Effects of nitrite concentration and exposure time on sulfide and methane production in sewer systems

Nitrite dosing is a promising technology to prevent sulfide and methane formation in sewers, due to the known inhibitory/toxic effect of nitrite on sulfate-reducing bacteria (SRB) and methanogenic Archaea (MA). The dependency of nitrite-induced inhibition on sulfide and methane producing activities of anaerobic sewer biofilms on nitrite levels and exposure time is investigated using a range of nitrite concentrations (40, 80, 120 mg-N/L) and exposure time up to 24 days. The recovery of these activities after the 24-day nitrite dosage was also monitored for more than two months. The inhibition level was found to be dependent on both nitrite concentration and exposure time, with stronger inhibition observed at higher nitrite concentrations and/or longer exposure time. However, the time required for achieving 50% recovery of both sulfate-reducing and methanogenic activities after the cessation of nitrite dosage only marginally depended on nitrite concentration. Model-based analysis of the recovery data showed that the recovery was likely due to the regrowth of SRB and methanogens. The lab studies and mathematical analysis supported the development of an intermittent dosing strategy, which was tested in a 1-km long rising main sewer. The field trial confirmed that intermittent dosing of nitrite can effectively reduce/prevent the formation of both sulfide and methane.     

Inhibition of sulfate-reducing and methanogenic activities of anaerobic sewer biofilms by ferric iron dosing

Ferric iron is commonly used for sulfide precipitation in sewers, thus achieving corrosion and odour control. Its impact on the activities of sulfate-reducing bacteria and methanogens in anaerobic sewer biofilms is investigated in this study. Two lab-scale rising main sewer systems fed with real sewage were operated for 8 months. One received Fe³⁺ dosage (experimental system) and the other was used as a control. In addition to precipitating sulfide from bulk water, Fe³⁺ dosage was found to significantly inhibit sulfate reduction and methane production by sewer biofilms. The experimental reactor discharged an effluent containing a higher concentration of sulfate and a lower concentration of methane in comparison with the reference reactor. Batch experiments showed that the addition of ferric ions reduced the sulfate reduction and methane production rates of the sewer biofilms by 60% and 80%, respectively. The batch experiments further showed that Fe³⁺ dosage changed the final products of sulfate reduction with sulfide accounting for only 54% of the sulfate reduced. The other products could not be confirmed, but were not dissolved inorganic sulfur species such as sulfite or thiosulfate. The results suggest the addition of Fe³⁺ at upstream locations would minimize the ferric salts required for achieving the same level of sulfide removal. Fe³⁺ dosing could also substantially reduce the formation of methane, a potent greenhouse gas, in sewers.     

Methane formation in sewer systems

Methane formation and emission in sewer systems has not received as much attention as hydrogen sulphide formation. Through field measurements from two rising mains, with an average sewage temperature of 28.4 and 26.6 degrees C, respectively, at the time of sampling, this study shows that a significant amount of methane can be produced in sewer systems, and that this production is positively correlated with the hydraulic retention time of wastewater in these systems. The experimental results from a laboratory-scale sewer system fed with real sewage with a temperature of approximately 21 degrees C confirmed these field observations and further revealed that methanogenesis and sulphate reduction occur simultaneously in sewers, with methane production contributing considerably more to the loss of soluble COD in sewers than sulphate reduction. The production of methane in sewers at levels revealed by this study is a serious environmental concern as it potentially results in greenhouse emissions that is comparable to that caused by the energy consumption for the treatment of the same wastewater. Further, methane production in sewers influences sulphide production and its management due to the competition between methanogens and sulphate-reducing bacteria for potentially the same electron donors. The potential interactions between sulphate-reducing and methanogenic bacteria in sewer networks are discussed.     

