Performance of a sulfide-oxidizing expanded-bed reactor supplied with dissolved oxygen

Using the sulfide-oxidizing bacterium Geobacillus thermodenitrificans to restrict H2S release during chicken manure composting

Hydrogen sulfide (H2S) is a toxic gas massively released during chicken manure composting. Diminishing its release requires efficient and low cost methods. In recent years, heterotrophic bacteria capable of rapid H2S oxidation have been discovered but their applications in environmental improvement are rarely reported. Herein, we investigated H2S oxidation activity of a heterotrophic thermophilic bacterium Geobacillus thermodenitrificans DSM465, which contains a H2S oxidation pathway composed by sulfide:quinone oxidoreductase (SQR) and persulfide dioxygenase (PDO). This strain rapidly oxidized H2S to sulfane sulfur and thiosulfate. The oxidation rate reached 5.73 μmol min-1·g-1 of cell dry weight. We used G. thermodenitrificans DSM465 to restrict H2S release during chicken manure composting. The H2S emission during composting process reduced by 27.5% and sulfate content in the final compost increased by 34.4%. In addition, this strain prolonged the high temperature phase by 7 days. Thus, using G. thermodenitrificans DSM465 to control H2S release was an efficient and economic method. This study provided a new strategy for making waste composting environmental friendly and shed light on perspective applications of heterotrophic H2S oxidation bacteria in environmental improvements.

Enhancement of bio-S0 recovery and revealing the inhibitory effect on microorganisms under high sulfide loading

Biodesulfurization is a mature technology, but obtaining biosulfur (S0) that can be easily settled naturally is still a challenge. Increasing the sulfide load is one of the known methods to obtain better settling of S0. However, the inhibitory effect of high levels of sulfide on microbes has also not been well studied. We constructed a high loading sulfide (1.55-10.86 kg S/m3/d) biological removal system. 100% sulfide removal and 0.56-2.53 kg S/m3/d S0 (7.0 ± 0.09-16.4 ± 0.25 μm) recovery were achieved at loads of 1.55-7.75 kg S/m3/d. Under the same load, S0 in the reflux sedimentation tank, which produced larger S0 particles (24.2 ± 0.73-53.8 ± 0.70 μm), increased the natural settling capacity and 45% recovery. For high level sulfide inhibitory effect, we used metagenomics and metatranscriptomics analyses. The increased sulfide load significantly inhibited the expression of flavin cytochrome c sulfide dehydrogenase subunit B (fccB) (Decreased from 615 ± 75 to 30 ± 5 TPM). At this time sulfide quinone reductase (SQR) (324 ± 185-1197 ± 51 TPM) was mainly responsible for sulfide oxidation and S0 production. When the sulfide load reached 2800 mg S/L, the SQR (730 ± 100 TPM) was also suppressed. This resulted in the accumulation of sulfide, causing suppression of carbon sequestration genes (Decreased from 3437 ± 842 to 665 ± 175 TPM). Other inhibitory effects included inhibition of microbial respiration, production of reactive oxygen species, and DNA damage. More sulfide-oxidizing bacteria (SOB) and newly identified potential SOB (99.1%) showed some activity (77.6%) upon sulfide accumulation. The main microorganisms in the sulfide accumulation environment were Thiomicrospiracea and Burkholderiaceae, whose sulfide oxidation capacity and respiration were not significantly inhibited. This study provides a new approach to enhance the natural sedimentation of S0 and describes new microbial mechanisms for the inhibitory effects of sulfide.

Machine learning-based model construction and identification of dominant factor for simultaneous sulfide and nitrate removal process

Accurate water quality prediction models are essential for the successful implementation of the simultaneous sulfide and nitrate removal process (SSNR). Traditional models, such as regression and analysis of variance, do not provide accurate predictions due to the complexity of microbial metabolic pathways. In contrast, Back Propagation Neural Networks (BPNN) has emerged as superior tool for simulating wastewater treatment processes. In this study, a generalized BPNN model was developed to simulate and predict sulfide removal, nitrate removal, element sulfur production, and nitrogen gas production in SSNR. Remarkable results were obtained, indicating the strong predictive performance of the model and its superiority over traditional mathematical models for accurately predicting the effluent quality. Furthermore, this study also identified the crucial influencing factors for the process optimization and control. By incorporating artificial intelligence into wastewater treatment modeling, the study highlights the potential to significantly enhance the efficiency and effectiveness of meeting water quality standards.

