Investigation of different nitrogen reduction routes and their key microbial players in wood chip-driven denitrification beds

Dissolved organic matter characteristics linked to bacterial community succession and nitrogen removal performance in woodchip bioreactors

Woodchip bioreactors are an eco-friendly technology for removing nitrogen (N) pollution. However, there needs to be more clarity regarding the dissolved organic matter (DOM) characteristics and bacterial community succession mechanisms and their association with the N removal performance of bioreactors. The laboratory woodchip bioreactors were continuously operated for 360 days under three influent N level treatments, and the results showed that the average removal rate of TN was 45.80 g N/(m3·day) when the influent N level was 100 mg N/L, which was better than 10 mg N/L and 50 mg N/L. Dynamic succession of bacterial communities in response to influent N levels and DOM characteristics was an important driver of TN removal rates. Medium to high N levels enriched a copiotroph bacterial module (Module 1) detected by network analysis, including Phenylobacterium, Xanthobacteraceae, Burkholderiaceae, Pseudomonas, and Magnetospirillaceae, carrying N-cycle related genes for denitrification and ammonia assimilation by the rapid consumption of DOM. Such a process can increase carbon limitation to stimulate local organic carbon decomposition to enrich oligotrophs with fewer N-cycle potentials (Module 2). Together, this study reveals that the compositional change of DOM and bacterial community succession are closely related to N removal performance, providing an ecological basis for developing techniques for N-rich effluent treatment.

Microbial succession and denitrifying woodchip bioreactor performance at low water temperatures

Mining activities are increasingly recognized for contributing to nitrogen (N) pollution and possibly also to emissions of the greenhouse gas nitrous oxide (N2O) due to undetonated, N-based explosives. A woodchip denitrifying bioreactor, installed to treat nitrate-rich leachate from waste rock dumps in northern Sweden, was monitored for two years to determine the spatial and temporal distribution of microbial communities, including the genetic potential for different N transformation processes, in pore water and woodchips and how this related to reactor N removal capacity. About 80 and 65 % of the nitrate was removed during the first and second operational year, respectively. There was a succession in the microbial community over time and in space along the reactor length in both pore water and woodchips, which was reflected in reactor performance. Nitrate ammonification likely had minimal impact on N removal efficiency due to the low production of ammonium and low abundance of the key gene nrfA in ammonifiers. Nitrite and N2O were formed in the bioreactor and released in the effluent water, although direct N2O emissions from the surface was low. That these unwanted reactive N species were produced at different times and locations in the reactor indicate that the denitrification pathway was temporally as well as spatially separated along the reactor length. We conclude that the succession of microbial communities in woodchip denitrifying bioreactors treating mining water develops slowly at low temperature, which impacts reactor performance.

The impact of perfluorooctanoic acid shock on hydrogen-driven nitrate and arsenate removal

Perfluorooctanoic acid (PFOA) is a type of toxic per- and poly-fluoroalkyl substance (PFAS) commonly found in groundwater due to its use in firefighting and industrial applications. The main purpose of this study was to investigate the influence of PFOA shock on the biological performance of a hydrogen-driven bioreactor for nitrate and arsenate removal. Four hydrogen-driven removal reactors (HdBRs) used for the simultaneous removal of nitrate and arsenal were operated with concentrations of either 0, 1, 5, and 10 mg/L of PFOA to induce shock on the systems and examine the corresponding bacterial response. Our results showed that PFOA shock inhibited and decreased the maximum hydrogen-driven arsenate removal rate. Principal Component Analysis (PCA) confirmed that this performance decrease occurred due to a bacterial strike triggered by PFOA shock. PFOA toxicity also led to protein secretion and sludge density decreases. Bacterial analyses showed shifts in the community population due to PFOA shock. The dominant bacteria phylum Proteobacteria became more abundant, from 41.24% originally to 48.29% after exposure to 10 mg/L of PFOA. Other phyla, such as Euryarchaeota and Bacteroidetes, were more tolerant to PFOA shock. Although some of the predominant species within the sludge of each HdBR exhibited a decline, other species with similar functions persisted and assumed the functional responsibilities previously held by the dominant species.

