Stratified microbial structure and activity in sulfide- and methane-producing anaerobic sewer biofilms

Effect of methane partial pressure on the performance of a membrane biofilm reactor coupling methane-dependent denitrification and anammox

Complete nitrogen removal has recently been demonstrated by integrating anaerobic ammonium oxidation (anammox) and denitrifying anaerobic methane oxidation (DAMO) processes. In this work, the effect of methane partial pressure on the performance of a membrane biofilm reactor (MBfR) consisting of DAMO and anammox microorganisms was evaluated. The activities of DAMO archaea and DAMO bacteria in the biofilm increased significantly with increased methane partial pressure, from 367 ± 9 and 58 ± 22 mg-N L^-1d^-1 to 580 ± 12 and 222 ± 22 mg-N L^-1d^-1, respectively, while the activity of anammox bacteria only increased slightly, when the methane partial pressure was elevated from 0.24 to 1.39 atm in the short-term batch tests. The results were supported by a long-term (seven weeks) continuous test, when the methane partial pressure was dropped from 1.39 to 0.78 atm. The methane utilization efficiency was always above 96% during both short-term and long-term tests. Taken together, nitrogen removal rate (especially the nitrate reduction rate by DAMO archaea) and methane utilization efficiency could be maintained at high levels in a broad range of methane partial pressure (0.24-1.39 atm in this study). In addition, a previously established DAMO/anammox biofilm model was used to analyze the experimental data. The observed impacts of methane partial pressure on biofilm activity were well explained by the modeling results. These results suggest that methane partial pressure can potentially be used as a manipulated variable to control reaction rates, ultimately to maintain high nitrogen removal efficiency, according to nitrogen loading rate.

Modelling the long-term effect of wastewater compositions on maximum sulfide and methane production rates of sewer biofilm

Reliable modelling of sulfide and methane production in sewer systems is required for efficient sewer emission management. Wastewater compositions affect sulfide and methane production kinetics through both its short-term variation influencing the substrate availability to sewer biofilms, and its long-term variation affecting the sewer biofilm structure. While the short-term effect is well considered in existing sewer models with the use of Monod or half-order equations, the long-term effect has not been explicitly considered in current sewer models suitable for network modelling. In this study, the long-term effect of wastewater compositions on sulfide and methane production activities in rising main sewers was investigated. A detailed biofilm model was firstly developed, and then calibrated and validated using experimental data measured during the entire biofilm development period of a laboratory sewer reactor. Based on scenario simulations using the detailed biofilm model, empirical equations describing the long-term effect of sulfate and sCOD (soluble chemical oxygen demand) concentrations on k_H2S (the maximum sulfide production rate of sewer biofilm) and k_CH4 (the maximum methane production rate of sewer biofilm) were proposed. These equations require further verification in future studies before their potential integration into network-wide sewer models.

Implications from distinct sulfate-reducing bacteria populations between cattle manure and digestate in the elucidation of H2S production during anaerobic digestion of animal slurry

Biogas produced from the anaerobic digestion of animal slurry consists mainly of methane (CH₄) and carbon dioxide (CO₂), but also includes other minor gases, such as hydrogen sulfide (H₂S). Since it can act as a potent corrosive agent and presents a health hazard even at low concentrations, H₂S is considered an undesirable by-product of anaerobic digestion. Sulfate-reducing bacteria (SRBs) have been identified as the main biological source of H₂S in a number of natural, biological, and human-made habitats, and thus represent likely candidate microorganisms responsible for the production of H₂S in anaerobic manure digesters. Phylogenetically, SRBs form a divergent group of bacteria that share a common anaerobic respiration pathway that allows them to use sulfate as a terminal electron acceptor. While the composition and activity of SRBs have been well documented in other environments, their metabolic potential remains largely uncharacterized and their populations poorly defined in anaerobic manure digesters. In this context, a combination of in vitro culture-based studies and DNA-based approaches, respectively, were used to gain further insight. Unexpectedly, only low to nondetectable levels of H₂S were produced by digestate collected from a manure biogas plant documented to have persistently high concentrations of H₂S in its biogas (2000-3000 ppm). In contrast, combining digestate with untreated manure (a substrate with comparatively lower sulfate and SRB cell densities than digestate) was found to produce elevated H₂S levels in culture. While a 16S rRNA gene-based community composition approach did not reveal likely candidate SRBs in digestate or untreated manure, the use of the dsrAB gene as a phylogenetic marker provided more insight. In digestate, the predominant SRBs were found to be uncharacterized species likely belonging to the genus Desulfosporosinus (Peptococcaceae, Clostridiales, Firmicutes), while Desulfovibrio-related SRBs (Desulfovibrionaceae, Desulfovibrionales, Proteobacteria) were the most highly represented in untreated manure. Intriguingly, the same species-level OTUs with a similar pattern of opposite relative abundance were also found in two other digesters with lower H₂S levels in their biogas. Together, our results suggest that elevated H₂S production in anaerobic digesters requires the combination of biological and nutritional factors from both untreated manure and digestate.

