Gaseous End Products Research Articles

Improving understanding of carbon storage in wood in landfills: Evidence from reactor studies

Approximately 1.5 million tonnes (Mt) of wood waste are disposed of in Australian landfills annually. Recent studies have suggested that anaerobic decay levels of wood in landfills are low, although knowledge of the decay of individual wood species is limited. The objective of this study was to establish the extent of carbon loss for wood species of commercial importance in Australia including radiata pine, blackbutt, spotted gum and mountain ash. Experiments were conducted under laboratory conditions designed to simulate optimal anaerobic biodegradation in a landfill. Bacterial degradation, identified by both light microscopy and electron microscopy, occurred to a varying degree in mountain ash and spotted gum wood. Fungal decay was not observed in any wood samples. Mountain ash, the species with the highest methane yield (20.9 mL CH4/g) also had the highest holocellulose content and the lowest acid-insoluble lignin and extractive content. As the decay levels for untreated radiata pine were very low, it was not possible to determine whether impregnation of radiata pine with chemical preservatives had any impact on decay. Carbon losses estimated from gas generation were below 5% for all species tested. Carbon losses as estimated by gas generation were lower than those derived by mass balance in most reactors, suggesting that mass loss does not necessarily equate to carbon emissions. There was no statistical difference between decay of blackbutt derived from plantation and older, natural forests. Addition of paper as an easily digestible feedstock did not increase carbon loss for the two wood species tested and the presence of radiata pine had an inhibitory effect on copy paper decay. Although differences between some of the wood types were found to be statistically significant, these differences were detected for wood with carbon losses that did not exceed 5%. The suggested factor for carbon loss for wood in landfills in Australia is 1.4%. This study confirms that disposal of wood in landfills in Australia results in long-term storage of carbon, with only minimal conversion of carbon to gaseous end products.

Evidence from nuclear magnetic resonance spectroscopy of the processes of soil organic carbon accumulation under long‐term fertilizer management

SummaryLong‐term application of fertilizers has a considerable effect on the accumulation of soil organic carbon (SOC), but the underlying processes remain unclear. We have examined the effects in the soil of two long‐term (> 20 years) field experiments of a double‐cropped maize (Zea mays L.)–wheat (Triticum aestivum L.) rotation on a Calcaric Fluvisol and paddy rice (Oryza sativa L) on a Hydragric Anthrosol. The chemical structures of the SOC were characterized with multiple cross‐polarization magic‐angle spinning 13C nuclear magnetic resonance spectroscopy. The treatments included organic fertilizer (OF) only, combined NPK (nitrogen, phosphorus and potassium) fertilizer with OF (NPKOF), mineral fertilizer pairings of NPK, NP and NK, and an unamended control. The continuous fertilizer treatments did not change the chemical composition of the Anthrosol SOC greatly except that the NPKOF treatment slightly enriched aromatic C and depleted O–alkyl C compared with the Control. All the treatments on the Anthrosol soil resulted in SOC accumulation, but to different extents. Principal component analysis of the Anthrosol and Fluvisol showed different relationships between the soil properties and functional groups. The Fluvisol SOC decreased in the order balanced fertilizer treatments (OF, NPKOF and NPK), NP, Control and NK treatments. The main structural changes of the Fluvisol SOC followed the same order with a decrease in the abundance of aromatic CO, aromatic C, anomeric C, O–alkyl C and OCH3/NCH, and an increase in COO/NCO and alkyl C, which led to an increase in alkyl–C/O–alkyl–C ratios and a decrease in aromaticity values. These results suggest that SOC accumulation with balanced fertilization on the Fluvisol can be attributed to less decomposition and more humification of the organic materials, whereas the smallest SOC content associated with P omission can be ascribed to more complete decomposition to gaseous end products. We consider that the chemical compositions of SOC may be altered by long‐term fertilizer treatments, depending on the soil type.Highlights How does soil organic carbon (SOC) change in response to long‐term fertilizer management? Improved understanding of SOC accumulation in Fluvisol and Anthrosol agricultural soils. Phosphorus omission in the Fluvisol retarded the accumulation and stimulated the decomposition of SOC. Fertilizer treatments affected chemical composition of SOC of the Fluvisol, but not the Anthrosol.