Denitrifying Woodchip Bioreactors: A Microbial Solution for Nitrate in Agricultural Wastewater-A Review

Nitrate (NO3-) is highly water-soluble and considered to be the main nitrogen pollutants leached from agricultural soils. Its presence in aquatic ecosystems is reported to cause various environmental and public health problems. Bioreactors containing microbes capable of transforming NO3- have been proposed as a means to remediate contaminated waters. Woodchip bioreactors (WBRs) are continuous flow, reactor systems located below or above ground. Below ground systems are comprised of a trench filled with woodchips, or other support matrices. The nitrate present in agricultural drainage wastewater passing through the bioreactor is converted to harmless dinitrogen gas (N2) via the action of several bacteria species. The WBR has been suggested as one of the most cost-effective NO3--removing strategy among several edge-of-field practices, and has been shown to successfully remove NO3- in several field studies. NO3- removal in the WBR primarily occurs via the activity of denitrifying microorganisms via enzymatic reactions sequentially reducing NO3- to N2. While previous woodchip bioreactor studies have focused extensively on its engineering and hydrological aspects, relatively fewer studies have dealt with the microorganisms playing key roles in the technology. This review discusses NO3- pollution cases originating from intensive farming practices and N-cycling microbial metabolisms which is one biological solution to remove NO3- from agricultural wastewater. Moreover, here we review the current knowledge on the physicochemical and operational factors affecting microbial metabolisms resulting in removal of NO3- in WBR, and perspectives to enhance WBR performance in the future.

Nitrogen Removal Capacity of Microbial Communities Developing in Compost- and Woodchip-Based Multipurpose Reactive Barriers for Aquifer Recharge With Wastewater

Global water supplies are threatened by climate changes and the expansion of urban areas, which have led to an increasing interest in nature-based solutions for water reuse and reclamation. Reclaimed water is a possible resource for recharging aquifers, and the addition of an organic reactive barrier has been proposed to improve the removal of pollutants. There has been a large focus on organic pollutants, but less is known about multifunctional barriers, that is, how barriers also remove nutrients that threaten groundwater ecosystems. Herein, we investigated how compost- and woodchip-based barriers affect nitrogen (N) removal in a pilot soil aquifer treatment facility designed for removing nutrients and recalcitrant compounds by investigating the composition of microbial communities and their capacity for N transformations. Secondary-treated, ammonium-rich wastewater was infiltrated through the barriers, and the changes in the concentration of ammonium, nitrate, and dissolved organic carbon (DOC) were measured after passage through the barrier during 1 year of operation. The development and composition of the microbial community in the barriers were examined, and potential N-transforming processes in the barriers were quantified by determining the abundance of key functional genes using quantitative PCR. Only one barrier, based on compost, significantly decreased the ammonium concentration in the infiltrated water. However, the reduction of reactive N in the barriers was moderate (between 21 and 37%), and there were no differences between the barrier types. All the barriers were after 1 year dominated by members of Alphaproteobacteria, Gammaproteobacteria, and Actinobacteria, although the community composition differed between the barriers. Bacterial classes belonging to the phylum Chloroflexi showed an increased relative abundance in the compost-based barriers. In contrast to the increased genetic potential for nitrification in the compost-based barriers, the woodchip-based barrier demonstrated higher genetic potentials for denitrification, nitrous oxide reduction, and dissimilatory reduction of nitrate to ammonium. The barriers have previously been shown to display a high capacity to degrade recalcitrant pollutants, but in this study, we show that most barriers performed poorly in terms of N removal and those based on compost also leaked DOC, highlighting the difficulties in designing barriers that satisfactorily meet several purposes.