Potential nanosilver impact on anaerobic digestion at moderate silver concentrations

Silver nanoparticles (AgNPs, nanosilver) entering the sewers and wastewater treatment plants (WWTPs) are mostly accumulated in the sludge. In this study, we determined the impact of AgNPs on anaerobic glucose degradation, sludge digestion and methanogenic assemblages. At ambient (22 °C) and mesophilic temperatures (37 °C), there was no significant difference in biogas and methane production between the sludge treated with AgNPs at the concentrations up to 40 mg Ag/L (13.2 g silver/Kg biomass COD) and the control. In these anaerobic digestion samples, acetate and propionic acid were the only detectable volatile fatty acids (VFAs) and they were depleted in 3 days. On the other hand, more than 90% of AgNPs was removed from the liquid phase and associated with the sludge while almost no silver ions were released from AgNPs under anaerobic conditions. Quantitative PCR results indicated that Methanosaeta and Methanomicrobiales were the dominant methanogens, and the methanogenic diversity and population remained largely unchanged after nanosilver exposure and anaerobic digestion. The results suggest that AgNPs at moderate concentrations (e.g., ≤40 mg/L) have negligible impact on anaerobic digestion and methanogenic assemblages because of little to no silver ion release.     

Methanogenic digestion using mixed substrate of acetic, propionic and butyric acids

Experiments on methanogenic digestion using high concentrations of mixed substrate were conducted. The major intermediate products of anaerobic digestion such as acetic, propionic and butyric acids were mixed in a ratio of 2:1:1 (COD basis), respectively, and used as a substrate for feeding into continuous-flow chemostat reactors maintained at 35°C. These reactors were operated stably at higher feed substrate concentrations and shorter hydraulic retention times (HRT) than those of using a single component of volatile fatty acids as a substrate. At an HRT of 4.43 days, the methanogenesis occurred normally up to a feed substrate concentration of 70,000 mg COD I⁻¹. At a feed substrate concentration of 20,000 mg COD I⁻¹, the methanogenesis occurred normally up to an HRT of 2.91 days and the minimum SRT for microbial populations was calculated to be 2.42 days. An increase in feed substrate concentration adversely affected the propionate degradation strikingly, while a decrease in HRT significantly adversely affected the acetate and propionate degradation. The methane production was 0.301 g⁻¹ COD utilized, and it was independent of the feed substrate concentration and HRT. Bacilli were predominant in all reactors, but sarcinae appeared in the reactors with high feed substrate concentrations and short HRTs. Phenomena in digester failure due to methanogen washout were also observed.     

Monitoring the role of aceticlasts in anaerobic digestion: activity and capacity

Aceticlastic methanogens are seen as a key to digester capacity and stability. This paper develops and applies an assay to measure digester stability by measuring the maximum aceticlastic methane production rate (V_max,ac). The V_max,ac in combination with acetate concentrations was found to be an effective digestion monitoring tool to indicate process upsets. At steady state, thermophilic, first stage and short SRT digesters generally had a greater V_max,ac than mesophilic, second stage or long SRT digesters. The ratio of the V_max,ac to the plant aceticlastic methane production rate, termed the Acetate Capacity Number (ACN), is a measure of the excess capacity of the digester. Either V_max,ac or ACN can be used to estimate the capability to handle higher organic loading rates. Monod modeling was used to predict V_max,ac, ACN and maximum VS loading rates for mesophilic and thermophilic digestion and for staged digesters to better understand expected digestion capacity and stability.     

Methanogenic community shift in anaerobic batch digesters treating swine wastewater