Anaerobic digestion without biogas?

Anaerobic digestion for the production of methane containing biogas is the classic example of a resource recovery process that combines stabilization of particulate organic matter or wastewater treatment with the production of a valuable end-product. Attractive features of the process include the production of a single end-product from a heterogeneous feedstock, and in-situ product separation of the gaseous end-product. Despite these intrinsic attractive properties of the process, the economic added value of the biogas produced is limited, enabling the development of alternative processes that yield higher-value end-products. Typically the production of higher value end-products from low value feedstock and industrial wastewater proceeds via intermediate production of organic acids (and carbon dioxide and molecular hydrogen). Optimization of organic acid production from particulate feedstocks and wastewater for development of the organic acid based resource recovery route receives significant research attention. The organic acid stream generated as such, has no economic value, but if organic acids can either be concentrated via membrane separation or (bio)converted to an end-product that can easily be separated from the liquid, an attractive biomass processing scheme can be developed. Attractive end-products of organic acid processing include polyhydroxyalkanoates, medium chain length fatty acids, or other organic molecules using bio-electrochemical systems. Overall we suggest that these novel bioprocessing routes for conversion of low value feedstock to higher added value products will contribute to a sustainable future and will change the economic status of organic waste.

Temperature effects on N2O and N2 denitrification end-products for a New Zealand pasture soil

Denitrification results in the formation of two gaseous end-products: radiatively active greenhouse gas nitrous oxide (N2O) and radiatively inert di-nitrogen (N2). Measuring N2 produced by soil microorganisms has historically been difficult because of interference by high background concentrations (780,840 ppm) of atmospheric N2. Here, we applied a new automated technique for simultaneous measurement of the denitrification end-products N2O and N2. We tested the hypothesis that the temperature response function (defined here as Q10) for N2O was greater than for N2 under anaerobic conditions in the laboratory. The Q10 for N2O was 2.0 and the Q10 for N2 was 1.4, indicating differences in sensitivity to temperature. Denitrified-N shifted towards N2O as temperature increased. While moisture strongly controls the relative abundance of denitrification end-products, additional experiments are needed to determine how a wider range of temperatures may affect N2O and N2. Results could enhance understanding of temperature controls on N2O production and consumption processes.

Development of a highly specific and productive process for n-caproic acid production: applying lessons from methanogenic microbiomes

High productivity and specificity in anaerobic digesters arise because complex microbiomes organize into a metabolic cascade to maximize energy recovery and to utilize the advantage that the gaseous end product methane freely bubbles out of the system. These lessons were applied to ascertain whether a reactor microbiome could be shaped to produce a different end product. The liquid product n-caproic acid was chosen, which is a 6-carbon-chain carboxylic acid that is valuable and that has a relatively low maximum solubility concentration for product recovery. Acetoclastic methanogenesis was inhibited by pH control and a route was provided for n-caproic acid extraction by implementing selective, in-line recovery. Next, ethanol was supplemented to promote chain elongation, which is a pathway in which short-chain carboxylic acids are elongated sequentially into medium-chain carboxylic acids with two-carbon units derived from ethanol. The reactor microbiome developed accordingly with the terminal process catalyzed by chain-elongating bacteria. As a result, n-caproic acid production rates increased to levels comparable to anaerobic digestion systems for solid waste treatment.