Comparison of microbial communities in replicated woodchip bioreactors

Denitrification in woodchip bioreactors is a microbial process, but the effects of variations in bioreactors operation on microbial community structure are not well understood. Here, our goals were to understand hydraulic retention time (HRT) as a factor that influences woodchip bioreactor microbial community variation and structure in replicated field bioreactors and to evaluate relationships between microbial community membership and marker genes for denitrification. We used a combination of quantitative polymerase chain reaction of nirS, nirK, nosZI, and nosZII and 16S rRNA amplicon sequencing to characterize the microbial communities of nine pilot-scale woodchip bioreactors located at Iowa State University. Our results showed dynamic microbial communities but with persistent taxa between two sampling years and three HRTs. Similarities between functional gene copy numbers across sampling year and HRT indicate that the potential for denitrification is conserved despite differences in the microbial communities. These results are evidence that there are specific and persistent taxa within replicated bioreactors. Woodchip bioreactor microbial community membership is recommended to be the focus of future studies to better understand the relationship between microbial community functions and bioreactor management.

Microbial communities in full-scale woodchip bioreactors treating aquaculture effluents

Woodchip bioreactors are being successfully applied to remove nitrate from commercial land-based recirculating aquaculture system (RAS) effluents. In order to understand and optimize the overall function of these bioreactors, knowledge on the microbial communities, especially on the microbes with potential for production or mitigation of harmful substances (e.g. hydrogen sulfide; H₂S) is needed. In this study, we quantified and characterized bacterial and fungal communities, including potential H₂S producers and consumers, using qPCR and high throughput sequencing of 16S rRNA gene. We took water samples from bioreactors and their inlet and outlet, and sampled biofilms growing on woodchips and on the outlet of the three full-scale woodchip bioreactors treating effluents of three individual RAS. We found that bioreactors hosted a high biomass of both bacteria and fungi. Although the composition of microbial communities of the inlet varied between the bioreactors, the conditions in the bioreactors selected for the same core microbial taxa. The H₂S producing sulfate reducing bacteria (SRB) were mainly found in the nitrate-limited outlets of the bioreactors, the main groups being deltaproteobacterial Desulfobulbus and Desulfovibrio. The abundance of H₂S consuming sulfate oxidizing bacteria (SOB) was 5-10 times higher than that of SRB, and SOB communities were dominated by Arcobacter and other genera from phylum Epsilonbacteraeota, which are also capable of autotrophic denitrification. Indeed, the relative abundance of potential autotrophic denitrifiers of all denitrifier sequences was even 54% in outlet water samples and 56% in the outlet biofilm samples. Altogether, our results show that the highly abundant bacterial and fungal communities in woodchip bioreactors are shaped through the conditions prevailing within the bioreactor, indicating that the bioreactors with similar design and operational settings should provide similar function even when conditions in the preceding RAS would differ. Furthermore, autotrophic denitrifiers can have a significant role in woodchip biofilters, consuming potentially produced H₂S and removing nitrate, lengthening the operational age and thus further improving the overall environmental benefit of these bioreactors.

Resistance genes and extracellular proteins relieve antibiotic stress on the anammox process

The anaerobic ammonium oxidation (anammox) process is regarded as a promising approach to treat antibiotic-containing wastewater. Therefore, it is urgent to elucidate the effects of various antibiotics on the anammox process. Moreover, the mechanism of extracellular polymeric substance (EPS) as protective barriers to relieve antibiotic stress remain unclear. Therefore, the single and combined effects of erythromycin (ETC) and sulfamethoxazole (SMZ), and interactions between EPS and antibiotics were investigated in this study. Based on a 228-day continuous flow experiment, high concentrations of ETC and SMZ had significant inhibitory effects on the nitrogen removal performance of the anammox process, with the abundances of corresponding antibiotic resistance genes (ARGs) increasing. In addition, the combined inhibitory effect of the two antibiotics on the anammox process was more significant and longer-lasting than that of the single. However, the anammox process was able to quickly recover from deterioration. The tolerance of anammox granules to the stress of low-concentration antibiotics was probably attributed to the increase in ARGs and secretion of EPS. Molecular docking simulation results showed that proteins in EPS could directly bind with SMZ and ETC at the sites of GLU-307, HYS-191, ASP-318 and THR-32, respectively. These findings improved our understanding of various antibiotic effects on the anammox process and the interaction mechanism between antibiotics and proteins in EPS.