Modelling Methane Production and Sulfate Reduction in Anaerobic Granular Sludge Reactor with Ethanol as Electron Donor

In this work, a mathematical model based on growth kinetics of microorganisms and substrates transportation through biofilms was developed to describe methane production and sulfate reduction with ethanol being a key electron donor. The model was calibrated and validated using experimental data from two case studies conducted in granule-based Upflow Anaerobic Sludge Blanket reactors. The results suggest that the developed model could satisfactorily describe methane and sulfide productions as well as ethanol and sulfate removals in both systems. The modeling results reveal a stratified distribution of methanogenic archaea, sulfate-reducing bacteria and fermentative bacteria in the anaerobic granular sludge and the relative abundances of these microorganisms vary with substrate concentrations. It also indicates sulfate-reducing bacteria can successfully outcompete fermentative bacteria for ethanol utilization when COD/SO₄²⁻ ratio reaches 0.5. Model simulation suggests that an optimal granule diameter for the maximum methane production efficiency can be achieved while the sulfate reduction efficiency is not significantly affected by variation in granule size. It also indicates that the methane production and sulfate reduction can be affected by ethanol and sulfate loading rates, and the microbial community development stage in the reactor, which provided comprehensive insights into the system for its practical operation.

Indirect sulfur reduction via polysulfide contributes to serious odor problem in a sewer receiving nitrate dosage

Nitrate dosing is commonly used to control hydrogen sulfide production in sewer systems. However, quick rebound of the sulfide concentration after nitrate depletion has been observed and results in more serious odor and corrosion problem. To investigate the mechanism of sulfide regeneration in the nitrate-free period, a laboratory-scale sewer reactor was run for 30 days to simulate sulfide production and oxidation with intermittent nitrate addition. The results show that nitrate addition substantially reduced the sulfide concentration, but the produced elemental sulfur was then quickly reduced back to sulfide in nitrate-free periods. This induced more and more sulfide production in the sewer reactor. Elemental sulfur and polysulfide reductions were found in the sewage in nitrate-free periods, showing their contributions to the sulfide regeneration. Through batch tests, polysulfide was confirmed as the key intermediate for accelerating sulfur reduction during the nitrate-free period in the sewer. Sulfide production rates significantly increased by 65% and 59% in the presences of tetrasulfide and sulfur with sulfide, respectively, at the beginning of the test. While polysulfide formation was prevented by the ferrous chloride addition, the sulfur reduction rate remarkably decreased from 12.8 mgS/L-h to 1.8 mgS/L-h. This indicates that direct sulfur reduction was significantly slower than the indirect sulfur reduction via polysulfide; the latter process could be the cause for the quick rebound of the sulfide concentration in the sewer with intermittent nitrate dosing. Thus, the pathways of sulfur transformations in a sewer, both in the presence and absence of nitrate, were proposed. Microbial community analysis results reveal that some common sulfate-reducing bacteria (SRB) genera in sewer sediment were possible sulfur reducers. According to this finding, the effect and strategy of nitrate dosing for hydrogen sulfide control in sewers should be re-evaluated and re-considered.