Performance of a commercial-scale DiCOM™ demonstration facility treating mixed municipal solid waste in comparison with laboratory-scale data

The current paper describes the performance of a commercial-scale (20,000tpa) demonstration facility of the DiCOM™ process, a biological treatment for the organic fraction of municipal solid waste (OFMSW). The 21-d process combines aerobic composting and high-solids (30%DM), thermophilic (55°C) anaerobic digestion (AD), within a single vessel. Mechanically sorted OFMSW, derived from mixed household MSW (324t), was exposed to sequential aerobic/anaerobic/aerobic treatment. The AD, initiated by adding anaerobic inoculum from a previous trial, was stable (without pH intervention) and the onset of methanogenesis, rapid (<3h). Volatile fatty acids formed during AD (including propionate) were exhausted prior to reuse of the inoculum. As measured by an electron flux from solids to gaseous end-products, AD accounted for the greatest portion of solids degradation (86%=160m3CH4/drytOFMSW). However, unlike laboratory trials, limited degradation occurred during initial aerobic treatment. The discharged solids were classified as a composted soil conditioner.

The reaction of methionine with hydroxyl radical: reactive intermediates and methanethiol production

The mechanisms of reaction of methionine with hydroxyl radical are not fully understood. Here, we unequivocally show using electron paramagnetic resonance spin-trapping spectroscopy and GC-FID and GC-MS, the presence of specific carbon-, nitrogen- and sulfur-centered radicals as intermediates of this reaction, as well as the liberation of methanethiol as a gaseous end product. Taking into account the many roles that methionine has in eco- and biosystems, our results may elucidate redox chemistry of this amino acid and processes that methionine is involved in.

Biohydrogen production behavior of moderately thermophile Thermoanaerobacterium thermosaccharolyticum W16 under different gas-phase conditions

In biohydrogen producing processes, cumulative gaseous end products can cause negative effects on hydrogen production. This study investigated inhibitory effects of hydrogen and carbon dioxide on biohydrogen production behavior of moderately thermophile Thermoanaerobacterium thermosaccharolyticum W16 with glucose or xylose as the carbon source. Experimental results indicated that high hydrogen partial pressure largely decreased the hydrogen yield by 16.9% and 18.8% for glucose and xylose respectively (from 2.07 to 1.72 mol H 2/mol glucose and from 1.80 to 1.46 mol H 2/mol xylose). Under high carbon dioxide partial pressure, the hydrogen yield was slightly dropped by 6.8% and 5.6% to 1.93 mol H 2/mol glucose and 1.70 mol H 2/mol xylose respectively. H 2 production rate, cell growth, and liquid end components were also affected by varied gaseous components, which led to the shifts of metabolic pathways and corresponding NADH flows due to gaseous feedback inhibition. Transcription levels of functional [FeFe] hydrogenases, which were determined by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), performed corresponding susceptibilities under high hydrogen and carbon dioxide partial pressures.

Warming increases the proportion of primary production emitted as methane from freshwater mesocosms

AbstractMethane (CH4) and carbon dioxide (CO2) are the dominant gaseous end products of the remineralization of organic carbon and also the two largest contributors to the anthropogenic greenhouse effect. We investigated whether warming altered the balance of CH4 efflux relative to gross primary production (GPP) and ecosystem respiration (ER) in a freshwater mesocosm experiment. Whole ecosystem CH4 efflux was strongly related to temperature with an apparent activation energy of 0.85 eV. Furthermore, CH4 efflux increased faster than ER or GPP with temperature, with all three processes having sequentially lower activation energies. Warming of 4 °C increased the fraction of GPP effluxing as CH4 by 20% and the fraction of ER as CH4 by 9%, in line with the offset in their respective activation energies. Because CH4 is 21 times more potent as a greenhouse gas, relative to CO2, these results suggest freshwater ecosystems could drive a previously unknown positive feedback between warming and the carbon cycle.

