Microbial denitrification by immobilized bacteria Pseudomonas denitrificans stimulated by constant electric field

Effects of Weak Electric Fields on the Denitrification Performance of Pseudomonas stutzeri: Insights into Enzymes and Metabolic Pathways

Enhanced denitrification has been reported under weak electric fields. However, it is difficult to investigate the mechanism of enhanced denitrification due to the complex interspecific interactions of mixed-culture systems. In this study, Pseudomonas stutzeri, capable of denitrification under anaerobic conditions, was selected for treating low COD/N (2.0, ratio between concentration of chemical oxygen demand and NO3--N) artificial wastewater under constant external voltages of 0.2, 0.4, and 0.6 V. The results revealed that P. stutzeri exhibited the highest efficiency in nitrate reduction at 0.2 V. Moreover, the maximum nitrate removal rate was 15.96 mg/(L·h) among the closed-circuit groups, 19.39% higher than that under the open-circuit group. Additionally, a notable reduction in nitrite accumulation was observed under weak electric fields. Enzyme activity analysis showed that the nitrate reductase activities were significantly increased among the closed-circuit groups, while nitrite reductase activities were inhibited. Transcriptomic analysis indicated that amino acid metabolism, carbohydrate metabolism, and energy metabolism were increased, enhancing the resistance of P. stutzeri to environmental stress and the efficiency of carbon source utilization for denitrification. The current study examined the impacts of weak electric fields on enzyme activities and microbial metabolic pathways and offers valuable insights into the mechanism by which denitrification is enhanced by weak electric fields.

Long-term electricity generation and denitrification performance of MFCs with different exchange membranes and electrode materials

Different biocathode electrode materials (graphite felt and carbon brush, GF and CB) and exchange membranes (proton exchange membrane and cation exchange membrane, PEM and CEM) were used in three microbial fuel cell (MFC) configurations operated for 300-days to investigate the power generation and the COD and N removal performance. Results showed no effect on the COD removal (all above 96%); however, the power generation (46.11 mW·h) and denitrification performance (68.0 ± 1.6%) of the MFC-B (GF + PEM) system were higher than those of the other systems (MFC-A: CB + PEM; MFC-C: CB + CEM) (P < 0.01), and the power generation and denitrification performance of all three systems decreased with time (P < 0.01). By analyzing the physicochemical properties of the exchange membrane and cathode electrode materials, the reasons that affect the power generation performance of the system were clarified. Furthermore, the increase in bioelectricity enhanced the electricity-related nitrification and denitrification reactions. The average 300-day unit denitrification cost of MFC-A was 4.2 and 6.3 times that of MFC-B and MFC-C, respectively. Comprehensive consideration of electricity generation, denitrification, and service life, combined with cost analysis and better selection of construction materials, provides a theoretical basis for the long-term stable operation and sustainable application of MFCs.

Technologies for biological removal and recovery of nitrogen from wastewater

Water contamination is a growing environmental issue. Several harmful effects on human health and the environment are attributed to nitrogen contamination of water sources. Consequently, many countries have strict regulations on nitrogen compound concentrations in wastewater effluents. Wastewater treatment is carried out using energy- and cost-intensive biological processes, which convert nitrogen compounds into innocuous dinitrogen gas. On the other hand, nitrogen is also an essential nutrient. Artificial fertilizers are produced by fixing dinitrogen gas from the atmosphere, in an energy-intensive chemical process. Ideally, we should be able to spend less energy and chemicals to remove nitrogen from wastewater and instead recover a fraction of it for use in fertilizers and similar applications. In this review, we present an overview of various technologies of biological nitrogen removal including nitrification, denitrification, anaerobic ammonium oxidation (anammox), as well as bioelectrochemical systems and microalgal growth for nitrogen recovery. We highlighted the nitrogen removal efficiency of these systems at different temperatures and operating conditions. The advantages, practical challenges, and potential for nitrogen recovery of different treatment methods are discussed.

