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In biofilm electrode reactors (BER), good nitrate removal performance can be achieved through cooperation of heterotrophic and hydrogen autotrophic denitrification under low carbon/nitrogen conditions. In this study, we proposed a more multifunctional composite cathode, which combine immobilized anthraquinone-2,6-disulphonic disodium salt (AQDS) with polypyrrole (PPy) by electrochemical polymerization-doping method. The nitrate removal performance in BER with PPy/AQDS composite cathode was obviously improved, the nitrate removal rate (4.96 mg/L·h) was almost 2.0 times higher than the control BER system, and relatively stabled nitrate removal efficiency (≥90.0%) was also achieved even as the COD/N of 2.50. Compared with the bare graphite felt, PPy/AQDS coating cathode showed much better electrocatalytic activities, which was more advantageous for in situ production of H

Hexavalent chromium (Cr(VI)) may inhibit denitrification in biological wastewater treatment systems, and the inhibited denitrification process is difficult to recover in a short time. This study explored Cr(VI) cascade impact (20–125 mg L−1) on denitrification and developed one nontoxic biological accelerant (combination of L -cysteine, flavin adenine dinucleotide, biotin and cytokinin) for denitrification recovery. The results showed that NO3−-N removal efficiency decreased from 75.7% to 21.5% when Cr(VI) concentration increased from 80 to 125 mg L−1. Addition of accelerant could effectively promote the removal of NO3--N, and observably reduce the recovery time (42 T) compared with natural recovery (63 T). Furthermore, the main site of Cr(VI) reduction and Cr(III) immobilization was located in the intercellular compartment of the biofilm. Microbes produced more tightly bound extracellular polymeric substances (TB-EPS) to protect them from toxicity under the low Cr(VI) concentrations, while low EPS was secreted when Cr(VI) concentration was higher than 60 mg L−1. Compared to natural recovery system, bio-accelerant addition was beneficial to the recovery of denitrifiers activities, especially for the bacteria containing nirS gene. The results facilitated an understanding of Cr(VI) impact on denitrification, and the proposed bio-accelerant can be potentially applied to heavy metal shock-loading emergency situations.

BACKGROUND: A biofilm‐electrode reactor (BER) was developed on the basis of hydrogenotrophic denitrification. The BER can be strengthened by adding organic matter, and the nitrate removal will be enhanced by a combination of heterotrophic and autotrophic denitrification. Denitrification in a biological system can be affected by operating parameters, and the microbial community and functional genes can be changed at the same time. In this study, the effect of hydraulic retention time (HRT) and current on the denitrification of a low chemical oxygen demand to nitrogen ratio (COD/N), saline wastewater in a continuous‐flow BER was evaluated and compared to a conventional biofilm reactor (BR). RESULTS: The highest extent of nitrate removal was obtained at the optimum HRT (8 h) and current (10 mA). Low levels of ammonia (0.5–4 mg L⁻¹), produced through dissimilatory nitrate reduction to ammonia (DNRA), were observed. Quantitative real‐time polymerase chain reaction analysis revealed that the proportion of nirS‐type denitrifiers (3.75–8.30%) exceeded nirK‐type denitrifiers (0.24–0.90%) in both BR and BER. The genera of T78, Paracoccus and Azoarcus were identified as the dominant denitrifying bacteria in the BER. The cooperative activity of nitrate, nitrite and nitrous oxide reductase is necessary to reduce nitrate to dinitrogen in the denitrification process. The highest ratio of nosZ/nir genes in the bacterial community was 0.25 when the applied current was 10 mA. CONCLUSION: Compared to other denitrification reactors, the BER is more effective to treat low‐COD/N, saline wastewater with short HRT and low current. © 2018 Society of Chemical Industry

A dicyclic-type electrode-based biofilm-electrode reactor (BER) was investigated for simultaneous removal of nitrate and Cr(VI). In the absence of Cr(VI), almost complete denitrification of 50 mg/L NO3−–N was achieved at a very low C/N ratio of 0.8 with the optimal current of 50 mA. Cr(VI) was removed by biological reduction and co-precipitation when Cr(VI) was taken as the only electron acceptor, and the removal efficiencies of Cr(VI) were 99.8%. In the coexistent system of nitrate and Cr(VI), nitrate removal was the result of the cooperation of hydrogenotrophic denitrification and heterotrophic denitrification. The methanol and H2 were also used as electron donors for biological reduction Cr(VI). The denitrification process was slightly inhibited by 1.00 mg/L Cr(VI) and 94.15% removal efficiency was achieved at current = 50 mA and HRT = 8 h. The present results show that the biofilm-electrode reactor is an effective way to simultaneous remove co-contaminants.