Predicting VFA formation by dark fermentation of particulate substrates

This study presents a mathematical model for predicting volatile fatty acid (VFA) production by dark fermentation of complex particulate substrates of biomass origin. This model is based on the premise that degradable components of such substrates are first hydrolyzed into soluble monomers, which are then fermented to liquid and gaseous end products. The model considered cellulose and hemicellulose as the degradable components of typical biomass-based organic substrates; and volatile fatty acids, hydrogen, and carbon dioxide as the end products. Hydrolysis of the particulate forms of the components is modeled by a surface-limiting rate equation, and the formation of end products is modeled following the guidelines of the anaerobic digestion model (ADM1). The model was calibrated and validated using experimental COD and VFA data from six batch reactors where, cattle manure was tested as a representative biomass-based substrate. The model predictions agreed well with the temporal trend in the experimental data, with overall r 2 of 0.85 for COD, 0.82 for acetic acid, 0.72 for butyric acid, and 0.42 for propionic acid.

Comparison of static, in-vessel composting of MSW with thermophilic anaerobic digestion and combinations of the two processes

The biological stabilisation of the organic fraction of municipal solid waste (OFMSW) into a form stable enough for land application can be achieved via aerobic or anaerobic treatments. To investigate the rates of degradation (e.g. via electron equivalents removed, or via carbon emitted) of aerobic and anaerobic treatment, OFMSW samples were exposed to computer controlled laboratory-scale aerobic (static in-vessel composting), and anaerobic (thermophilic anaerobic digestion with liquor recycle) treatment individually and in combination. A comparison of the degradation rates, based on electron flow revealed that provided a suitable inoculum was used, anaerobic digestion was the faster of the two waste conversion process. In addition to faster maximum substrate oxidation rates, anaerobic digestion (followed by post-treatment aerobic maturation), when compared to static composting alone, converted a larger fraction of the organics to gaseous end-products (CO 2 and CH 4), leading to improved end-product stability and maturity, as measured by compost self-heating and root elongation tests, respectively. While not comparable to windrow and other mixed, highly aerated compost systems, our results show that in the thermophilic, in-vessel treatment investigated here, the inclusion of a anaerobic phase, rather than using composting alone, improved hydrolysis rates as well as oxidation rates and product stability. The combination of the two methods, as used in the DiCOM ® process, was also tested allowing heat generation to thermophilic operating temperature, biogas recovery and a low odour stable end-product within 19 days of operation.

Evaluation of constructed wetlands by wastewater purification ability and greenhouse gas emissions

Domestic wastewater is a significant source of nitrogen and phosphorus, which cause lake eutrophication. Among the wastewater treatment technologies, constructed wetlands are a promising low-cost means of treating point and diffuse sources of domestic wastewater in rural areas. However, the sustainable operation of constructed wetland treatment systems depends upon a high rate conversion of organic and nitrogenous loading into their metabolic gaseous end products, such as N2O and CH4. In this study, we examined and compared the performance of three typical types of constructed wetlands: Free Water Surface (FWS), Subsurface Flow (SF) and Vertical Flow (VF) wetlands. Pollutant removal efficiency and N2O and CH4 emissions were assessed as measures of performance. We found that the pollutant removal rates and gas emissions measured in the wetlands exhibited clear seasonal changes, and these changes were closely associated with plant growth. VF wetlands exhibited stable removal of organic pollutants and NH3-N throughout the experiment regardless of season and showed great potential for CH4 adsorption. SF wetlands showed preferable T-N removal performance and a lower risk of greenhouse gas emissions than FWS wetlands. Soil oxidation reduction potential (ORP) analysis revealed that water flow structure and plant growth influenced constructed wetland oxygen transfer, and these variations resulted in seasonal changes of ORP distribution inside wetlands that were accompanied by fluctuations in pollutant removal and greenhouse gas emissions.

