Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) as a technology platform for greenhouse gas abatement in wastewater treatment plants: State-of-the-art and challenges.

Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) is a metabolic process recently discovered and partially characterized in terms of the microorganisms and pathways involved. The N-AOM process can be a powerful tool for mitigating the impacts of greenhouse gas emissions from wastewater treatment plants by coupling the reduction of nitrate or nitrite with the oxidation of residual dissolved methane. Besides specific anaerobic methanotrophs such as bacteria members of the phylum NC10 and archaea belonging to the lineage ANME-2d, recent reports suggested that other methane-oxidizing bacteria in syntrophy with denitrifiers can also perform the N-AOM process, which facilitates the application of this metabolic process for the oxidation of residual methane under realistic scenarios. This work constitutes a state-of-art review that includes the fundamentals of the N-AOM process, new information on process microbiology, bioreactor configurations, and operating conditions for process implementation in WWTP. Potential advantages of the N-AOM process over aerobic methanotrophic biotechnologies are presented, including the potential interrelation of the N-AOM with other nitrogen removal processes within the WWTP, such as the anaerobic ammonium oxidation. This work also addressed the challenges of this biotechnology towards its application at full scale, identifying and discussing critical research niches.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities.

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H2S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N2O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Multi-Probe Measurement System Based on Single-Cut Transformation for Fast Testing of Linear Arrays.

This paper introduces a near-field measurement system concept for the fast testing of linear arrays suited for mass production scenarios where a high number of nominally identical antennas needs to be measured. The proposed system can compute the radiation pattern, directivity and gain on the array plane, as well as the array complex feeding coefficients in a matter of seconds. The concept is based on a multi-probe antenna array arranged in a line which measures the near field of the antenna under test in its array plane. This linear measurement is postprocessed with state-of-the-art single-cut transformation techniques. To compensate the lack of full 3D information, a previous complete characterization of a "Gold Antenna" is performed. This antenna is nominally identical to the many ones that will be measured with the proposed system. Therefore, the data extracted from this full characterization can be used to complement the postprocessing steps of the single-cut measurements. An X-band 16-probe demonstrator of the proposed system is implemented and introduced in this paper, explaining all the details of its architecture and operation steps. Finally, some measurement results are given to compare the developed demonstrator with traditional anechoic measurements, and show the potential capabilities of the proposed concept to perform fast and reliable measurements.

Continuous anaerobic oxidation of methane: Impact of semi-continuous liquid operation and nitrate load on N2O production and microbial community.

This work proves the feasibility of employing regular secondary activated sludge for the enrichment of a microbial community able to perform the anaerobic oxidation of methane coupled to nitrate reduction (N-AOM). After 96 days of activated sludge enrichment, a clear N-AOM activity was observed in the resulting microbial community. The methane removal potential of the enriched N-AOM culture was then studied in a stirred tank reactor (STR) operated in continuous mode for methane supply and semi-continuous mode for the liquid phase. The effect of applying nitrate loads of ∼22, 44, 66, and 88 g NO3- m-3 h-1 on (i) STR methane and nitrate removal performance, (ii) N2O emission, and (iii) microbial composition was investigated. Methane elimination capacities from 21 ± 13.3 to 55 ± 12 g CH4 m-3 h-1 were recorded, coupled to nitrate removal rates ranging from 6 ± 3.2 to 43 ± 14.9 g NO3- m-3 h-1. N2O production was not detected under the three nitrate loading rates applied for the assessment of potential N2O emission in the continuous N-AOM process (i.e. ∼22-66 g NO-3 m-3 h-1). The lack of N2O emissions during the process was attributed to the N2O reducing capacity of the bacterial taxa identified and the rigorous control of dissolved O2 and pH implemented (dissolved O2 values ≤ 0.07 g m-3 and pH of 7.6 ± 0.4). Microbial characterization showed that the N-AOM process was performed in absence of putative N-AOM archaea and bacteria (ANME-2d, M. oxyfera). Instead, microbial activity was driven by methane-oxidizing bacteria and denitrifying bacteria (Bacteroidetes, α-, and γ-proteobacteria).

H2S oxidation coupled to nitrate reduction in a two-stage bioreactor: Targeting H2S-rich biogas desulfurization.