A dicyclic-type electrode based biofilm electrode reactor (BER) was developed for advanced nitrate removal from saline municipal wastewater. The denitrification efficiency was evaluated with a synthetic feed (NO3−-N, 20 mg L−1) under different salinity and COD to nitrogen ratios (COD/N). As the salinity increased from 0% to 1.0%, the denitrification performance of both the traditional biofilm reactor (BR) and BER was inhibited; however, the BER showed better adaptation and ability to recover. The BER achieved a high nitrate removal efficiency (≥90%) at a salinity of 1.0% and a low COD/N of 2.5 (theoretical stoichiometric 2.86 ignoring microbial growth). The abundance of Methylotenera mobilis in BR and Clostridium sticklandii in BER was higher than in the initial sludge sample used as inoculum. Likewise, the abundance of napA, nirS and nosZ genes increased as the COD/N further decreased. Under high salinity stress, the BER had a higher denitrification efficiency and the consumption of the organic carbon source (i.e., methanol) was reduced compared to BR. The cooperation between heterotrophic and autotrophic denitrifiers in the BER system provides a more efficient and feasible solution for nitrate removal from saline municipal wastewater.

The transient hexavalent chromium (Cr(VI)) shock may directly inhibit the denitrification process of municipal wastewater treatment plants (WWTPs), which is difficult to recover in a short time. This study developed four nontoxic bio-promoters (combination of L -cysteine, flavin adenine dinucleotide (FAD), biotin, cytokinin and different redox mediators) to quickly restore the denitrification performance after high-loading Cr(VI) suppressing. After feeding with 100 mg/L of Cr(VI) for 42 cycles (T, 4 h), the removal efficiency of nitrate was reduced by 85.00%, and nitrite was accumulated simultaneously. The denitrification performance was recovered quickly with the addition of bio-promoters, introducing redox mediators showed noticeable superiority on the bio-inhibition release. Compared with sodium humate and riboflavin, the AQDS induced bio-promoter achieved the best nitrate removal recovery performance within only 28 T, and the recovery rate was 2.16 times faster than the natural recovery. Microbial analysis showed that Cr(VI) specially inhibited napA-type denitrifiers, and the OTU numbers sharply dropped by 48.74%. Redox mediators induced bio-promoters could effectively recover the abundance of napA-type and nirS-type denitrifying microorganisms, which was consistent with the change of nitrate removal efficiency. This study offers a cost-effective approach to deal with Cr(VI) shock problem, which may promote the development of bio-promoters for WWTPs.

A spiral fiber based biofilm reactor was developed to remove nitrate and chromate simultaneously. The denitrification and Cr(VI) removal efficiency was evaluated with synthetic groundwater (NO3--N=50mg/L) under different Cr(VI) concentrations (0-1.0mg/L), carbon nitrogen ratios (C/N) (0.8-1.2), hydraulic retention times (HRT) (2-16h) and initial pHs (4-10). Nitrate and Cr(VI) were completely removed without nitrite accumulation when the Cr(VI) concentration was lower than 0.4mg/L. As Cr(VI) up to 1.0mg/L, the system was obviously inhibited, but it recovered rapidly within 6days due to the strong adaption and domestication of microorganisms in the biofilm reactor. The results demonstrated that high removal efficiency of nitrate (≥99%) and Cr(VI) (≥95%) were achieved at lower C/N=0.9, HRT=8h, initial pH=7, and Cr(VI)=1.0mg/L. The technology proposed in present study can be alternative for simultaneous removal of co-contaminants in groundwater.

High salinity suppresses denitrification by inhibiting microorganism activities. The shift of microbial community and denitrification functional genes under salinity gradient was systematically investigated in a biofilm electrode reactor (BER) and biofilm reactor (BR) systems. Denitrification efficiency of both BER and BR was not significantly inhibited during the period of low salinity (0–2.0%). As the salinity increased to 2.5%, BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance, and the effluent NO3−-N was lower than 1.5 mg/L. High salinity (>2.5%) impoverished microbial diversity and altered the microbial community in both BER and BR. However, two genera Methylophaga and Methyloexplanations were enriched in BER due to electrochemical stimulation, which can tolerate high salinity (>3.0%). The relative abundance of Methylophaga in BER was almost 10 times as much as in BR. Paracoccus is a hydrogen autotrophic denitrifier, which was obviously inhibited with 1.0% NaCl. The hetertrophic denitrifiers were primarily responsible for the nitrate removal in the BER compared to the autotrophic denitrifiers. The abundance and proportion of denitrifying functional genes confirmed that main denitrifiers shift to salt-tolerant species (nirK-type denitrifiers) to reduce the toxic effects. The napA (2.2 × 108 to 6.5 × 108 copies/g biofilm) and nosZ (2.2 × 107 to 4.4 × 107 copies/g biofilm) genes were more abundant in BER compared to BR's, which was attributed to the enrichment of Methylophaga alcalica and Methyloversatilis universalis FAM5 in the BER. The results proved that BER had greater denitrification potential under high salinity (>2.0%) stress at the molecular level.