Influence of gaseous end-products inhibition and nutrient limitations on the growth and hydrogen production by hydrogen-producing fermentative bacterial B49

Hydrogen-producing bacterial strain B49 (HPB B49, AF481148 in EMBL) was grown in a synthetic medium with glucose as substrate, enabling the precise quantification of each nutrient's contribution to growth. Hydrogen production had a correlation with cell growth and the consumption of glucose and soluble protein. Liquid end-products of fermentation, predominantly ethanol, acetate and in minor amounts lactate, accumulated in the medium. Gaseous end-products comprised hydrogen and carbon dioxides. Growth of the bacterium in fresh media sparged with hydrogen and carbon dioxide showed that the hydrogen partial pressure in the fermentor had no influence on the growth rate and hydrogen production, however, the carbon dioxide affected the hydrogen fermentation and growth of HPB B49. As regards nutrients, iron and magnesium ion could induce metabolic change and affect the growth and the hydrogen production by HPB B49.

Denitrification rates and excess nitrogen gas concentrations in the Arabian Sea oxygen deficient zone

Rates of canonical, i.e. heterotrophic, water-column denitrification were measured by 15N incubation techniques at a number of coastal and open ocean stations in the Arabian Sea. Measurements of N 2:Ar gas ratios were also made to obtain independent estimates of N 2 excess resulting from denitrification. Measured denitrification rates ( 15NO 3 − → 15 − 14N 2) at open ocean stations averaged 9.1±1.0 nmol N l −1 d −1 ( n = 15 ), and coastal rates averaged 33.2±12.4 nmol N l −1 d −1 ( n = 18 ). When extrapolated to the entire Arabian Sea, deep measurements within the offshore perennial suboxic zone indicate an overall denitrification rate of 41 Tg N a −1±18 Tg N a −1, which is within the range (10–44 Tg N a −1) of previous estimates for canonical denitrification in the region based on stoichiometric calculations and electron transport system activity. Nitrogen excess gas measurements predict a larger nitrogen anomaly than estimated by classical stoichiometric methods (maximum anomaly=23 μg at N l −1 vs. 13 μg at N l −1, respectively). This mismatch may result from incorrect assumptions of Redfield stoichiometry inherent in the nitrate deficit calculation, inputs of new nitrogen through N-fixation, N 2 contributions from sedimentary denitrification along continental margins, the anammox reaction, and metal catalyzed denitrification reactions. Nevertheless, if denitrification is defined as the conversion of combined nitrogen to a gaseous end product, then the data suggest that denitrification in the Arabian Sea may have been underestimated so far.

Influence of inoculum to substrate ratio on the biochemical methane potential of maize in batch tests

Completely mixed batch reactors were used to compare the methane production from maize at inoculum to substrate volatile solids ratios ( r I/S) of 3, 2, 1.5 and 1. The tests were carried out at 35 °C and run against a control of inoculum without substrate. The stability and progress of the reaction from solid substrate to gaseous end product was monitored by measuring the pH, soluble chemical oxygen demand (CODs), total alkalinity (TA), volatile fatty acids (VFA) and gas composition on a daily basis. The results showed that the yield coefficient showed little variation over the range used, with an average value of 211 ± 6 ml CH 4 at standard temperature and pressure (STP) conditions g V S added − 1 . The methane production curves were further analysed to give values for the maximum specific methane production rate, ranging from 10 ml CH 4 g VSS −1 day −1 for a r I/S of 3–23 ml CH 4 g VSS −1 day −1 for a r I/S of 1. The initial buffering capacity of the system was enhanced by addition of NaHCO 3 to 9100 mg CaCO 3 l −1 which allowed digestion to proceed within the pH range 7.2–8.0 and at TVFA to TA ratio below the critical limit of 0.4. The largest proportion of the CODs was in the form of VFA and the reactor with the highest VS loading showed an accumulation of longer chain acids.

Anaerobic nitrate-dependent iron(II) bio-oxidation by a novel lithoautotrophic betaproteobacterium, strain 2002.