Microbiome Structure and Function in Woodchip Bioreactors for Nitrate Removal in Agricultural Drainage Water

Woodchip bioreactors are increasingly used to remove nitrate (NO₃^–) from agricultural drainage water in order to protect aquatic ecosystems from excess nitrogen. Nitrate removal in woodchip bioreactors is based on microbial processes, but the microbiomes and their role in bioreactor efficiency are generally poorly characterized. Using metagenomic analyses, we characterized the microbiomes from 3 full-scale bioreactors in Denmark, which had been operating for 4–7 years. The microbiomes were dominated by Proteobacteria and especially the genus Pseudomonas, which is consistent with heterotrophic denitrification as the main pathway of NO₃^– reduction. This was supported by functional gene analyses, showing the presence of the full suite of denitrification genes from NO₃^– reductases to nitrous oxide reductases. Genes encoding for dissimilatory NO₃^– reduction to ammonium were found only in minor proportions. In addition to NO₃^– reducers, the bioreactors harbored distinct functional groups, such as lignocellulose degrading fungi and bacteria, dissimilatory sulfate reducers and methanogens. Further, all bioreactors harbored genera of heterotrophic iron reducers and anaerobic iron oxidizers (Acidovorax) indicating a potential for iron-mediated denitrification. Ecological indices of species diversity showed high similarity between the bioreactors and between the different positions along the flow path, indicating that the woodchip resource niche was important in shaping the microbiome. This trait may be favorable for the development of common microbiological strategies to increase the NO₃^– removal from agricultural drainage water.

Inhibition effects of long-term calcium-magnesia phosphate fertilizer application on Cd uptake in rice: Regulation of the iron-nitrogen coupling cycle driven by the soil microbial community

Cadmium (Cd) pollution in paddy soil seriously endangers food safety production. To investigate the effects and microbiological mechanisms of calcium-magnesium-phosphate (CMP) fertilizer application on Cd reduction in rice, field experiments were conducted in Cd-contaminated paddy soil. Compared with conventional compound fertilizer, CMP fertilizer treatments inhibited Cd uptake through plant roots, significantly decreasing Cd content in rice grains from 0.340 to 0.062 mg/kg. Soil pH and total Ca, Mg and P contents increased after CMP fertilizer application, resulting in a further decrease in soil available Cd content from 0.246 to 0.181 mg/kg. Specific extraction analysis recorded a decrease in both available Fe content and the ratio of nitrate to ammonium nitrogen, indicating that the soil Fe-N cycle was affected by the addition of CMP fertilizer. This finding was also recorded using soil bacterial community sequencing, with CMP fertilizer promoting the progress of nitrate-dependent Fe-oxidation driven by Thiobacillus (1.60-2.83%) and subsequent dissimilatory nitrate reduction to ammonium (DNRA) driven by Ignavibacteriae (1.01-1.92%); Fe-reduction driven by Anaeromyxobacter (3.09-2.23%) was also inhibited. Our results indicate that CMP fertilizer application regulates the Fe-N coupling cycle driven by the soil microbial community to benefit remediation of Cd contaminated paddy soil.

Temperature Sensitivity and Composition of Nitrate-Reducing Microbiomes from a Full-Scale Woodchip Bioreactor Treating Agricultural Drainage Water