Interspecific Bacterial Interactions are Reflected in Multispecies Biofilm Spatial Organization

Interspecies interactions are essential for the persistence and development of any kind of complex community, and microbial biofilms are no exception. Multispecies biofilms are structured and spatially defined communities that have received much attention due to their omnipresence in natural environments. Species residing in these complex bacterial communities usually interact both intra- and interspecifically. Such interactions are considered to not only be fundamental in shaping overall biomass and the spatial distribution of cells residing in multispecies biofilms, but also to result in coordinated regulation of gene expression in the different species present. These communal interactions often lead to emergent properties in biofilms, such as enhanced tolerance against antibiotics, host immune responses, and other stresses, which have been shown to provide benefits to all biofilm members not only the enabling sub-populations. However, the specific molecular mechanisms of cellular processes affecting spatial organization, and vice versa, are poorly understood and very complex to unravel. Therefore, detailed description of the spatial organization of individual bacterial cells in multispecies communities can be an alternative strategy to reveal the nature of interspecies interactions of constituent species. Closing the gap between visual observation and biological processes may become crucial for resolving biofilm related problems, which is of utmost importance to environmental, industrial, and clinical implications. This review briefly presents the state of the art of studying interspecies interactions and spatial organization of multispecies communities, aiming to support theoretical and practical arguments for further advancement of this field.

Antimicrobial Effects of Free Nitrous Acid on Desulfovibrio vulgaris: Implications for Sulfide-Induced Corrosion of Concrete

Hydrogen sulfide produced by sulfate-reducing bacteria (SRB) in sewers causes odor problems and asset deterioration due to the sulfide-induced concrete corrosion. Free nitrous acid (FNA) was recently demonstrated as a promising antimicrobial agent to alleviate hydrogen sulfide production in sewers. However, details of the antimicrobial mechanisms of FNA are largely unknown. Here, we report the multiple-targeted antimicrobial effects of FNA on the SRB Desulfovibrio vulgaris Hildenborough by determining the growth, physiological, and gene expression responses to FNA exposure. The activities of growth, respiration, and ATP generation were inhibited when exposed to FNA. These changes were reflected in the transcript levels detected during exposure. The removal of FNA was evident by nitrite reduction that likely involved nitrite reductase and the poorly characterized hybrid cluster protein, and the genes coding for these proteins were highly expressed. During FNA exposure, lowered ribosome activity and protein production were detected. Additionally, conditions within the cells were more oxidizing, and there was evidence of oxidative stress. Based on an interpretation of the measured responses, we present a model depicting the antimicrobial effects of FNA on D. vulgaris These findings provide new insight for understanding the responses of D. vulgaris to FNA and will provide a foundation for optimal application of this antimicrobial agent for improved control of sewer corrosion and odor management.IMPORTANCE Hydrogen sulfide produced by SRB in sewers causes odor problems and results in serious deterioration of sewer assets that requires very costly and demanding rehabilitation. Currently, there is successful application of the antimicrobial agent free nitrous acid (FNA), the protonated form of nitrite, for the control of sulfide levels in sewers (G. Jiang et al., Water Res 47:4331-4339, 2013, http://dx.doi.org/10.1016/j.watres.2013.05.024). However, the details of the antimicrobial mechanisms of FNA are largely unknown. In this study, we identified the key responses (decreased anaerobic respiration, reducing FNA, combating oxidative stress, and shutting down protein synthesis) of Desulfovibrio vulgaris Hildenborough, a model sewer corrosion bacterium, to FNA exposure by examining the growth, physiological, and gene expression changes. These findings provide new insight and underpinning knowledge for understanding the responses of D. vulgaris to FNA exposure, thereby benefiting the practical application of FNA for improved control of sewer corrosion and odor.

Control of sulfide and methane production in anaerobic sewer systems by means of Downstream Nitrite Dosage