A two-stage bioreactor operated under anoxic denitrifying conditions was evaluated for desulfurization of synthetic biogas laden with H2S concentrations between 2500 and 10,000 ppmv. H2S removal efficiencies higher than 95% were achieved for H2S loads ranging from 16.2 to 51.9 gS mliquid-3h-1. Average H2S oxidation performance (fraction of S-SO42- produced per gram of S-H2S absorbed) ranged between 8.2 ± 1.2 and 18.7 ± 5.3% under continuous liquid operation. Nitrogen mass balance showed that only 2-6% of the N-NO3- consumed was directed to biomass growth and the rest was directed to denitrification. Significant changes in the bacterial community composition did not hinder the H2S removal efficiency. The bioreactor configuration proposed avoided clogging issues due to elemental sulfur accumulation as commonly occurs in packed bed bioreactors devoted to H2S-rich biogas desulfurization.

Reduced graphene oxide decorated with magnetite nanoparticles enhance biomethane enrichment.

The addition of magnetite nanoparticles (MNPs), reduced graphene oxide (rGO), and reduced graphene oxide decorated with magnetite nanoparticles (rGO-MNPs) was evaluated during biomethane enrichment process. rGO-MNPs presented the highest beneficial impact on the hydrogenotrophic assays with an improvement of 47 % in CH4 production. The improvement was linked to the increase of the electron shuttling capacity (ESC) by rGO-MNPs addition, which boosted the hydrogenotrophic activity of microorganisms, to the rGO and rGO-MNPs, which served as reservoirs of hydrogen, improving H⁠2 transport from the gas to the liquid phase, and to the iron ions released, which acted as a dietary supply for microorganisms. Raman and XRD confirmed a greater disorder and lower crystallinity of rGO-MNPs after the hydrogenotrophic assays, with a lower effect at a nanoparticle concentration of 50 mg/L. Moreover, FTIR analysis indicated that rGO-MNPs were oxidized during the hydrogenotrophic tests. This study highlights the advantages of adding rGO-MNPs as a magnetic nanocomposite. Furthermore, rGO-MNPs can be easily recovered, minimizing their release to the environment.

Evaluation of bioaerosols by flow cytometry and removal performance in a biofilter treating toluene/ethyl acetate vapors.

The removal efficiency (RE) and bioaerosol emission of a perlite biofilter treating vapors of toluene (T) and/or ethyl acetate (EA) were assessed, under different operating conditions, during 171 days. Under the first stages of operation, a mixture of EA and T was treated, with equivalent inlet loads (ILs) of each compound (ranging from 26 to 84 g m-3 h-1), achieving a 100% RE of EA, and a maximum elimination capacity (EC) of T of 58.7 g m-3 h-1. An inhibition of T removal was noted in presence of EA, as T was treated subsequently to EA, along biofilter depth. A 17 days starvation period induced no global deterioration of performance regarding EA removal, but a 50% lower RE of T. Suspension of one contaminant, with interspersed feeding of only one component of the mixture, caused a permanent drop of the RE of EA (to 87.3%), after a T only feeding of 41 days. Flow cytometry (FC) was applied for quantification of bioaerosols, allowing for differentiation between viable, dead and damaged cells. During the overall biofilter operation, bioaerosol emission was not statistically different from bioaerosol retention. However, the biofilter significantly emitted bioaerosols (mostly viable cells) during start-up and IL increase, whereas a global retention of dead cells was observed during the interspersed feeding of one contaminant. Bioaerosols measured by FC (107 Cells m-3) were three orders of magnitude greater than with plate counting dishes, indicating that FC does not underestimate bioaerosols as culture dependent techniques.

Theoretical framework for the estimation of H2S concentration in biogas produced from complex sulfur-rich substrates.

A theoretical framework was developed and validated for the estimation of H2S concentration in biogas produced from complex sulfur-rich effluents. The modeling approach was based on easy-to-obtain data such as biological biogas potential (BBP), chemical oxygen demand, and total sulfur content. Considering the few data required, the model fitted well the experimental H2S concentrations obtained from BBP tests and continuous bioreactors reported in the literature. The model supported a correlation coefficient (R2) of 0.989 over the experimental data, obtaining average and maximum errors of ~ 25 and ~ 35%, respectively. The theoretical framework yielded good estimations for a wide range of experimental H2S concentrations (0.2 to 4.5% in biogas). This modeling approach is, therefore, a useful tool towards anticipating the H2S concentration in biogas produced from sulfur-rich substrates and deciding whether the installation of a desulfurization technology is required or not.

Ammonium influences kinetics and structure of methanotrophic consortia.