Microbial nitrate-dependent Fe(II) oxidation is known to contribute to iron biogeochemical cycling; however, the microorganisms responsible are virtually unknown. In an effort to elucidate this microbial metabolic process in the context of an environmental system, a 14-cm sediment core was collected from a freshwater lake and geochemically characterized concurrently with the enumeration of the nitrate-dependent Fe(II)-oxidizing microbial community and subsequent isolation of a nitrate-dependent Fe(II)-oxidizing microorganism. Throughout the sediment core, ambient concentrations of Fe(II) and nitrate were observed to coexist. Concomitant most probable number enumeration revealed the presence of an abundant nitrate-dependent Fe(II)-oxidizing microbial community (2.4 x 10(3) to 1.5 x 10(4) cells g(-1) wet sediment) from which a novel anaerobic, lithoautotrophic, Fe(II)-oxidizing bacterium, strain 2002, was isolated. Analysis of the complete 16S rRNA gene sequence revealed that strain 2002 was a member of the beta subclass of the proteobacteria with 94.8% similarity to Chromobacterium violaceum, a bacterium not previously recognized for the ability to oxidize nitrate-dependent Fe(II). Under nongrowth conditions, both strain 2002 and C. violaceum incompletely reduced nitrate to nitrite with Fe(II) as the electron donor, while under growth conditions nitrate was reduced to gaseous end products (N2 and N2O). Lithoautotrophic metabolism under nitrate-dependent Fe(II)-oxidizing conditions was verified by the requirement of CO2 for growth as well as the assimilation of 14C-labeled CO2 into biomass. The isolation of strain 2002 represents the first example of an anaerobic, mesophilic, neutrophilic Fe(II)-oxidizing lithoautotroph isolated from freshwater samples. Our studies further demonstrate the abundance of nitrate-dependent Fe(II) oxidizers in freshwater lake sediments and provide further evidence for the potential of microbially mediated Fe(II) oxidation in anoxic environments.

Anaerobic redox cycling of iron by freshwater sediment microorganisms

The potential for microbially mediated anaerobic redox cycling of iron (Fe) was examined in a first-generation enrichment culture of freshwater wetland sediment microorganisms. Most probable number enumerations revealed the presence of significant populations of Fe(III)-reducing (approximately 10(8) cells ml(-1)) and Fe(II)-oxidizing, nitrate-reducing organisms (approximately 10(5) cells ml(-1)) in the freshwater sediment used to inoculate the enrichment cultures. Nitrate reduction commenced immediately following inoculation of acetate-containing (approximately 1 mM) medium with a small quantity (1% v/v) of wetland sediment, and resulted in the transient accumulation of NO(2)(-) and production of a mixture of gaseous end-products (N(2)O and N(2)) and NH(4)(+). Fe(III) oxide (high surface area goethite) reduction took place after NO(3)(-) was depleted and continued until all the acetate was utilized. Addition of NO(3)(-) after Fe(III) reduction ceased resulted in the immediate oxidation of Fe(II) coupled to reduction of NO(3)(-) to NH(4)(+). No significant NO(2)(-) accumulation was observed during nitrate-dependent Fe(II) oxidation. No Fe(II) oxidation occurred in pasteurized controls. Microbial community structure in the enrichment was monitored by denaturing gradient gel electrophoresis analysis of polymerase chain reaction-amplified 16S rDNA and reverse transcription polymerase chain reaction-amplified 16S rRNA, as well as by construction of 16S rDNA clone libraries for four different time points during the experiment. Strong similarities in dominant members of the microbial community were observed in the Fe(III) reduction and nitrate-dependent Fe(II) oxidation phases of the experiment, specifically the common presence of organisms closely related (>or= 95% sequence similarity) to the genera Geobacter and Dechloromonas. These results indicate that the wetland sediments contained organisms such as Geobacter sp. which are capable of both dissimilatory Fe(III) reduction and oxidation of Fe(II) with reduction of NO(3)(-) to NH(4)(+). Our findings suggest that microbially catalysed nitrate-dependent Fe(II) oxidation has the potential to contribute to a dynamic anaerobic Fe redox cycle in freshwater sediments.