Qualitative and quantitative molecular analysis techniques were used to determine associations between differences in methanogenic microbial communities and the efficiency of batch anaerobic digesters. Two bioreactors were initially seeded with anaerobic sludge originating from a local municipal wastewater treatment plant and then supplemented with swine wastewater. Differences were observed in the total amount of methane produced in the two bioreactors (7.9 L/L, and 4.5 L/L, respectively). To explain these differences, efforts were taken to characterize the microbial populations present using a PCR-based DGGE analysis with methanogenic primer and probe sets. The groups Methanomicrobiales (MMB), Methanobacteriales (MBT), and Methanosarcinales (MSL) were detected, but Methanococcales (MCC) was not detected. Following this qualitative assay, real-time PCR was used to investigate quantitative differences in the populations of these methanogenic orders. MMB was found to be the dominant order present and its abundance patterns were different in the two digesters. The population profiles of the other methanogenic groups also differed. Through redundancy analysis, correlations between the concentrations of the different microbes and chemical properties such as volatile fatty acids were calculated. Correlations between MBT and MSL populations and chemical properties were found to be consistent in both digesters, however, differences were observed in the correlations between MMB and propionate. These results suggest that interactions between populations of MMB and other methanogens affected the final methane yield, despite MMB remaining the dominant group overall. The exact details of why changes in the MMB community caused different profiles of methane production could not be ascertained. However, this research provides evidence that microbial behavior is important for regulating the performance of anaerobic processes.     

Characterization of sulfate-reducing granular sludge in the SANI process

Hong Kong practices seawater toilet flushing covering 80% of the population. A sulfur cycle-based biological nitrogen removal process, the Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANI^®) process, had been developed to close the loop between the hybrid water supply and saline sewage treatment. To enhance this novel process, granulation of a Sulfate-Reducing Up-flow Sludge Bed (SRUSB) reactor has recently been conducted for organic removal and provision of electron donors (sulfide) for subsequent autotrophic denitrification, with a view to minimizing footprint and maximizing operation resilience. This further study was focused on the biological and physicochemical characteristics of the granular sulfate-reducing sludge. A lab-scale SRUSB reactor seeded with anaerobic digester sludge was operated with synthetic saline sewage for 368 days. At 1 h nominal hydraulic retention time (HRT) and 6.4 kg COD/m³-d organic loading rate, the SRUSB reactor achieved 90% COD and 75% sulfate removal efficiencies. Granular sludge was observed within 30 days, and became stable after 4 months of operation with diameters of 400–500 μm, SVI₅ of 30 ml/g, and extracellular polymeric substances of 23 mg carbohydrate/g VSS. Fluorescence in situ hybridization (FISH) analysis revealed that the granules were enriched with abundant sulfate-reducing bacteria (SRB) as compared with the seeding sludge. Pyrosequencing analysis of the 16S rRNA gene in the sulfate-reducing granules on day 90 indicated that the microbial community consisted of a diverse SRB genera, namely Desulfobulbus (18.1%), Desulfobacter (13.6%), Desulfomicrobium (5.6%), Desulfosarcina (0.73%) and Desulfovibrio (0.6%), accounting for 38.6% of total operational taxonomic units at genera level, with no methanogens detected. The microbial population and physicochemical properties of the granules well explained the excellent performance of the granular SRUSB reactor.     

Quantitative analysis of methanogenic community dynamics in three anaerobic batch digesters treating different wastewaters

Quantitative changes in methanogenic community structures, associated with performance data, were investigated in three anaerobic batch digesters treating synthetic glucose medium, whey permeate, and liquefied sewage sludge. All digesters were initially seeded with anaerobic sludge obtained from a local municipal wastewater treatment plant. Dynamics of methanogenic populations were monitored, at order and family levels, using real-time PCR based on the 16S rRNA gene. The molecular monitoring revealed that, in each digester, the quantitative structure of methanogenic community varied continuously over treatment time and the variation corresponded well to the changes in chemical profiles. Biphasic production of methane, associated with successive increases in aceticlastic (mainly Methanosarcinaceae) and hydrogenotrophic (mainly Methanomicrobiales) methanogenic groups, was observed in each digester. This corresponded to the diauxic utilization of acetate and longer-chain volatile fatty acids (C₃-C₆), mainly propionate. Additionally, the non-metric multidimensional scaling (NMDS) analysis of the quantification results demonstrated that the community shift patterns in three digesters were totally different from each other. Considering that the operating conditions in all trials were identical except substrates, the differences in quantitative shift profiles were suggested to be due to the different substrate compositions. This implied that the composition of wastewater could affect the evolution of quantitative methanogenic community structure in an anaerobic process. Overall, our results suggested that more attention to quantitative as well as qualitative approaches on microbial communities is needed for fundamental understanding of anaerobic processes, particularly under dynamic or transitional conditions.     

