Biological treatment of synthetic hypersaline wastewater by conventional sequencing batch reactors (SBRs) and integrated fixed film activated sludge reactor (SBR-IFAS)

The rising production of saline wastewater presents significant treatment challenges. The oil and gas sector is particularly involved, producing large volumes of wastewater known as “produced water” (PW) during hydrocarbons extraction and processing operations. These effluents often contain a complex mixture of salts (0.1-320 gNaCl/L), hydrocarbons and additives, including glycols, making them particularly challenging to treat. Effective treatment of produced water is crucial both for meeting regulatory standards and for protecting natural ecosystems from salinity and pollutant related stress. While physicochemical treatments are generally effective, they are typically expensive, energy intensive, and environmentally unsustainable. Biological treatment represents a more sustainable and cost-effective alternative; however, its application under saline conditions is limited by several bottlenecks, including the low salt tolerance of conventional microbial consortia, poor biomass settling, and reduced degradation kinetics. These limitations highlight the need to explore alternative types of microorganisms specifically, halophilic and halotolerant, and reactor configurations that can better handle the difficulties of treating wastewater with high salt content. This doctoral research is aimed to address these challenges through a comprehensive experimental investigation focused on the development and optimization of biological treatment processes for hypersaline wastewater, with specific attention to the removal of organic matter under aerobic condition. The study was structured into two main experimental phases. In the first experiment, a long-term study (583 days) was carried out using a sequencing batch reactor (SBR) inoculated with sediments collected from the solar saltern in Trapani, Italy. The reactor treated synthetic hypersaline wastewater (110 gNaCl/L), using sodium acetate and yeast extract as organic substrates. Results demonstrated high and stable organic carbon removal efficiencies under increasing organic loading rate (OLR), from 294 ± 72 to 1286 ± 187 mgCOD/(L·d). A maximum OLR of 1586 mgCOD/(L·d) and a corresponding organic removal efficiency of 93% were achieved. Notably, the halophilic microbial community developed without the need for a conventional acclimation phase, and was dominated by Halomonas genus. During the initial period, the biomass showed a preliminary ability to settle. However, settling performance was lost during SBR operation. In addition, a tendency to form biofilm was observed, as evidenced by the formation of a layer on the reactor walls. Based on this observation, the subsequent experiment was designed to take advantage from this properties with the aim to try to retain biomass inside the reactor in attached form. Notably, filamentous microorganisms, were observed during the experiment (based on the literature review, the strain is supposedly related to the genus Halomonas), contradicting the common belief of their scarcity or absence in hypersaline condition. The second main experiment of this PhD thesis investigated the comparative performance of a conventional SBR (SBR I) and an SBR II-IFAS system operated in sequencing batch mode. The reactors, both inoculated with hypersaline sediments from the Trapani solar saltern, were operated for 257 days under hypersaline conditions and fed with synthetic wastewater containing easy biodegradable compound (sodium acetate, yeast extract) and diethylene glycol (DEG). Dissolved organic carbon (DOC) and DEG were measured to monitor the performance of the bioreactors. Nuclear magnetic resonance (NMR) was also employed to detect possible intermediates of DEG during the degradation process. The reactors were operated under progressively increasing organic loading rates (OLRs), ranging from approximately 157 to 677 mgCarbon/(L·d). Different sodium acetate/DEG ratios were investigated, while yeast extract accounted for 10% of the influent carbon during the entire experimental period. At the maximum loading rate, removal efficiencies were about 60% when DEG accounted for 35% of the influent carbon, and about 10% when DEG accounted for 92%. Volatile solids concentration in the SBR were 487 ± 25 mgVSS/L while the use of biofilm carriers in the IFAS system significantly enhanced biomass retention with suspended solids concentrations in SBR II-IFAS reaching 8617 ± 1564 mgVSS/L. During the last experimental period, DEG was analyzed and no removal was observed in SBR I, while a removal efficiency of approximately 30% was achieved in SBR II-IFAS. Batch tests confirmed that the biomass from SBR I was unable to degrade DEG effectively, whereas complete DEG removal was observed in the batch assay using biomass from SBR II-IFAS. However, this removal (detected in both the SBR II-IFAS reactor and the batch test) was not reflected by a decrease in dissolved organic carbon (DOC), as the intermediate metabolite produced during degradation preserves the same carbon atom count as DEG (i.e. DOC did not change). The metabolic compound was identified through NMR as 2-hydroxyethoxyacetic acid (HEAA), which represents an initial oxidation stage of DEG and contains the same number of carbon atoms as DEG. To evaluate whether, after overcoming the metabolic block, the biomass from the SBR II-IFAS reactor could degrade additional intermediates formed during the process, monoethylene glycol (MEG) (a possible product of HEAA oxidation) was tested in batch experiments with yeast extract as a co-substrate. Initial MEG concentrations (≈2200 mg/L) decreased to <440 mg/L within 15 days, achieving ≈83% removal efficiency with average removal rates of 152 ± 53 and 102 ± 60 mg/(L·d). In parallel, dissolved organic carbon removal reached 82%, confirming the degradative potential of the biomass toward MEG. In conclusion, this research provides a robust scientific basis for the development of environmentally sustainable and biological treatment solutions for hypersaline wastewaters, especially for produced water. The results highlight the critical importance of microbial inoculum selection, reactor configuration, and process monitoring in overcoming the limitations of biological treatment under hypersaline condition. Future research should focus on microbial selection to enable the complete mineralization of recalcitrant compounds such as DEG, and on scaling up hybrid systems as SBR II-IFAS for application in real industrial environments.