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Competition for electrons among different steps of denitrification has previously been shown to occur, and to play an important role in the accumulation and emission of N2O in wastewater treatment. However, this electron competition is not recognized in the current denitrification models, limiting their ability to predict N2O accumulation during denitrification. In this work, a new denitrification model is developed for wastewater treatment processes. It describes electron competition among the four steps of denitrification, through modeling the carbon oxidation and nitrogen reduction processes separately, in contrast to the existing models that directly couple these two types of processes. Electron carriers are introduced to link carbon oxidation, which donates electrons to carriers, and nitrogen oxides reduction, which receives electrons from these carriers. The relative ability of each denitrification step to compete for electrons is modeled through the use of different affinity constants with reduced carriers. Model calibration and validation results demonstrate that the developed model is able to reasonably describe the nitrate, nitrite, and N2O reduction rates of a methanol-utilizing denitrifying culture under various carbon and nitrogen oxides supplying conditions. The model proposed, while subject to further validation, is expected to enhance our ability to predict N2O accumulation in denitrification. [ABSTRACT FROM AUTHOR]

Sulfide is produced in sewer networks, and previous studies suggest that sulfide in sewage could alter the activity of heterotrophic denitrification and lead to N2O accumulation during biological wastewater treatment. However, the details of this phenomenon are poorly understood. In this study, the potential inhibitory effects of sulfide on nitrate, nitrite, and N2O reduction were assessed with a methanol-utilizing denitrifying culture both prior to and after its exposure and adaptation to sulfide. Hydrogen sulfide was found to be strongly inhibitory to N2O reduction, with 50% inhibition observed at H2S concentrations of 0.04 mg H2S-S/L and 0.1 mg H2S-S/L for the unadapted and adapted cultures, respectively. In comparison, both nitrate and nitrite reduction was more tolerant to H2S. A 50% inhibition of nitrite reduction was observed at approximately 2.0 mg H2S-S/L for both unadapted and adapted cultures, while no inhibition of nitrate reduction occurred at the highest H2S concentrations applied (2.0 mg H2S-S/L) to either culture. N2O accumulation was observed during nitrate and nitrite reduction by the adapted culture when H2S concentrations were above 0.5 and 0.2 mg H2S-S/L, respectively. Additionally, we reveal that hydrogen sulfide (H2S), rather than sulfide, was likely the true inhibitor of N2O reduction, and the inhibitory effect was reversible. These findings suggest that sulfide management in sewers could potentially have a significant impact on N2O emission from wastewater treatment plants. [ABSTRACT FROM AUTHOR]

The treatment of wastewater containing sulfides in bioelectrochemical systems (BES) causes deposition of sulfur on the anode as a result of a solely electrochemical process. In this study, we investigate whether microorganisms can use this sulfur, rather than the anode or soluble sulfate, as an electron acceptor for the oxidation of acetate. Our results indicate that microorganisms use electrodeposited sulfur as preferable electron acceptor over the anode and sulfate and produce sulfide irrespective of electrochemical conditions. Bioelectrochemical and biological sulfide generation pathways were studied under different electrochemical conditions. The obtained results show that the sulfide generation rate at open circuit condition (anode potential -235 ± 5 mV versus standard hydrogen electrode, SHE) was higher in comparison to the electrochemical sulfide generation even at a lower potential of -275 mV (vs SHE), confirming that sulfide is produced through biological processes without any current generation. However, during closed circuit operation, the overall Coulombic efficiency (97% ± 2%) is not affected as the produced sulfide (originating from the reduction of deposited sulfur) is spontaneously reoxidized to sulfur when a favorable potential is maintained. This confirms the mediator role of sulfur during acetate oxidation in BES. A diagrammatic representation of the mechanism is proposed to characterize the interactions between acetate oxidation and sulfur conversions on the anode. [ABSTRACT FROM AUTHOR]

"Sludge-drying lagoons" are a preferred sludge treatment and drying method in tropical and subtropical areas due to the low construction and operational costs. However, this method may be a potential significant source of methane (CH4) because some of the organic matter would be microbially metabolized under anaerobic conditions in the lagoon. The quantification of CH4 emissions from lagoons is difficult due to the expected temporal and spatial variations over a lagoon maturing cycle of several years. Sporadic ebullition of CH4, which cannot be easily quantified by conventional methods such as floating hoods, is also expected. In this study, a novel method based on mass balances was developed to estimate the CH4 emissions and was applied to a full-scale sludge-drying lagoon over a three year operational cycle. The results revealed that processes in a sludge-drying lagoon would emit 6.5 kg CO2-e per megaliter of treated sewage. This would represent a quarter to two-thirds of the overall greenhouse gas (GHG) emissions from wastewater-treatment plants (WWTPs). This work highlights the fact that sludge-drying lagoons are a significant source of CH4 that adds substantially to the overall GHG footprint of WWTPs despite being recognized as a cheap and energy-efficient means of drying sludge. [ABSTRACT FROM AUTHOR]

Autotrophic ammonia oxidizing bacteria (AOB) have been recognized as a major contributor to N2O production in wastewater treatment systems. However, so far N2O models have been proposed based on a single N2O production pathway by AOB, and there is still a lack of effective approach for the integration of these models. In this work, an integrated mathematical model that considers multiple production pathways is developed to describe N2O production by AOB. The pathways considered include the nitrifier denitrification pathway (N2O as the final product of AOB denitrification with NO2- as the terminal electron acceptor) and the hydroxylamine (NH2OH) pathway (N2O as a byproduct of incomplete oxidation of NH2OH to NO2-). In this model, the oxidation and reduction processes are modeled separately, with intracellular electron carriers introduced to link the two types of processes. The model is calibrated and validated using experimental data obtained with two independent nitrifying cultures. The model satisfactorily describes the N2O data from both systems. The model also predicts shifts of the dominating pathway at various dissolved oxygen (DO) and nitrite levels, consistent with previous hypotheses. This unified model is expected to enhance our ability to predict N2O production by AOB in wastewater treatment systems under varying operational conditions. [ABSTRACT FROM AUTHOR]

The application of aerobic sludge granules in wastewater treatment could increase the intensity of wastewater treatment processes because of their greater density and size relative to conventional sludge flocs. It has been suggested that granules are distinguished from flocs by gel forming exopolysaccharides. In this study, evidence is presented linking a specific exopolysaccharide component with granule extracellular polymeric substance (EPS) gelation. Granular EPS comprised three components: high-molecular-weight (MW) exopolysaccharide, medium-MW proteins and glycosides, and low-MW proteins and glycosides. The high-MW fraction was separated by fractional precipitation and preparatory-scale gel permeation chromatography (GPC). The MW profile of this fraction appears to be exclusively attributable to high-MW polysaccharide. The exopolysaccharide exists as a gel at normal wastewater treatment operating pH (i.e., 6.0-8.5), whereas the low/medium-MW material does not. Conformational analysis by atomic force microscopy (AFM) of the dried material showed that the polysaccharide forms pearl-necklace-like, intramolecularly condensed structures when dissolved in Milli-Q water and partially relaxed helical aggregates when in alkali solution. Consequently, the gel-forming property of EPS in the aerobic sludge granules tested is probably associated with high-MW polysaccharide components. [ABSTRACT FROM AUTHOR]