Mixture of different Pseudomonas aeruginosa SD-1 strains in the efficient bioaugmentation for synthetic livestock wastewater treatment

Strains selection for inoculation is the key to the successful construction of a bioaugmentation system, a promising strategy for specific pollutant removal. Pseudomonas aeruginosa SD-1 wild-type (WT) strain exhibited high capacity for biofilm formation but low efficiency for nitrate (NO₃^-) removal. Meanwhile, quorum sensing deficient strain ΔlasR showed excellent efficiency for NO₃^- removal but poor capability for colonization in activated sludge. The opposite effect of biofilm formation and NO₃^- removal exist in WT or ΔlasR, which limits the construction of bioaugmentation system of strain SD-1 and its application. To solve this issue, a mixture of WT and ΔlasR (v/v = 1:1) was used to construct a bioaugmentation system. Compared with the inoculation of WT or ΔlasR alone, the mixed inoculation not only was beneficial for activated sludge development but also for pollutant removal. The indicators for activated sludge including the abundance of P. aeruginosa, the sludge volume index and the average particle size in mixed inoculated reactors were close to those of reactors with single and repeated inoculation of WT. The effluent of chemical oxygen demand (COD) and NO₃^--N were stable at 3.9-22.6 mg L^-1 and 0-5.53 mg L^-1 after d 3, respectively. This study presents a detailed case on the ecological tradeoff of colonization and pollutant removal of inoculated strains during bioaugmentation. The results provide information on the appropriate conditions for application of P. aeruginosa SD-1 for livestock wastewater treatment and further enrich our ecological understanding of bioaugmentation.

Improving Substrate Consumption and Decrease of Growth Yield in Aerobic Cultures of Pseudomonas denitrificans By Applying Low Voltages in Bioelectric Systems

It is well known that activated sludge treatment systems generate a lot of surplus sludge having environmental and economic impacts. Although several approaches have been proposed for the treatment/reuse of the excess of sludge, there are few studies focused on decreasing the biomass yield without affecting the metabolic activity. This work reports the effect of low magnitude electrical fields (0.07 to 0.2 V/cm) on the growth yield of a pure strain of Pseudomonas denitrificans (used as model microorganism). Cell potentials between 0.2 and 0.57 V were measured during 24 h to the aerobic culture; biomass production and substrate consumption were evaluated at regular intervals. Results indicated that the substrate (lactate) consumption efficiency increased with the applied potential, up to 100%, while the yield diminished 31% (0.34 g biomass/g lactate consumed) at 0.7 V vs. NHE. Bioenergetics showed that the fraction of electron equivalents toward biomass synthesis decreased from 0.68 (when no potential was applied) to 0.47 at 0.57 V, pointing out the redirection of the energy flow toward maintenance to cope with the stress caused by the imposed voltage. Therefore, the electrical stimulus could be used as control of biomass growth in aerobic wastewater treatment lines.

Post-denitrification using alginate beads containing organic carbon and activated sludge microorganisms

Nitrate concentration in the final effluent is a key issue in pre-denitrification biological treatment systems. This study investigated post-denitrification with alginate beads containing immobilized activated sludge microorganisms and organic carbon source. A batch study was first performed to identify suitable carbon sources among acetate, glucose, calcium tartrate, starch and canola oil on the basis of nitrate removal and bead stability. Canola oil and starch beads exhibited significantly higher denitrification rates, greater bead stability and lower nitrite accumulation (6 mg/L and 10 mg/L, respectively). Glucose and acetate beads showed longer acclimation phases and degraded faster whereas tartrate beads had higher nitrite build-up (39 mg/L) and degraded due to brittleness. Post-denitrification with canola oil and starch beads was investigated in the final clarifier of a coupled upflow bioreactor and aerobic system treating synthetic dairy farm wastewater, and showed a denitrification efficiency of >90%. Beads faded in 12 days due to alginate degradation. Therefore, enhancement in bead strength or use of more stable nontoxic gel would be required to further prolong the treatment. Moreover, this study was conducted at laboratory scale and further research is needed for application in real systems.