The literature is conflicted on the influence of ammonium on the kinetics and microbial ecology of methanotrophy. In this study, methanotrophic cultures were enriched, under ammonium concentrations ranging from 0 to 200 mM, from an inoculum comprising leachate and top-cover soil from a landfill. Specific CH4 biodegradation rates were highest (7.8 × 10-4 ±â€¯6.0 × 10-5 gCH4 gX-1 h-1) in cultures enriched at 4 mM NH4+, which were mainly dominated by type II methanotrophs belonging to Methylocystis spp. Lower specific CH4 oxidation rates (average values of 1.8-3.6 × 10-4 gCH4 gX-1 h-1) were achieved by cultures enriched at higher NH4+ concentrations (20 and 80 mM), and had higher affinity for CH4 compared to 4 mM enrichments. These lower affinities were attributed to lower diversity dominated by type I methanotrophs, of the Methylosarcina, Methylobacter and Methylomicrobium genera, encountered with increasing concentrations of NH4+. The study indicates that CH4 oxidation biotechnologies applied at low NH4+ concentrations can support efficient abatement of CH4 and high diversity of methanotrophic consortia, whilst enriching type II methanotrophs.

A state-of-the-art review on nitrous oxide control from waste treatment and industrial sources.

This review aims at holistically analyzing the environmental problems associated with nitrous oxide (N2O) emissions by evaluating the most important sources of N2O and its environmental impacts. Emissions from wastewater treatment processes and the industrial production of nitric and adipic acid represent nowadays the most important anthropogenic point sources of N2O. Therefore, state-of-the-art strategies to mitigate the generation and release to the atmosphere of this greenhouse and O3-depleting gas in the waste treatment and industrial sectors are also reviewed. An updated review of the end-of-the-pipe technologies for N2O abatement, both in the waste treatment and industrial sectors, is herein presented and critically discussed for the first time. Despite the consistent efforts recently conducted in the development of cost-efficient and eco-friendly N2O abatement technologies, physical/chemical technologies still constitute the most popular treatments for the control of industrial N2O emissions at commercial scale. The recent advances achieved on biological N2O abatement based on heterotrophic denitrification have opened new opportunities for the development of eco-friendly alternatives for the treatment of N2O emissions. Finally, the main limitations and challenges faced by these novel N2O abatement biotechnologies are identified in order to pave the way for market implementation.

From mesophilic to thermophilic conditions: one-step temperature increase improves the methane production of a granular sludge treating agroindustrial effluents.

OBJECTIVES: To assess the effect of one-step temperature increase, from 35 to 55 °C, on the methane production of a mesophilic granular sludge (MGS) treating wine vinasses and the effluent of a hydrogenogenic upflow anaerobic sludge blanket (UASB) reactor. RESULTS: One-step temperature increase from mesophilic to thermophilic conditions improved methane production regardless of the substrate tested. The biomethane potentials obtained under thermophilic conditions were 1.8-2.9 times higher than those obtained under mesophilic conditions. The MGS also performed better than an acclimated thermophilic digestate, producing 2.2-2.5 times more methane than the digestate under thermophilic conditions. Increasing the temperature from 35 to 55 °C also improved the methane production rate of the MGS (up to 9.4 times faster) and reduced the lag time (up to 1.9 times). Although the temperature increase mediated a decrease in the size of the sludge granules, no negative effects on the performance of the MGS was observed under thermophilic conditions. CONCLUSIONS: More methane is obtained from real agroindustrial effluents at thermophilic conditions than under mesophilic conditions. One-step temperature increase (instead of progressive sequential increases) can be used to implement the thermophilic anaerobic digestion processes with MGS.

Biotrickling filter modeling for styrene abatement. Part 2: Simulating a two-phase partitioning bioreactor.

A dynamic model describing styrene abatement was developed for a two-phase partitioning bioreactor operated as a biotrickling filter (TPPB-BTF). The model was built as a coupled set of two different systems of partial differential equations depending on whether an irrigation or a non-irrigation period was simulated. The maximum growth rate was previously calibrated from a conventional BTF treating styrene (Part 1). The model was extended to simulate the TPPB-BTF based on the hypothesis that the main change associated with the non-aqueous phase is the modification of the pollutant properties in the liquid phase. The three phases considered were gas, a water-silicone liquid mixture, and biofilm. The selected calibration parameters were related to the physical properties of styrene: Henry's law constant, diffusivity, and the gas-liquid mass transfer coefficient. A sensitivity analysis revealed that Henry's law constant was the most sensitive parameter. The model was successfully calibrated with a goodness of fit of 0.94. It satisfactorily simulated the performance of the TPPB-BTF at styrene loads ranging from 13 to 77 g C m-3 h-1 and empty bed residence times of 30-15 s with the mass transfer enhanced by a factor of 1.6. The model was validated with data obtained in a TPPB-BTF removing styrene continuously. The experimental outlet emissions associated to oscillating inlet concentrations were satisfactorily predicted by using the calibrated parameters. Model simulations demonstrated the potential improvement of the mass-transfer performance of a conventional BTF degrading styrene by adding silicone oil.