Bioproduction of hydrogen sulfide (H2S) and methane (CH4) under anaerobic conditions in sewer pipes causes detrimental effects on both sewer facilities and surrounding environment. Among the strategies used to mitigate the production of both compounds, the addition of nitrite (NO2(-)) has shown a greater long-term inhibitory effect compared with other oxidants such as nitrate or oxygen. The aim of this study was to determine the effectiveness of a new method, the Downstream Nitrite Dosage strategy (DNO2D), to control H2S and CH4 emissions in sewers. Treatment effectiveness was assessed on H2S and CH4 abatement on the effluent of a laboratory sewer pilot plant that mimics a full-scale anaerobic rising sewer. The experiment was divided in three different periods: system setup (period 1), nitrite addition (period 2) and system recovery (period 3). Different process and molecular methods were combined to investigate the impact of NO2(-) addition on H2S and CH4 production. Results showed that H2S load was reduced completely during nitrite addition when compared to period 1 due to H2S oxidation but increased immediately after nitrite addition stopped. The H2S overproduction during recovery period was associated with the bacterial reduction of different sulfur species (elemental sulfur/thiosulfate/sulfite) accumulated within the sewer biofilm matrix. Oxidation of CH4 was also detected during period 2 but, contrary to sulfide production, re-establishment of methanogenesis was not immediate after stopping nitrite dosing. The analysis of bulk and active microbial communities along experimental treatment showed compositional changes that agreed with the observed dynamics of chemical processes. Results of this study show that DNO2D strategy could significantly reduce H2S and CH4 emissions from sewers during the addition period but also suggest that microbial agents involved in such processes show a high resilience towards chemical stressors, thus favoring the re-establishment of H2S and CH4 production after stopping nitrite addition.

Sulfide and methane production in sewer sediments: Field survey and model evaluation

Sewer sediment processes have been reported to significantly contribute to overall sulfide and methane production in sewers, at a scale comparable to that of sewer biofilms. The physiochemical and biological characteristics of sewer sediments are heterogeneous; however, the variability of in-sediments sulfide and methane production rates among sewers has not been assessed to date. In this study, five sewer sediment samples were collected from two cities in Australia with different climatic conditions. Batch assays were conducted to determine the rates of sulfate reduction and methane production under different flow velocity (shear stress) conditions as well as under completely mixed conditions. The tests showed substantial and variable sulfate reduction and methane production activities among different sediments. Sulfate reduction and methane production from sewer sediments were confirmed to be areal processes, and were dependent on flow velocity/shear stress. Despite of the varying characteristics and reactions kinetics, the sulfate reduction and methane production processes in all sediments could be well described by a one-dimensional sewer sediment model recently developed based on results obtained from a laboratory sewer sediment reactor. Model simulations indicated that the in-situ contribution of sewer sediment emissions could be estimated without the requirement of measuring the specific sediment characteristics or the sediment depths.

Methane emission from sewers

Recent studies have shown that sewer systems produce and emit a significant amount of methane. Methanogens produce methane under anaerobic conditions in sewer biofilms and sediments, and the stratification of methanogens and sulfate-reducing bacteria may explain the simultaneous production of methane and sulfide in sewers. No significant methane sinks or methanotrophic activities have been identified in sewers to date. Therefore, most of the methane would be emitted at the interface between sewage and atmosphere in gravity sewers, pumping stations, and inlets of wastewater treatment plants, although oxidation of methane in the aeration basin of a wastewater treatment plant has been reported recently. Online measurements have also revealed highly dynamic temporal and spatial variations in methane production caused by factors such as hydraulic retention time, area-to-volume ratio, temperature, and concentration of organic matter in sewage. Both mechanistic and empirical models have been proposed to predict methane production in sewers. Due to the sensitivity of methanogens to environmental conditions, most of the chemicals effective in controlling sulfide in sewers also suppress or diminish methane production. In this paper, we review the recent studies on methane emission from sewers, including the production mechanisms, quantification, modeling, and mitigation.

Changes in Microbial Biofilm Communities during Colonization of Sewer Systems

The coexistence of sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) in anaerobic biofilms developed in sewer inner pipe surfaces favors the accumulation of sulfide (H2S) and methane (CH4) as metabolic end products, causing severe impacts on sewerage systems. In this study, we investigated the time course of H2S and CH4 production and emission rates during different stages of biofilm development in relation to changes in the composition of microbial biofilm communities. The study was carried out in a laboratory sewer pilot plant that mimics a full-scale anaerobic rising sewer using a combination of process data and molecular techniques (e.g., quantitative PCR [qPCR], denaturing gradient gel electrophoresis [DGGE], and 16S rRNA gene pyrotag sequencing). After 2 weeks of biofilm growth, H2S emission was notably high (290.7±72.3 mg S-H2S liter(-1) day(-1)), whereas emissions of CH4 remained low (17.9±15.9 mg COD-CH4 liter(-1) day(-1)). This contrasting trend coincided with a stable SRB community and an archaeal community composed solely of methanogens derived from the human gut (i.e., Methanobrevibacter and Methanosphaera). In turn, CH4 emissions increased after 1 year of biofilm growth (327.6±16.6 mg COD-CH4 liter(-1) day(-1)), coinciding with the replacement of methanogenic colonizers by species more adapted to sewer conditions (i.e., Methanosaeta spp.). Our study provides data that confirm the capacity of our laboratory experimental system to mimic the functioning of full-scale sewers both microbiologically and operationally in terms of sulfide and methane production, gaining insight into the complex dynamics of key microbial groups during biofilm development.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