Interactions between methanogenic,sulfate-reducing and syntrophic acetogenic bacteria in the anaerobic degradation of benzoate

Effect of sulfate on the anaerobic degradation of benzoate was investigated by using the chemostat-type reactors at 35°C. The benzoate concentrations were equivalent to 1250–10000 mg.l⁻¹ in COD (chemical oxygen demand) and the sulfate concentrations were equivalent to 167–1670 mg.l⁻¹ in sulfur (S). Interactions between the methane-producing bacteria (MPB) and sulfate-reducing bacteria (SRB) were dependent strongly on the ratio of COD/S in wastewater. The MPB consumed 99% of the available electron donors at COD/S ratio of 60, but consumed only 69% at ratio of 1.5, and 13% at 0.75. The biochemical reactions and the bacterial composition in the biomass were also governed by the COD/S ratio. At high COD/S ratios (3.0 or higher), benzoate was degraded mainly to methane via acetate and hydrogen/formate. The degradation of benzoate required the syntrophic association between the hydrogen-producing acetogens such as Syntrophus buswellii and hydrogen-consuming MPB, plus Methanothrix-like MPB. On the other hand, at low COD/S ratio (1.5 or lower), benzoate was consumed mainly by SRB, converting sulfate into sulfide and suppressing the methane production. The anaerobic degradation of benzoate was partially inhibited when sulfide concentration was high.     

Kinetic modelling and microbial community assessment of anaerobic biphasic fixed film bioreactor treating distillery spent wash

Anaerobic digestion, microbial community structure and kinetics were studied in a biphasic continuously fed, upflow anaerobic fixed film reactor treating high strength distillery wastewater. Treatment efficiency of the bioreactor was investigated at different hydraulic retention times (HRT) and organic loading rates (OLR 5-20 kg COD m⁻³ d⁻¹). Applying the modified Stover-Kincannon model to the reactor, the maximum removal rate constant (U_max) and saturation value constant (K_B) were found to be 2 kg m⁻³ d⁻¹ and 1.69 kg m⁻³ d⁻¹ respectively. Bacterial community structures of acidogenic and methanogenic reactors were assessed using culture-independent analyses. Sequencing of 16S rRNA genes exhibited a total of 123 distinct operational taxonomic units (OTUs) comprising 49 from acidogenic reactor and 74 (28 of eubacteria and 46 of archaea) from methanogenic reactor. The findings reveal the role of Lactobacillus sp. (Firmicutes) as dominant acid producing organisms in acidogenic reactor and Methanoculleus sp. (Euryarchaeotes) as foremost methanogens in methanogenic reactor.     

A biofilm model for prediction of pollutant transformation in sewers

This study developed a new sewer biofilm model to simulate the pollutant transformation and biofilm variation in sewers under aerobic, anoxic and anaerobic conditions. The biofilm model can describe the activities of heterotrophic, autotrophic, and sulfate-reducing bacteria (SRB) in the biofilm as well as the variations in biofilm thickness, the spatial profiles of SRB population and biofilm density. The model can describe dynamic biofilm growth, multiple biomass evolution and competitions among organic oxidation, denitrification, nitrification, sulfate reduction and sulfide oxidation in a heterogeneous biofilm growing in a sewer. The model has been extensively verified by three different approaches, including direct verification by measurement of the spatial concentration profiles of dissolved oxygen, nitrate, ammonia, and hydrogen sulfide in sewer biofilm. The spatial distribution profile of SRB in sewer biofilm was determined from the fluorescent in situ hybridization (FISH) images taken by a confocal laser scanning microscope (CLSM) and were predicted well by the model.     