Microalgal-bacterial aggregates: Applications and perspectives for wastewater treatment.

Research on wastewater treatment by means of microalgal-bacterial processes has become a hot topic worldwide during the last two decades. Owing to the lower energy demand for oxygenation, the enhanced nutrient removal and the potential for resource recovery, microalgal-based technologies are nowadays considered as a good alternative to conventional activated sludge treatments in many instances. Nevertheless, biomass harvesting still constitutes one of the major challenges of microalgal-bacterial systems for wastewater treatment, which is hindered by the poor settleability of microalgal biomass. In this review, the use of microalgal-bacterial aggregates (MABAs) to overcome harvesting issues and to enhance resource recovery is presented. The fundamentals of MABAs-based technologies, the operational strategies and factors affecting the formation of MABAs, the microbiology and the methanogenic potential of the aggregates are addressed and critically discussed. The most recent findings and the challenges facing this technology towards its consolidation are also presented.

Biogas-based denitrification in a biotrickling filter: Influence of nitrate concentration and hydrogen sulfide.

The feasibility of NO3- removal by the synergistic action of a prevailing denitrifying anoxic methane oxidising (DAMO), and nitrate-reducing and sulfide-oxidising bacterial (NR-SOB) consortium, using CH4 and H2 S from biogas as electron donors in a biotrickling filter was investigated. The influence of NO3- concentration on N2 O production during this process was also evaluated. The results showed that NO3- was removed at rates up to 2.8 g mreactor-3 h-1 using CH4 as electron donor. N2 O production rates correlated with NO3- concentration in the liquid phase, with a 10-fold increase in N2 O production as NO3- concentration increased from 50 to 200 g m-3 . The use of H2 S as co-electron donor resulted in a 13-fold increase in NO3- removal rates (∼18 gNO3- m-3 h-1 ) and complete denitrification under steady-state conditions, which was supported by higher abundances of narG, nirK, and nosZ denitrifying genes. Although the relative abundance of the DAMO population in the consortium was reduced from 60% to 13% after H2 S addition, CH4 removals were not compromised and H2 S removal efficiencies of 100% were achieved. This study confirmed (i) the feasibility of co-oxidising CH4 and H2 S with denitrification, as well as (ii) the critical need to control NO3- concentration to minimize N2 O production by anoxic denitrifiers. Biotechnol. Bioeng. 2017;114: 665-673. © 2016 Wiley Periodicals, Inc.

Enhancing the biomethane potential of liquid dairy cow manure by addition of solid manure fractions.

OBJECTIVES: To assess the effect of adding solid manure fractions on the biomethane potential (BMP) of liquid dairy cow manure and on the biokinetic parameters of the process. RESULTS: The methanogenic potential of liquid dairy cow manure was strongly effected by adding a solid manure fraction. The 90/10 % (w/w) liquid/solid manure fraction mixture was the best substrate for CH4 production. This substrate mixture improved by 50 % the final CH4 production per g substrate and decreased the lag time by 220 % relative to the reference BMP test without the addition. Moreover, the addition of 20 % solid manure fraction adversely affected both the final CH4 production and the maximum methane production rate, while increased the lag time by 400 % compared to the reference BMP test without addition. CONCLUSIONS: Liquid dairy cow manure should be supplemented with no more than 10 % of solid manure fraction in order to improve the biomethane potential of this important agro-industrial residue.

A fundamental study on biological removal of N2O in the presence of oxygen.

The biodegradation of N2O by a non-acclimated secondary activated sludge in the presence of O2 was studied. Batch tests with a headspace containing an initial N2O concentration of ∼400 mg m(-3) (∼200 ppmv) and initial O2 gas concentrations of 0%, 1%, 2%, 5% and 21% were investigated. The effect of O2 on the biokinetic parameters qmax (maximum specific N2O uptake rate) and KS (half-saturation constant), as well as on the bacterial population structure, was evaluated. A complete N2O removal was recorded in the presence of up to 2% O2, while O2 at 5% and 21% mediated inhibitions of 37% and 95% in the removal of N2O compared with the control without O2. The elemental analysis of the biomass obtained at the end of the batch tests strongly suggested that NN2O was not used as a nitrogen source. The presence of O2 mediated decreases of up to 12.6- and 4.8-fold in qmax and KS, respectively, compared to the control without O2. Likewise, the presence of O2 induced changes in the structure of the bacterial population. The predominant microorganisms in the presence of O2 belonged to the phyla Proteobacteria, Firmicutes and Chlamydiae. Bacteria belonging to the Proteobacteria phylum, particularly the Dokdonella genus, were predominant at 2% O2, which was the highest O2 concentration without inhibitory effects on N2O biodegradation.