Effects of nitrate dosing on sulfidogenic and methanogenic activities in sewer sediment

Nitrate dosing is widely used to control sulfide and methane formation in sewers. The impact of nitrate on sulfide and methane production by sewer biofilms in rising mains has been elucidated recently. However, little is known about the effect of nitrate on biologically active sewer sediment, which is substantially thicker than rising main biofilms (centimeters vs. hundreds of micrometers, respectively). In this study, we investigated the effect of nitrate addition to sewer sediment cultivated in lab-scale sewer sediment reactors. Batch test results showed that nitrate addition does not suppress sulfide production in sewer sediment, but it reduces sulfide accumulation through anoxic sulfide oxidation in the sediment and hence, also reduces sulfide accumulation in the bulk water. Microsensor measurement of sediment sulfide revealed the presence of sulfide oxidation and sulfide production zones with the interface dynamically regulated by the depth of nitrate penetration. In contrast, the methane production activity of sewer sediment was substantially reduced, likely due to the long-term inhibitory effects of nitrate on methanogens. Pore water measurements showed that methane production activity in the sediment zone with frequent nitrate exposure was completely suppressed, and consequently, the methane production zone re-established deeper in the sediment where nitrate penetration was infrequent.

Impact of reduced water consumption on sulfide and methane production in rising main sewers

Reduced water consumption (RWC), for water conservation purposes, is expected to change the wastewater composition and flow conditions in sewer networks and affect the in-sewer transformation processes. In this study, the impact of reduced water consumption on sulfide and methane production in rising main sewers was investigated. Two lab-scale rising main sewer systems fed with wastewater of different strength and flow rates were operated to mimic sewers under normal and RWC conditions (water consumption reduced by 40%). Sulfide concentration under the RWC condition increased by 0.7-8.0 mg-S/L, depending on the time of a day. Batch test results showed that the RWC did not change the sulfate-reducing activity of sewer biofilms, the increased sulfide production being mainly due to longer hydraulic retention time (HRT). pH in the RWC system was about 0.2 units lower than that in the normal system, indicating that more sulfide would be in molecular form under the RWC condition, which would result in increased sulfide emission to the atmosphere as confirmed by the model simulation. Model based analysis showed that the cost for chemical dosage for sulfide mitigation would increase significantly per unit volume of sewage, although the total cost would decrease due to a lower sewage flow. The dissolved methane concentration under the RWC condition was over two times higher than that under the normal flow condition and the total methane discharge was about 1.5 times higher, which would potentially result in higher greenhouse gas emissions. Batch tests showed that the methanogenic activity of sewer biofilms increased under the RWC condition, which along with the longer HRT, led to increased methane production.

Degradation of methanethiol in anaerobic sewers and its correlation with methanogenic activities

Methanethiol (MT) is considered one of the predominant odorants in sewer systems. Therefore, understanding MT transformation in sewers is essential to sewer odor assessment and abatement. In this study, we investigated the degradation of MT in laboratory anaerobic sewers. Experiments were carried out in seven anaerobic sewer reactors with biofilms at different stages of development. MT degradation was found to be strongly dependent on the methanogenic activity of sewer biofilms. The MT degradation rate accelerated with the increase of methanogenic activity of sewer biofilms, resulting in MT accumulation (i.e. net production) in sewer reactors with relatively low methanogenic activities, and MT removal in reactors with higher methanogenic activities. A Monod-type kinetic expression was developed to describe MT degradation kinetics in anaerobic sewers, in which the maximum degradation rate was modeled as a function of the maximum methane production rate through a power function. It was also found that MT concentration had a linear relationship with acetate concentration, which may be used for preliminary assessment of MT presence in anaerobic sewers.