Denitrifying woodchip bioreactors (WBR), which aim to reduce nitrate (NO3-) pollution from agricultural drainage water, are less efficient when cold temperatures slow down the microbial transformation processes. Conducting bioaugmentation could potentially increase the NO3- removal efficiency during these specific periods. First, it is necessary to investigate denitrifying microbial populations in these facilities and understand their temperature responses. We hypothesized that seasonal changes and subsequent adaptations of microbial populations would allow for enrichment of cold-adapted denitrifying bacterial populations with potential use for bioaugmentation. Woodchip material was sampled from an operating WBR during spring, fall, and winter and used for enrichments of denitrifiers that were characterized by studies of metagenomics and temperature dependence of NO3- depletion. The successful enrichment of psychrotolerant denitrifiers was supported by the differences in temperature response, with the apparent domination of the phylum Proteobacteria and the genus Pseudomonas. The enrichments were found to have different microbiomes' composition and they mainly differed with native woodchip microbiomes by a lower abundance of the genus Flavobacterium. Overall, the performance and composition of the enriched denitrifying population from the WBR microbiome indicated a potential for efficient NO3- removal at cold temperatures that could be stimulated by the addition of selected cold-adapted denitrifying bacteria.

Substrate type determines microbial activity and community composition in bioreactors for nitrate removal by denitrification at low temperature

High levels of nitrogen originating from blasting operations, for example at mining sites or quarries, risk contaminating water bodies through leaching from waste rock dumps. Woodchip bioreactors can be a simple and cost-effective way of reducing nitrate concentrations in the leachate. In this study we investigated how bottle sedge, barley straw, and pine woodchips used as electron donors for denitrification influenced microbial community composition and nitrate removal in lab-scale bioreactors during 270 days. The reactors were operated to ensure that nitrate was never limiting and to achieve similar nitrate removal (%). Distinct bacterial communities developed due to the different substrates, as determined by sequencing of the 16S rRNA gene. Sedge and straw reactors shared more taxa with each other than with woodchips and throughout the experimental period, sedge and straw were more diverse than woodchips. Cellulose degrading bacteria like Fibrobacteres and Verrucomicrobia were detected in the substrates after 100-150 days of operation. Nitrate removal rates were highest in the sedge and straw reactors. After initial fluctuations, these reactors removed 5.1-6.3 g N m^-3 water day^-1, which was 3.3-4.4 times more than in the woodchip reactors. This corresponded to 48%, 42%, and 44% nitrate removal for the sedge, straw, and woodchip reactors respectively. The functional communities were characterized by quantitative PCR and denitrification was the major nitrate removing process based on genetic potential and water chemistry, although sedge and straw developed a capacity for ammonification. Gene ratios suggested that denitrification was initially incomplete and terminating with nitrous oxide. An increase in abundances of nitrous oxide reducing capacity in all substrate types towards the end increased the potential for less emissions of the greenhouse gas nitrous oxide.

Microbial controls on net production of nitrous oxide in a denitrifying woodchip bioreactor

Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO₃ ^- ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO₂ ^- ), nitrous oxide (N₂ O), or ammonium (NH₄ ⁺ ). To identify the factors controlling the relative accumulation of NO₂ ^- , N₂ O, and/or NH₄ ⁺ in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO₃ ^- from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N₂ O reduction, and dissimilatory NO₃ ^- reduction to NH₄ ⁺ were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N₂ O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N₂ O. Temperature and the total organic carbon/NO₃ ^- ratio were strongly correlated with NH₄ ⁺ concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO₃ ^- transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6 d demonstrate the temporal development in the microbial community and indicate an increased potential for N₂ O emissions with time from the denitrifying woodchip bioreactor.

Nitrate removal microbiology in woodchip bioreactors: A case-study with full-scale bioreactors treating aquaculture effluents

