High salinity is becoming more common in industrial process water and final effluents, particularly when striving to close water loops. There is limited knowledge on the anaerobic treatment of chemical wastewaters characterized by distinct salinity fluctuations. This study investigates the high and fluctuating salinity effects on the conversion capacity and membrane filtration performance of an anaerobic membrane bioreactor (AnMBR) in treating phenol-containing wastewater. The AnMBR operated for 180 days with sodium concentrations between 8 and 37 gNa^+.L⁻¹. At ≤ 26 gNa^+.L⁻¹, approximately 99% COD and phenol removal efficiencies were achieved. At 37 gNa^+.L⁻¹, phenol and COD removal efficiencies decreased to 86 and 82%, respectively, while the biomass specific methanogenic activity was 0.12 ± 0.05 gCOD-CH₄.gVSS^−1.d⁻¹. Due to large salinity fluctuations, phenol and COD removal efficiencies reduced to ≤ 45% but recovered to ≥ 88%. Compared to phenol conversion, methanogenesis was more severely affected. Calculations showed a maximum in-situ phenol conversion rate of 25.5 mgPh.gVSS^−1.d⁻¹. Concomitantly, biomass integrity was compromised, and the median particle size severely dropped from 65.6 to 4.3 μm, resulting in a transmembrane pressure increase above 400 mbar. Cake layer resistance to filtration contributed to 85% of the total resistance. Nonetheless, all biomass was effectively retained in the AnMBR. A change in salinity ≥ 14 gNa^+.L⁻¹ substantially reduced the microbial richness and diversity. The microbial community was dominated by Bacteria belonging to Clostridiales and Archaea of the orders Methanosarcinales and Methanobacteriales. Our findings demonstrate AnMBRs as suitable techniques for treating chemical process water, with possible subsequent reclamation, characterized by high phenols concentrations and largely fluctuating salinity levels.

Hydrolysis is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge (WAS). In this study, an innovative 4 stages cascade anaerobic digestion system was researched to (1) comprehensively clarify whether cascading configuration enhances WAS hydrolysis, and to (2) better understand the governing hydrolysis kinetics in this system. The cascade system consisted of three 2.2 L ultra-short solids retention times (SRT) continuous stirred tank reactors (CSTRs) and one 15.4 L CSTR. The cascade system was compared with a reference conventional CSTR digester (22 L) in terms of process performance, hydrolytic enzyme activities and microbial community dynamics under mesophilic conditions (35 °C). The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40–42%, even at 12 days total SRT that corresponded to only 1.2 days SRT each in the first three reactors of the cascade. The reference-CSTR converted only 31% tCOD into biogas and suffered process deterioration at the applied low SRTs. Calculated specific hydrolysis rates in the first reactors of the cascade system were significantly higher compared to the reference-CSTR, especially at the lowest applied SRTs. The activities of several hydrolytic enzymes produced in the different stages revealed that protease, cellulase, amino peptidases, and most of the tested glycosyl-hydrolases had significantly higher activities in the first three small digesters of the cascade system, compared to the reference-CSTR. This increase in hydrolytic enzyme production by far exceeded the increase in specific hydrolysis rate, indicating that hydrolysis was limited by solids-surface availability for enzymatic attack. Correspondingly, high relative abundances of hydrolytic-fermentative bacteria and hydrogenotrophic methanogens as well as the presence of syntrophic bacteria were found in the first three digesters of the cascade system. However, in the fourth reactor, acetoclastic methanogens dominated, similarly as in the reference-CSTR. Overall, the results concluded that using multiple CSTRs that are operated at low SRTs in a cascade mode of operation significantly improved the enzymatic hydrolysis rate and extend in anaerobic WAS digestion. Moreover, the governing hydrolysis kinetics in the cascading reactors were far more complex than the generally assumed simplified first-order kinetics.

Industrial wastewaters are becoming increasingly associated with extreme conditions such as the presence of refractory compounds and high salinity that adversely affect biomass retention or reduce biological activity. Hence, this study evaluated the impact of long-term salinity increase to 20 gNa⁺.L⁻¹ on the bioconversion performance and microbial community composition in anaerobic membrane bioreactors treating phenolic wastewater. Phenol removal efficiency of up to 99.9% was achieved at 14 gNa⁺.L⁻¹. Phenol conversion rates of 5.1 mgPh.gVSS⁻¹.d⁻¹, 4.7 mgPh.gVSS⁻¹.d⁻¹, and 11.7 mgPh.gVSS⁻¹.d⁻¹ were obtained at 16 gNa⁺.L⁻¹,18 gNa⁺.L⁻¹ and 20 gNa⁺.L⁻¹, respectively. The AnMBR's performance was not affected by short-term step-wise salinity fluctuations of 2 gNa⁺.L⁻¹ in the last phase of the experiment. It was also demonstrated in batch tests that the COD removal and methane production rate were higher at a K⁺:Na⁺ ratio of 0.05, indicating the importance of potassium to maintain the methanogenic activity. The salinity increase adversely affected the transmembrane pressure likely due to a particle size decrease from 185 μm at 14 gNa⁺.L⁻¹ to 16 μm at 20 gNa⁺.L⁻¹. Microbial community was dominated by bacteria belonging to the Clostridium genus and archaea by Methanobacterium and Methanosaeta genus. Syntrophic phenol degraders, such as Pelotomaculum genus were found to be increased when the maximum phenol conversion rate was attained at 20 gNa⁺.L⁻¹. Overall, the observed robustness of the AnMBR performance indicated an endured microbial community to salinity changes in the range of the sodium concentrations applied.

This study examined the temperature susceptibility of a continuous-flow lab-scale anaerobic membrane bioreactor (AnMBR) to temperature shifts from 35 °C to 55 °C and its bioconversion robustness treating synthetic phenolic wastewater at 16 gNa^+.L⁻¹. During the experiment, the mesophilic reactor was subjected to stepwise temperature increases by 5 °C. The phenol conversion rates of the AnMBR decreased from 3.16 at 35 °C to 2.10 mgPh^.gVSS^−1.d⁻¹ at 45 °C, and further decreased to 1.63 mgPh^.gVSS^−1.d⁻¹ at 50 °C. At 55 °C, phenol conversion rate stabilized at 1.53 mgPh^.gVSS^−1.d⁻¹ whereas COD removal efficiency was 38% compared to 95.5% at 45 °C and 99.8% at 35 °C. Interestingly, it was found that the phenol degradation process was less susceptible for the upward temperature shifts than the methanogenic process. The temperature increase implied twenty-one operational taxonomic units from the reactor's microbial community with significant differential abundance between mesophilic and thermophilic operation, and eleven of them are known to be involved in aromatic compounds degradation. Reaching the upper-temperature limits for mesophilic operation was associated with the decrease in microbial abundance of the phyla Firmicutes and Proteobacteria, which are linked to syntrophic phenol degradation. It was also found that the particle size decreased from 89.4 μm at 35 °C to 21.0 μm at 55 °C. The accumulation of small particles and higher content of soluble microbial protein-like substances led to increased transmembrane pressure which negatively affected the filtration performance. Our findings indicated that at high salinity a mesophilic AnMBR can tolerate a temperature up to 45 °C without being limited in the phenol conversion capacity.