Exploring the potential of fungi for methane abatement: Performance evaluation of a fungal-bacterial biofilter.

Despite several fungal strains have been retrieved from methane-containing environments, the actual capacity and role of fungi on methane abatement is still unclear. The batch biodegradation tests here performed demonstrated the capacity of Graphium sp. to co-metabolically biodegrade methane and methanol. Moreover, the performance and microbiology of a fungal-bacterial compost biofilter treating methane at concentrations of ∼2% was evaluated at empty bed residence times of 40 and 20 min under different irrigation rates. The daily addition of 200 mL of mineral medium resulted in elimination capacities of 36.6 ± 0.7 g m(-3) h(-1) and removal efficiencies of ≈90% at the lowest residence time. The indigenous fungal community of the compost was predominant in the final microbial population and outcompeted the inoculated Graphium sp. during biofilter operation.

Continuous nitrous oxide abatement in a novel denitrifying off-gas bioscrubber.

The potential of a bioscrubber composed of a packed bed absorption column coupled to a stirred tank denitrification bioreactor (STR) was assessed for 95 days for the continuous abatement of a diluted air emission of N2O at different liquid recycling velocities. N2O removal efficiencies of up to 40 ± 1 % were achieved at the highest recirculation velocity (8 m h(-1)) at an empty bed residence time of 3 min using a synthetic air emission containing N2O at 104 ± 12 ppmv. N2O was absorbed in the packed bed column and further reduced in the STR at efficiencies >80 % using methanol as electron donor. The long-term operation of the bioscrubber suggested that the specialized N2O degrading community established was not able to use N2O as nitrogen source. Additional nitrification assays showed that the activated sludge used as inoculum was not capable of aerobically oxidizing N2O to nitrate or nitrite, regardless of the inorganic carbon concentration tested. Denitrification assays confirmed the ability of non-acclimated activated sludge to readily denitrify N2O at a specific rate of 3.9 mg N2O g VSS h(-1) using methanol as electron donor. This study constitutes, to the best of our knowledge, the first systematic assessment of the continuous abatement of N2O in air emission. A characterization of the structure of the microbial population in the absorption column by DGGE-sequencing revealed a high microbial diversity and the presence of heterotrophic denitrifying methylotrophs.

Treatment of O2-free toluene emissions by anoxic biotrickling filtration.

Toluene biotrickling filtration under anoxic denitrifying conditions was evaluated in two identical bioreactors (R1 and R2) operated at liquid recycling rates of 1.3, 2.7 and 5.3 m h−1 and liquid renewal rates of 0 and 0.17 d−1. R1 and R2 achieved a similar maximum elimination capacity (EC ∼30 g m−3 h−1) at the same toluene inlet load (∼50 g m−3 h−1), which was approximately 7 times higher compared with available literature on continuous toluene removal under anoxic conditions. Nevertheless, higher metabolite accumulation was observed in the bioreactor operated without periodical liquid phase renewal (R2), leading to intermittent drops in its toluene removal performance. This is the first work operating an anoxic biotrickling filter at empty bed residence time of 3 min, which is comparable with those employed in conventional aerobic systems. A characterization of the metabolites accumulated in the liquid phase revealed a dynamic metabolite production and degradation.

Assessment of methane biodegradation kinetics in two-phase partitioning bioreactors by pulse respirometry.

Biological methane biodegradation is a promising treatment alternative when the methane produced in waste management facilities cannot be used for energy generation. Two-phase partitioning bioreactors (TPPBs), provided with a non-aqueous phase (NAP) with high affinity for the target pollutant, are particularly suitable for the treatment of poorly water-soluble compounds such as methane. Nevertheless, little is known about the influence of the presence of the NAP on the resulting biodegradation kinetics in TPPBs. In this study, an experimental framework based on the in situ pulse respirometry technique was developed to assess the impact of NAP addition on the methane biodegradation kinetics using Methylosinus sporium as a model methane-degrading microorganism. A comprehensive mass transfer characterization was performed in order to avoid mass transfer limiting scenarios and ensure a correct kinetic parameter characterization. The presence of the NAP mediated significant changes in the apparent kinetic parameters of M. sporium during methane biodegradation, with variations of 60, 120, and 150% in the maximum oxygen uptake rate, half-saturation constant and maximum specific growth rate, respectively, compared with the intrinsic kinetic parameters retrieved from a control without NAP. These significant changes in the kinetic parameters mediated by the NAP must be considered for the design, operation and modeling of TPPBs devoted to air pollution control.