Recent advances in biological technologies for anoxic biogas desulfurization

Recovery of the energy contained in biogas will be essential in coming years to reduce greenhouse gas emissions and our current dependence on fossil fuels. The elimination of H₂S is a priority to avoid equipment corrosion, poisoning of catalytic systems and SO₂ emissions in combustion engines. This review describes the advances made in this technology using fixed biomass bioreactors (FBB) and suspended growth bioreactors (SGB) since the first studies in this field in 2008. Anoxic desulfurization has been studied mainly in biotrickling filters (BTF). Elimination capacities (EC) up to 287 gS m^-3 h^-1 have been achieved, with a removal efficiency (RE) of 99%. Both nitrate and nitrite have been successfully used as electron acceptor. SGBs can solve some operational problems present in FBBs, such as clogging or nutrient distribution issues. However, they present greater difficulties in gas-liquid mass transfer, although ECs of up to 194 gS m^-3 h^-1 have been reported in both gas-lift and stirred tank reactors. One of the major disadvantages of using anoxic biodesulfurization compared to aerobic biodesulfurization is the need to provide reagents (nitrates and/or nitrites), with the consequent increase in operating costs. A solution proposed in this respect is the use of nitrified effluents, some ammonium-rich effluents nitrified include landfill leachate and digested effluent from the anaerobic digester have been tested successfully. Among the microbial diversity found in the bioreactors, the genera Thiobacillus, Sulfurimonas and Sedimenticola play a key role in anoxic removal of H₂S. Finally, a summary of future trends in technology is provided.

Metagenomic analysis of microbial community and metabolic pathway of simultaneous sulfide and nitrite removal process exposed to divergent hydraulic retention times

The role of hydraulic retention time (HRT) on S⁰ production was assessed through metagenomics analyses. Considering comprehensive performance for the tested HRTs (0.25-13.33 h), the optimal HRT was 1 h, while respective sulfide and nitrite loading rate could reach 6.84 kg S/(m³·d) and 1.95 kg N/(m³·d), and total S⁰ yield was 0.36 kg S/(kg (VSS)·d). Bacterial community richness decreased along the shortening of HRT. Microbacterium, Sulfurimonas, Sulfurovum, Paracoccus and Thauera were highly abundant bacteria. During sulfur metabolism, high expression of sqr gene was the main reason of maintaining high desulfurization load, while lacking soxB caused the continuous increase of S⁰. Regarding nitrogen metabolism, the rapid decrease of nitrite transporter prevented nitrite to enter in cells, which caused a rapid decrease of nitrite removal under extreme HRT. Adjusting HRT is an effective way to enhance S⁰ production for the application of the simultaneous sulfide and nitrite removal process.

High-Rate Sulfate Removal Coupled to Elemental Sulfur Production in Mining Process Waters Based on Membrane-Biofilm Technology

It is anticipated that copper mining output will significantly increase over the next 20 years because of the more intensive use of copper in electricity-related technologies such as for transport and clean power generation, leading to a significant increase in the impacts on water resources if stricter regulations and as a result cleaner mining and processing technologies are not implemented. A key concern of discarded copper production process water is sulfate. In this study we aim to transform sulfate into sulfur in real mining process water. For that, we operate a sequential 2-step membrane biofilm reactor (MBfR) system. We coupled a hydrogenotrophic MBfR (H₂-MBfR) for sulfate reduction to an oxidizing MBfR (O₂-MBfR) for oxidation of sulfide to elemental sulfur. A key process improvement of the H₂-MBfR was online pH control, which led to stable high-rate sulfate removal not limited by biomass accumulation and with H₂ supply that was on demand. The H₂-MBfR easily adapted to increasing sulfate loads, but the O₂-MBfR was difficult to adjust to the varying H₂-MBfR outputs, requiring better coupling control. The H₂-MBfR achieved high average volumetric sulfate reduction performances of 1.7-3.74 g S/m³-d at 92-97% efficiencies, comparable to current high-rate technologies, but without requiring gas recycling and recompression and by minimizing the H₂ off-gassing risk. On the other hand, the O₂-MBfR reached average volumetric sulfur production rates of 0.7-2.66 g S/m³-d at efficiencies of 48-78%. The O₂-MBfR needs further optimization by automatizing the gas feed, evaluating the controlled removal of excess biomass and S⁰ particles accumulating in the biofilm, and achieving better coupling control between both reactors. Finally, an economic/sustainability evaluation shows that MBfR technology can benefit from the green production of H₂ and O₂ at operating costs which compare favorably with membrane filtration, without generating residual streams, and with the recovery of valuable elemental sulfur.