Woodchip bioreactors are viable low-cost nitrate (NO₃^-) removal applications for treating agricultural and aquaculture discharges. The active microbial biofilms growing on woodchips are conducting nitrogen (N) removal, reducing NO₃^- while oxidizing the carbon (C) from woodchips. However, bioreactor age, and changes in the operating conditions or in the microbial community might affect the NO₃^- removal as well as potentially promote nitrous oxide (N₂O) production through either incomplete denitrification or dissimilatory NO₃^- reduction to ammonium (DNRA). Here, we combined stable isotope approach, amplicon sequencing, and captured metagenomics for studying the potential NO₃^- removal rates, and the abundance and community composition of microbes involved in N transformation processes in the three different full-scale woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. We confirmed denitrification producing di‑nitrogen gas (N₂) to be the primary NO₃^- removal pathway, but found that 6% of NO₃^- could be released as N₂O under high NO₃^- concentrations and low amounts of bioavailable C, whereas DNRA rates tend to increase with the C amount. The abundance of denitrifiers was equally high between the studied bioreactors, yet the potential NO₃^- removal rates were linked to the denitrifying community diversity. The same core proteobacterial groups were driving the denitrification, while Bacteroidetes dominated the DNRA carrying microbes in all the three bioreactors studied. Altogether, our results suggest that woodchip bioreactors have a high genetic potential for NO₃^- removal through a highly abundant and diverse denitrifying community, but that the rates and dynamics between the NO₃^- removal pathways depend on the other factors (e.g., bioreactor design, operating conditions, and the amount of bioavailable C in relation to the incoming NO₃^- concentrations).

Characterization and comparison of groundwater quality and redox conditions in the Arakawa Lowland and Musashino Upland, southern Kanto Plain of the Tokyo Metropolitan area, Japan

Groundwater is essential for the Earth biosphere but is often contaminated by harmful chemical compounds due to both anthropogenic and natural causes. A key factor controlling the fate of harmful chemicals in groundwater is the reduction/oxidation (redox) conditions. The formation factors for the groundwater redox conditions are insufficiently understood. In this study, long-term groundwater quality beneath one of the world megacities was monitored and evaluated. We measured and compared hydrogeochemical conditions including groundwater quality (35 chemical parameters) and redox conditions of five aquifers in the Arakawa Lowland and Musashino Upland, southern Kanto Plain of the Tokyo Metropolitan area, Japan. Monitoring results suggested the following: The main origin of groundwater is precipitation in both the Lowland and Upland areas. The three aquifers in the Arakawa Lowland are likely fully separated, with one unconfined and two confined aquifers under iron reducing and methanogenic conditions, respectively. Oppositely, in the Musashino Upland, the water masses in the two aquifers are likely partly connected, under aerobic conditions, and undergoing the same groundwater recharge and flow processes under similar hydrogeological conditions. The different groundwater redox conditions observed are likely caused by the very different groundwater residence times for the Arakawa Lowland and Musashino Upland.

Agricultural management and plant selection interactively affect rhizosphere microbial community structure and nitrogen cycling

BACKGROUND

Rhizosphere microbial communities are key regulators of plant performance, yet few studies have assessed the impact of different management approaches on the rhizosphere microbiomes of major crops. Rhizosphere microbial communities are shaped by interactions between agricultural management and host selection processes, but studies often consider these factors individually rather than in combination. We tested the impacts of management (M) and rhizosphere effects (R) on microbial community structure and co-occurrence networks of maize roots collected from long-term conventionally and organically managed maize-tomato agroecosystems. We also explored the interaction between these factors (M × R) and how it impacts rhizosphere microbial diversity and composition, differential abundance, indicator taxa, co-occurrence network structure, and microbial nitrogen-cycling processes.

RESULTS

Host selection processes moderate the influence of agricultural management on rhizosphere microbial communities, although bacteria and fungi respond differently to plant selection and agricultural management. We found that plants recruit management-system-specific taxa and shift N-cycling pathways in the rhizosphere, distinguishing this soil compartment from bulk soil. Rhizosphere microbiomes from conventional and organic systems were more similar in diversity and network structure than communities from their respective bulk soils, and community composition was affected by both M and R effects. In contrast, fungal community composition was affected only by management, and network structure only by plant selection. Quantification of six nitrogen-cycling genes (nifH, amoA [bacterial and archaeal], nirK, nrfA, and nosZ) revealed that only nosZ abundance was affected by management and was higher in the organic system.

CONCLUSIONS

Plant selection interacts with conventional and organic management practices to shape rhizosphere microbial community composition, co-occurrence patterns, and at least one nitrogen-cycling process. Reframing research priorities to better understand adaptive plant-microbe feedbacks and include roots as a significant moderating influence of management outcomes could help guide plant-oriented strategies to improve productivity and agroecosystem sustainability.