Analysis of methane emissions in a tunnel excavated through Carboniferous strata based on underground coal mining experience

During the excavation of different segments of the Variante de Pajares tunnels by means of single shield TBMs (tunnel boring machines), methane has been found. The analysis of the recorded data has allowed to prove that the natural gas emission in a tunnel excavating through Carboniferous strata is similar to that which occurs in an underground coal mine. In this paper, two mining parameters for the characterisation of methane emission, the methane emission rate (volume of methane emitted per ton of excavated rock) and average methane flow rate (volume of methane emitted per day), are introduced. Following, using the measurements of TBM advancing rate, quantity air flow and methane concentration carried out during the tunnel excavation through San Emiliano geological formation these two parameters have been determined for the tunnel. The results were very similar to those observed in coal mines, which means that mining experience can be used in order to predict the methane inflow into a tunnel excavated through a Carboniferous rockmass which is very useful for the design of the ventilation system, something very significant for safety.     

Adaptation of methanogenic sludge to high ammonia-nitrogen concentrations

The influence of ammonia-nitrogen concentrations in excess of 1500 mg 1⁻¹ on the methane formation from volatile fatty acids by two types of methanogenic sludge was investigated in batch experiments. One was digested sewage sludge, acclimated to 815 mg 1⁻¹ ammonia-nitrogen and the other was digested piggery manure, acclimated to an ammonia-nitrogen concentration of 2420 mg 1⁻¹. In the experiment with digested sewage sludge, methane formation took place still at an ammonia-nitrogen concentration as high as 5 g 1⁻¹. However, an increasing lag-phase was observed at increasing ammonia-nitrogen concentrations in the range 730–4990 mg 1⁻¹. On the other hand in digested piggery manure methane formation immediately started without any lag-phase in the ammonia-nitrogen concentration range of 605–3075 mg 1⁻¹. In the experiments with both types of sludge the maximum methane formation rate slowly decreased with increasing ammonia-nitrogen concentrations.     

Classification and assessment of rock mass parameters in Choghart iron mine using P-wave velocity

Engineering rock mass classification,based on empirical relations between rock mass parameters and engineering applications,is commonly used in rock engineering and forms the basis for designing rock structures.The basic data required may be obtained from visual observation and laboratory or field tests.However,owing to the discontinuous and variable nature of rock masses,it is difficult for rock engineers to directly obtain the specific design parameters needed.As an alternative,the use of geophysical methods in geomechanics such as seismography may largely address this problem.In this study,25 seismic profiles with the total length of 543 m have been scanned to determine the geomechanical properties of the rock mass in blocks Ⅰ,Ⅲ and Ⅳ-2 of the Choghart iron mine.Moreover,rock joint measurements and sampling for laboratory tests were conducted.The results show that the rock mass rating(RMR) and Q values have a close relation with P-wave velocity parameters,including P-wave velocity in field(V_(PF)).P-wave velocity in the laboratory(V_(PL)) and the ratio of V_(PF) V_(PL)(i.e.K_p = V_(PF)/V_(PL).However,Q value,totally,has greater correlation coefficient and less error than the RMR,In addition,rock mass parameters including rock quality designation(RQD),uniaxial compressive strength(UCS),joint roughness coefficient(JRC) and Schmidt number(RN) show close relationship with P-wave velocity.An equation based on these parameters was obtained to estimate the P-wave velocity in the rock mass with a correlation coefficient of 91%.The velocities in two orthogonal directions and the results of joint study show that the wave velocity anisotropy in rock mass may be used as an efficient tool to assess the strong and weak directions in rock mass.     

