 1 day of HRT), a NO3- removal efficiency of up to 99% was reached, and NO2- accumulation was minimised and kept constantly below the regulatory limit of 0.5 mg NO2-/L. Minimal greenhouse gas emissions, such as NO, N2O, and methane (CH4), were observed. Besides, the dissolved organic carbon (DOC) in the effluent was approximately 0.58 mg DOC/L, ensuring a high-quality output water. Microbial community, at the family level, revealed the presence of Comamonadaceae and Spirochaetaceae, both showing a relative abundance of up to 19.3%. The Comamonadaceae family is associated with the denitrification process, whereas the Spirochaetaceae family is typically found in anoxic environments. The last experimental study, the acute effects of selected ECs, i.e., acetaminophen (ACN) and chloramphenicol (CHP), both at a 200 mg/L concentration, was evaluated over 6 days of AnGS H2Den operated in batch mode. Exposure to ACN improved NO3- removal, likely because its metabolites acted as extra electron donors, reaching a NRE of 97.5% after only 3 days of operation. Nevertheless, in the presence of ACN, a NO accumulation in the headspace on day 6 peaked at 9.5% (i.e., 16.2·10-2 µg NO/min/g VS) was observed. Conversely, exposure to CHP initially inhibited denitrification, specifically after 3 days, a NRE of only 26.3% was reached. Subsequently, microbial adaptation likely occurred and led to recovery, achieving an NRE above 97%, although exposure to CHP caused NO3- accumulation up to 37 mg NO2-/L. The findings of this PhD thesis showed the applicability of AnGS as in H2Den, highlighting its potential as an attractive process for drinking water treatment plants. At the same time, the results pointed out the importance of considering the possible presence of ECs to better understand their effect on the process and on both liquid and gas-phase denitrification intermediates (i.e., NO2-, NO, N2O).

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