NO3 - removal efficiency in field denitrification beds: key controlling factors and main implications

Nitrate pollution is a growing environmental threat that affects both ground- and surface-water. The typically used technique for nitrate elimination in wastewater treatment plants cannot be applied for all water streams as it necessitates a highly developed technical infrastructure. Field denitrification beds comprise one strategy to treat surface water containing high nitrate loads, which typically is due to the increasing agricultural land use. Here, the water passes through a basin containing a cheap carbon material as electron donor that provides the environmental niche for a complex microbial biocenosis. The microorganisms catalyse the hydrolysis of the polymeric organic carbon substrate and a variety of fermentative and respiratory pathways that are in the end supposed to lead to an efficient denitrification process. This review article integrates our current knowledge on environmental and operating parameters of and within denitrification beds including biotic and abiotic factors influencing the nitrate removal efficiency. Steering of these two factors can allow to minimise pollution swapping and the formation of greenhouse gases.

Efficient Bioelectrochemical Conversion of Industrial Wastewater by Specific Strain Isolation and Community Adaptation

The aim of this study was the development of a specifically adapted microbial community for the removal of organic carbon from an industrial wastewater using a bioelectrochemical system. In a first step, ferric iron reducing microorganisms were isolated from the examined industrial wastewater. In a second step, it was tested to what extent these isolates or a cocultivation of the isolates with the exoelectrogenic model organism Geobacter sulfurreducens (G. sulfurreducens) were able to eliminate organic carbon from the wastewater. To establish a stable biofilm on the anode and to analyze the performance of the system, the experiments were conducted first under batch-mode conditions for 21 days. Since the removal of organic carbon was relatively low in the batch system, a similar experiment was conducted under continuous-mode conditions for 65 days, including a slow transition from synthetic medium to industrial wastewater as carbon and electron source and variations in the flow rate of the medium. The overall performance of the system was strongly increased in the continuous- compared to the batch-mode reactor and the highest average current density (1,368 mA/m2) and Coulombic efficiency (54.9%) was measured in the continuous-mode reactor inoculated with the coculture consisting of the new isolates and G. sulfurreducens. The equivalently inoculated batch-mode system produced only 82-fold lower current densities, which were accompanied by 42-fold lower Coulombic efficiencies.

Influence of the Potential Carbon Sources for Field Denitrification Beds on Their Microbial Diversity and the Fate of Carbon and Nitrate

Nitrogen based eutrophication of ecosystems is a global problem that gains momentum through a growing global population. The water quality of nitrate or ammonium contaminated rivers and streams cannot always be amended in centralized waste water treatment plants. Field denitrification plants were suggested as a solution for a decentralized reduction of nitrate to dinitrogen. Here, stable and cheap organic carbon sources serve as carbon and electron source for a microbial community. Still, our knowledge on the impact of these organic carbon sources on the development and diversity of these cultures is sparse. Moreover, the stability of these denitrification plants at different nitrate loading rates especially in the higher concentration regime were not tested so far. In this study, we compare the fate of carbon and nitrogen as well as the microbial community of wood pellet (WP) (pressed sawdust), wheat straw, and wood chips (WC) based laboratory denitrification reactors. Our study reveals that the diversity and composition of the community is strongly dependent on the carbon source. The diversity decreased in the order WC, wheat straw, and WPs. The three reactor types were characterized by different nitrate reduction kinetics and were affected differently by high nitrate loading rates. While the nitrate reduction kinetics were negatively influenced by higher nitrate doses in the wheat straw reactors, WPs as carbon source sustained the opposite trend and WC lead to an overall slower but concentration independent nitrate reduction rate. Counterintuitively, the concentration of soluble organic carbon was highest in the WP reactors but methane emission was not detectable. This is corroborated by the microbial diversity data in which methanogenic species were highly underrepresented compared to the other two reactor types. In contrary, the methane emissions in the wheat straw and WC reactors were comparable to each other.