Development and validation of a sampling and analysis method to determine biogenic sulfur in a desulfurization bioreactor by gas chromatography coupled with a pulsed flame photometric detector (GC-PFPD)

Suspended biomass bioreactors can be operated to remove H₂S from biogas under anoxic conditions and produce elemental sulfur, the commercial value of which has been demonstrated. In the present paper, a novel methodology comprising the optimization of a determination method performed in a gas chromatograph equipped with a pulsed flame photometric detector (GC-PFPD), combined with a simple preparation based on filtration and extraction with toluene, is proposed. The injector temperature and carrier gas flow rate (Q_He) values were optimized using a response surface methodology based on a face-centred composite central design. This optimization revealed that the optimum conditions were an injector temperature and carrier gas flow rate of 222 °C and 7 mL min^-1, respectively. The chromatographic method shows an analysis time of 48 min, a detection limit of more than 5.9 mg L^-1, a relative standard deviation of less than 3.71%, and a sulfur recovery percentage of more than 98%. These values provide excellent linearity and a reasonable concentration range (10-200 mg L^-1). Finally, a measurement error of 4.45% was obtained when using the present method in a selectivity test.

Response difference of simultaneous sulfide and nitrite removal process to different cooling modes

The effects of various cooling modes (sudden cooling (25℃→10℃) and step cooling (25℃→20℃→15℃→10℃)) on the performance of simultaneous sulfide and nitrite removal process were reported. Regardless of cooling mode adopted, the process maintained good sulfide removal performance, and removal percentage was 100.00%. Considering nitrite removal percentage, the process was more sensitive to step cooling mode (k = 0.06707) in comparison to sudden cooling mode (k = 0.02760). Lowering temperature promoted the transformation from sulfate to elemental sulfur, and it was easier to increase the proportion of elemental sulfur (79.90%) by means of step cooling. The sulfide oxidation rate and nitrite reduction rate were 0.01540 mg /(L∙min) and 0.00354 mg /(L∙min), respectively, in the sudden cooling mode, and 0.01168 mg /(L∙min) and 0.00138 mg /(L∙min), respectively, in the step cooling mode. Low temperature reduced the diversity of microbial community, and Sulfurovum was still a dominant bacterial member in both cooling modes.

Production and characteristics of elemental sulfur during simultaneous nitrate and sulfide removal

The production and characteristics of elemental sulfur were examined during simultaneous sulfide and nitrate removal, with abiotic assays as control. The biotic assay showed good sulfide and nitrate removal, with the respective removal percentage of which were 90.67-96.88% and 100%. Nitrate reduction resulted in the production of nitrogen gas, while sulfate formed due to sulfide oxidation. The concentration of elemental sulfur in the effluent was greater than that in the sludge, which accounted for 73.70-86.28% of total elemental sulfur produced. Furthermore, the elemental sulfur of the effluent and sludge from the biotic assays was orthorhombic crystal S₈. Elemental sulfur was normally distributed in the effluent, but its average diameter increased with the increasing influent sulfide concentration (60-300 mg S/L), where the average diameter increased from 10 (60 mg S/L) to 29 μm (300 mg S/L).

Removal of dimethyl sulphide via a bio-scrubber under anoxic conditions

The removal performance of dimethyl sulphide (DMS) by anoxic laboratory-scale bio-scrubber was studied under different operation conditions for 315 days. DMS removal in bio-scrubber system was performed by controlling and changing the operation parameters, including inlet concentration, empty bed residence time (EBRT) and spraying density (SD) of irrigation. Best conditions in the system were achieved for SD of 0.18 m³/m² h within EBRT of 40 s at an inlet gas concentration of 150 mg/m³ in which 93% of waste gas stream was removed in the bio-scrubber column and bio-degradation in the bio-reactor tank led to 89% of DMS removal from the transferred bio-reactor, while 91.5% of input chemical oxygen demand (COD) was successfully removed. The use of closer values of the average experimental yield to the theoretical value (Y_NO3/NO3 -) of 0.74 led to the production of elemental sulphur (S°) and other sulphur forms rather than sulphate (SO₄^2-) , which was also was recognized as a pale-yellow coloured substance of S° that appeared within the biomass.

Effect of sulfate removal in a high sulfate volumetric loading micro-aerobic bio-reactor and study of subsequent bio-sulfur adsorption by iron-modified activated carbon