Two-step nitrification in a pure moving bed biofilm reactor-membrane bioreactor for wastewater treatment: nitrifying and denitrifying microbial populations and kinetic modeling

The moving bed biofilm reactor-membrane bioreactor (MBBR-MBR) is a novel solution to conventional activated sludge processes and membrane bioreactors. In this study, a pure MBBR-MBR was studied. The pure MBBR-MBR mainly had attached biomass. The bioreactor operated with a hydraulic retention time (HRT) of 9.5 h. The kinetic parameters for heterotrophic and autotrophic biomasses, mainly nitrite-oxidizing bacteria (NOB), were evaluated. The analysis of the bacterial community structure of the ammonium-oxidizing bacteria (AOB), NOB, and denitrifying bacteria (DeNB) from the pure MBBR-MBR was carried out by means of pyrosequencing to detect and quantify the contribution of the nitrifying and denitrifying bacteria in the total bacterial community. The relative abundance of AOB, NOB, and DeNB were 5, 1, and 3%, respectively, in the mixed liquor suspended solids (MLSS), and these percentages were 18, 5, and 2%, respectively, in the biofilm density (BD) attached to carriers. The pure MBBR-MBR had a high efficiency of total nitrogen (TN) removal of 71.81±16.04%, which could reside in the different bacterial assemblages in the fixed biofilm on the carriers. In this regard, the kinetic parameters for autotrophic biomass had values of YA=2.3465 mg O2 mg N(-1), μm, A=0.7169 h(-1), and KNH=2.0748 mg NL(-1).

Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor

Excess nitrogenous compounds are detrimental to natural water systems and to human health. To completely realize autohydrogenotrophic nitrogen removal, a novel 3-dimensional biofilm-electrode reactor was designed. Titanium was electroplated with ruthenium and used as the anode. Activated carbon fiber felt was used as the cathode. The reactor was separated into two chambers by a permeable membrane. The cathode chamber was filled with granular graphite and glass beads. The cathode and cathode chamber were inhabited with domesticated biofilm. In the absence of organic substances, a nitrogen removal efficiency of up to 91% was achieved at DO levels of 3.42 +/- 0.37 mg/L when the applied current density was only 0.02 mA/cm2. The oxidation of ammonium in biofilm-electrode reactors was also investigated. It was found that ammonium could be oxidized not only on the anode but also on particle electrodes in the cathode chamber of the biofilm-electrode reactor. Oxidation rates of ammonium and nitrogen removal efficiency were found to be affected by the electric current loading on the biofilm-electrode reactor. The kinetic model of ammonium at different electric currents was analyzed by a first-order reaction kinetics equation. The regression analysis implied that when the current density was less than 0.02 mA/cm2, ammonium removal was positively correlated to the current density. However, when the current density was more than 0.02 mA/cm2, the electric current became a limiting factor for the oxidation rate of ammonium and nitrogen removal efficiency.

Biological removal of nitrate by an oil reservoir culture capable of autotrophic and heterotrophic activities: kinetic evaluation and modeling of heterotrophic process

Kinetics of heterotrophic denitrification was investigated using an oil reservoir culture with the ability to function under both autotrophic and heterotrophic conditions. In the batch system nitrate at concentrations up to 30 mM did not influence the kinetics but with 50mM slower growth and removal rates were observed. A kinetic model, representing the denitrification as reduction of nitrate to nitrite, and subsequent reduction of nitrite to nitrous oxides and nitrogen gas was developed. The value of various kinetic coefficients, including maximum specific growth rate, saturation constant, yield and activation energy for nitrate and nitrite reductions were determined by fitting the experimental data into the developed model. In continuous bioreactors operated with 10 or 30 mM nitrate, complete removal of nitrate (no residual nitrite) and linear dependency between nitrate loading and removal rates were observed for loading rates up to 0.21 and 0.58 mM h(-1), respectively. The highest removal rates of 0.31 and 0.94 mM h(-1) observed at loading rates of 0.42 mM h(-1) and 1.26 mM h(-1), with corresponding removal percentages of nitrate and total nitrogen being 75.4, 54.4%, and 74.4 and 17.9%, respectively. Developed kinetic model predicted the performance of the continuous bioreactors with accuracy.