Biological oxidation of dissolved methane in effluents from anaerobic reactors using a down-flow hanging sponge reactor

Anaerobic wastewater treatment plants discharge dissolved methane, which is usually not recovered. To prevent emission of methane, which is a greenhouse gas, we utilized an encapsulated down-flow hanging sponge reactor as a post-treatment to biologically oxidize dissolved methane. Within 3 weeks after reactor start-up, methane removal efficiency of up to 95% was achieved with a methane removal rate of 0.8 kg COD m⁻³ day⁻¹ at an HRT of 2 h. After increasing the methane-loading rate, the maximum methane removal rate reached 2.2 kg COD m⁻³ day⁻¹ at an HRT of 0.5 h. On the other hand, only about 10% of influent ammonium was oxidized to nitrate during the first period, but as airflow was increased to 2.5 L day⁻¹, nitrification efficiency increased to approximately 70%. However, the ammonia oxidation rate then decreased with an increase in the methane-loading rate. These results indicate that methane oxidation occurred preferentially over ammonium oxidation in the reactor. Cloning of the 16S rRNA and pmoA genes as well as phylogenetic and T-RFLP analyses revealed that type I methanotrophs were the dominant methane oxidizers, whereas type II methanotrophs were detected only in minor portion of the reactor.     

Improved prediction of shear wave velocity for clastic sedimentary rocks using hybrid model with core data

Accurate measurement of acoustic velocities of sedimentary rocks is essential for prediction of rock elastic constants and well failure analysis during drilling operations. Direct measurement by advanced logging tools such as dipole sonic imager is not always possible. For older wells, such data are not available in most cases. Therefore, it is an alternate way to develop a reliable correlation to estimate the shear wave velocity from existing log and/or core data. The objective of this research is to investigate the nature of dependency of different reservoir parameters on the shear wave velocity (V_s) of clastic sedimentary rocks, and to identify the parameter/variable which shows the highest level of dependency. In the study, data-driven connectionist models are developed using machine learning approach of least square support vector machine (LSSVM). The coupled simulated annealing (CSA) approach is utilized to optimize the tuning and kernel parameters in the model development. The performance of the simulation-based model is evaluated using statistical parameters. It is found that the most dependency predictor variable is the compressional wave velocity, followed by the rock porosity, bulk density and shale volume in turn. A new correlation is developed to estimate V_s, which captures the most influential parameters of sedimentary rocks. The new correlation is verified and compared with existing models using measured data of sandstone, and it exhibits a minimal error and high correlation coefficient (R² = 0.96). The hybridized LSSVM-CSA connectionist model development strategy can be applied for further analysis to predict rock mechanical properties. Additionally, the improved correlation of V_s can be adopted to estimate rock elastic constants and conduct wellbore failure analysis for safe drilling and field development decisions, reducing the exploration costs.     

Molecular assessment of the sensitivity of sulfate-reducing microbial communities remediating mine drainage to aerobic stress

Sulfate-reducing permeable reactive zones (SR-PRZs) are microbially-driven anaerobic systems designed for the removal of heavy metals and sulfate in mine drainage. Environmental perturbations, such as oxygen exposure, may adversely affect system stability and long-term performance. The objective of this study was to examine the effect of two successive aerobic stress events on the performance and microbial community composition of duplicate laboratory-scale lignocellulosic SR-PRZs operated using the following microbial community management strategies: biostimulation with ethanol or carboxymethylcellulose; bioaugmentation with sulfate-reducing or cellulose-degrading enrichments; inoculation with dairy manure only; and no inoculation. A functional gene-based approach employing terminal restriction fragment length polymorphism and quantitative polymerase chain reaction targeting genes of sulfate-reducing (dsrA), cellulose-degrading (cel5, cel48), fermentative (hydA), and methanogenic (mcrA) microbes was applied. In terms of performance (i.e., sulfate removal), biostimulation with ethanol was the only strategy that clearly had an effect (positive) following exposure to oxygen. In terms of microbial community composition, significant shifts were observed over the course of the experiment. Results suggest that exposure to oxygen more strongly influenced microbial community shifts than the different microbial community management strategies. Sensitivity to oxygen exposure varied among different populations and was particularly pronounced for fermentative bacteria. Although the community structure remained altered after exposure, system performance recovered, indicating that SR-PRZ microbial communities were functionally redundant. Results suggest that pre-exposure to oxygen might be a more effective strategy to improve the resilience of SR-PRZ microbial communities relative to bioaugmentation or biostimulation.