Rhizospheric exudation of Eriophorum vaginatum L. — Potential link to methanogenesis

Root exudates are a direct link between primary production in higher plants and methanogenesis. The relationship has been widely studied on rice paddies, but less is known about its role in wetlands populated by naturally occurring species. This study provides information about the amount and composition of root exudates produced by a widespread mire plant, Eriophorum vaginatum L. For this purpose, E. vaginatum plants were grown in quartz sand in pots from April to October, and root exudates were collected once a month by percolation of the cultivation substrate. In June and October, a set of plants was labelled with 14CO2 for two days and subsequently harvested for the determination of dry weight and for root exudates collected by the dipping method. The study supports earlier findings that natural wetland plants can enhance methanogenesis in their rhizosphere via active and seasonally varying exudation, but that the amount of exuded carbon (C) is many times lower than that delivered via litter formation. At both harvests in June and October, the proportion of incorporated radioactivity in shoots, roots and exudates was 92–96%, 4–8%, and 0.2%, respectively. New C was primarily fixed in the metabolically important carbohydrates, as well as acid anions that composed the main compounds of the new exudates. However, microbes seemed to rapidly metabolise the exudates into other substances like acetate. This was the dominant compound in the rhizoplane and rhizosphere, and it was the only detected substance that occurred in higher amounts outside the roots than inside them. Further studies in the field, including the quantification of gaseous end products, are necessary to complete our understanding of the carbon cycling in E. vaginatum-soil-microbe-system.

Evidence for Fungal Dominance of Denitrification and Codenitrification in a Grassland Soil

Fungi are capable of nitrification and denitrification and often dominate the microbial biomass of temperate grassland soils. We determined the contributions of bacteria and fungi to N2O and N2 production in a grassland soil from Northern Ireland by combining the substrate‐induced respiration inhibition method and the 15N gas‐flux method. Streptomycin (C21H39N7O12) was used as the bacterial inhibitor and cycloheximide (C15H23NO4) as the fungal inhibitor. By labeling the NH4 and NO3 pools, we tested the hypothesis that fungi produce N2O and N2 solely by the reduction of NO3 Cycloheximide decreased the flux of N2O by 89% and streptomycin decreased the flux by 23%, indicating that fungi were responsible for most of the N2O production. All of the N2O was derived from NO3 reduction. Labeled N2 was only detected in control and streptomycin treatments. The distribution of the 15N atoms in the labeled N2 indicated that the source of the labeling was predominantly the NO3 pool, but that the process of formation was not dominated by denitrification. Codenitrification, where a 15N atom from labeled nitrogen dioxide (NO2) combines with a 14N atom from a natural abundance source, was proposed as the process forming labeled N2 About 92% of the labeled N2 was estimated to be due to codenitrification and 8% due to denitrification. The flux of N2O was always greater than the flux of N2, the mole fraction of N2O averaging 0.7. Fungal denitrification could be of ecological significance because N2O is the dominant gaseous end product.

Balancing of nitrogen conversion in deammonifying biofilms through batch tests and GC/MS

Growth carriers from a technical deammonifying moving-bed WWTP were used in batch tests to determine possible N-conversion reactions under varying oxygen and substrate conditions. Deammonification, denitrification, and nitrification reactions could be proved using conventional analysis, combined with gas chromatography/mass spectrometry analysis to get additional information about 15N-isotope labelled gaseous end products of the different reactions. In this orientating study N2O could be observed in some cases up to 12% of the total gas production. N2O production came from incomplete denitrification processes under anoxic or oxygen-limiting conditions and in the absence of organic substrate, as if structural components of deammonifying biofilms play a crucial role for the portion of side-reactions, leading to undesirable gaseous end products.