Microaeration improves the removal/biotransformation of organic micropollutants in anaerobic wastewater treatment systems.

This work assessed the effect of increasing microaeration flow rates (1-6 mL min-1 at 28 °C and 1 atm, equivalent to 0.025-0.152 L O2 L-1 feed) on the removal/biotransformation of seven organic micropollutants (OMPs) (three hormones, one xenoestrogen, and three pharmaceuticals), at 200 µg L-1 each, in a lab-scale upflow anaerobic sludge blanket reactor operated at a hydraulic retention time (HRT) of 7.4 h. Additionally, the operational stability of the system and the evolution of its microbial community under microaerobic conditions were evaluated. Microaeration was demonstrated to be an effective strategy to improve the limited removal/biotransformation of the evaluated OMPs in short-HRT anaerobic wastewater treatment systems. The rise in the airflow rate considerably increased the removal efficiencies of all OMPs. However, there seems to be a saturation limit for the biochemical reactions. Then, the best results were obtained with 4 mL air min-1 (0.101 L O2 L-1 feed) (~90%) because, above this flow rate, the efficiency increase was negligible. The long-term exposure to microaerobic conditions (249 days) led the microbiota to a gradual evolution. Consequently, there was some enrichment with species potentially associated with the biotransformation of OMPs, which may explain the better performance at the end of the microaerobic term even with the lowest airflow rate tested.

Redox mediator, microaeration, and nitrate addition as engineering approaches to enhance the biotransformation of antibiotics in anaerobic reactors.

The present work assessed some engineering approaches, such as the addition of the redox mediator anthraquinone-2,6-disulfonate (AQDS) (50 and 100 µM), microaeration (1 mL air min-1), and nitrate (100-400 mg L-1), for enhancing the biotransformation of the antibiotics sulfamethoxazole (SMX) and trimethoprim (TMP) (200 µg L-1 each) in anaerobic reactors operated at a short hydraulic retention time (7.4 h). Initially, very low removal efficiencies (REs) of SMX and TMP were obtained under anaerobic conditions (∼6%). After adding AQDS, the anaerobic biotransformation of these antibiotics significantly improved, with an increase of approximately 70% in the REs with 100 µM of AQDS. Microaeration also enhanced the biotransformation of SMX and TMP, especially when associated with AQDS, which provided REs above 70%, particularly for TMP (∼91% with 1 mL air min-1 and 50 µM of AQDS). Concerning nitrate, the higher the added concentration, the higher the REs of the antibiotics (∼86% with 400 mg L-1). Therefore, all the assessed approaches were demonstrated to be very effective in improving the limited biotransformation of SMX and TMP in anaerobic reactors, ensuring REs comparable to those found in higher-cost wastewater treatment technologies, such as conventional activated sludge, membrane bioreactors, and hybrid processes.

Autotrophic denitrification via nitrate as an effective approach for removal of dissolved sulfide in anaerobic reactors.

This work assessed the effect of adding different concentrations of nitrate (50-300 mg NO3 -·L-1) on the removal of dissolved and gaseous sulfide in an anaerobic reactor treating synthetic effluent containing sulfate (100 mg SO4 2-·L-1) and organic matter (1 g COD·L-1). Autotrophic denitrification, stimulated by the addition of nitrate, was demonstrated to be a very effective approach for removal of dissolved sulfide even in the presence of a high concentration of organic matter (complete removal with 50 mg NO3 - mg·L-1). However, it had a minor effect on H2S(g). Sulfide remained partially oxidized to elemental sulfur even with excess nitrate (100-300 mg NO3 - mg·L-1). Therefore, the competition for this electron acceptor between the autotrophic and heterotrophic denitrification pathways may have prevented the conversion of the generated sulfide into sulfate again. No evidence of inhibition of methanogenesis and sulfidogenesis was found during nitrate supplementation.

Reductive decolourisation of sulphonated mono and diazo dyes in one- and two-stage anaerobic systems.

This work assessed the application of one- and two-stage mesophilic anaerobic systems to colour removal of sulphonated mono and diazo dyes with ethanol as electron donor. The dyes Congo Red (CR), Reactive Black 5 (RB5) and Reactive Red 2 (RR2) were selected as model compounds and tested separately in seven different periods. The one-stage system (R(1)) consisted of a single up-flow anaerobic sludge blanket (UASB) reactor, whereas the two-stage system (R(2)) consisted of an acidogenic UASB reactor (R(A)), a settler and a methanogenic UASB reactor (R(M)). For CR and RB5, no remarkable difference was observed between the colour removal performance of both anaerobic systems R(1) and R(2). The experiments with RR2 revealed that R(2) was more efficient on colour removal than R(1), showing efficiencies almost 2-fold (period VI) and 2.5-fold (period VII) higher than those found by R(1). Additionally, R(2) showed a higher stability, giving a good prospect for application to textile wastewaters. Finally, the acidogenic reactor (R(A)) had an important role in the overall decolourisation achieved by R(2) during the experiments with CR and RB5 (>78 %), whereas for RR2, a more recalcitrant dye, R(A) was responsible for up to 38 % of the total colour removal.

Impact of the redox mediator sodium anthraquinone-2,6-disulphonate (AQDS) on the reductive decolourisation of the azo dye Reactive Red 2 (RR2) in one- and two-stage anaerobic systems.

This work assessed the impact of the redox mediator sodium anthraquinone-2,6-disulphonate (AQDS) on the reductive decolourisation of the azo dye Reactive Red 2 (RR2) in one- and two-stage anaerobic systems (R(1) and R(2), respectively). The two-stage system achieved better colour removal efficiencies (52-62%) than the single-stage system (23-33%) in the absence of AQDS. Addition of AQDS accelerated the electrons transfer from the substrate (ethanol) to the dye, which increased the colour removal efficiency of both anaerobic systems (≈ 85%). Finally, the impact of acidogenic and methanogenic phases separation was masked by AQDS supplementation.