Removal of sulfide from a micro-aerobic bio-reactor was studied at 10 000 mg L⁻¹ chemical oxygen demand (COD) of inlet water, with the sulfate volumetric loading 0.75, 1.0, 1.5 and 2.0 kg (m⁻³ d⁻¹), respectively. Tentatively, activated carbon (AC) as an adsorbent was modified in positively charged iron to adsorb bio-sulfur through electrostatic interaction. At an O₂/S molar ratio of 8–10, the reactor was sufficient to decrease the sulfide in the effluent and biogas to low levels at the sulfate volumetric loading of 2 kg (m⁻³ d⁻¹). After iron-modified, the specific surface area of AC was form 32.4 m² g⁻¹ to 65.0 m² g⁻¹, and the zeta potential was 25.3 mV at pH 7.0. The XRD pattern of the iron-modified activated carbon (FeAC) explained that the metal species of iron was Fe₃O₄. It could be clearly seen that there was Fe₃O₄ on the surface of the FeAC, and sulfur particles with a large particle size were adsorbed by the FeAC on the SEM figures. And the XRD pattern of the bio-sulfur explained that the bio-sulfur was made up of S₈ (91.444%), C₃H₄N₂OS (1.491%) and CH₅N₃S (7.075%). The zeta potential of bio-sulfur was −25 mV and the particle size was mainly distributed at the average diameter of 1935 nm at pH 7.0.

Removal of sulfide from a micro-aerobic bio-reactor was studied at 10 000 mg L⁻¹ chemical oxygen demand (COD) of inlet water, with the sulfate volumetric loading 0.75, 1.0, 1.5 and 2.0 kg (m⁻³ d⁻¹), respectively.

Effect of dimethyl disulfide on the sulfur formation and microbial community composition during the biological H2S removal from sour gas streams

Removal of organic and inorganic sulfur compounds from sour gases is required because of their toxicity and atmospheric pollution. The most common are hydrogen sulfide (H₂S) and methanethiol (MT). Under oxygen-limiting conditions about 92 mol% of sulfide is oxidized to sulfur by haloalkaliphilic sulfur-oxidizing bacteria (SOB), whilst the remainder is oxidized either biologically to sulfate or chemically to thiosulfate. MT is spontaneously oxidized to dimethyl disulfide (DMDS), which was found to inhibit the oxidation of sulfide to sulfate. Hence, we assessed the effect of DMDS on product formation in a lab-scale biodesulfurization setup. DMDS was quantified using a newly, in-house developed analytical method. Subsequently, a chemical reaction mechanism was proposed for the formation of methanethiol and dimethyl trisulfide from the reaction between sulfide and DMDS. Addition of DMDS resulted in significant inhibition of sulfate formation, leading to 96 mol% of sulfur formation. In addition, a reduction in the dominating haloalkaliphilic SOB species, Thioalkalivibrio sulfidiphilus, was observed in favor of Thioalkaibacter halophilus as a more DMDS-tolerant with the 50 % inhibition coefficient at 2.37 mM DMDS.

Sulfur recovery in the sulfide-oxidizing membrane aerated biofilm reactor: experimental investigation and model simulation

The production of sulfur (S) from the biological oxidization of sulfide (S^2-) by SOB (sulfide-oxidizing bacteria) allows for resource recovery. Past researches have indicated that S recovery from S^2- oxidation MABR (the membrane aerated biofilm reactor) was feasible. The process was complicated by the requirement of maintaining appropriate oxygen supply to prevent the produced S from being further oxidized into sulfate ( SO42- ) and by the presence of heterotrophic biomass. In this study, a multispecies biofilm model was developed and experimentally validated to gain insight for the S recovery process in MABR. The developed model was capable of predicting the S recovery performance in the MABR. The optimal conditions involved in maintaining the appropriate oxygen flux and the biofilm thickness according to the hydraulic and S^2- loading rate. The low anoxic heterotrophic growth rate using SO42- and S as electron donors could explain why the impact of heterotrophic growth was insignificant.

Ethanethiol gas removal in an anoxic bio-scrubber

The performance of ethanethiol removal in an anoxic lab-scale bio-scrubber was investigated under different operating parameters and conditions for 300 days. The removal efficiency (RE) of ethanethiol was examined as a function of inlet concentration, empty bed residence time (EBRT) and spray density of irrigation. The results showed the best operation conditions and operation characteristics of the bio-scrubber for this study were at an inlet concentration of 150 mg/m³, a spray density of 0.23 m³/m² h and an EBRT of 90 s. An average RE of 91% and elimination capacity (EC) of 24.74 g/m³ h was found for all inlet ethanethiol concentrations. Variations in spray density higher than 0.23 m³/m² h had no effect on ethanethiol RE at different ethanethiol concentrations. The average experimental yield values were closer to the Y_ET/NO3^- theoretical value of 0.74 when the main product was elemental sulphur (S^o). This indicates that S^o and other forms of sulphur were formed rather than sulphate (SO₄^2-) as the end product. Furthermore, growth kinetics for bio-degradation were evaluated in batch culture experiments using the Monod model, and bio-kinetic parameters of μ_max, K_s, Y_xs and q_max were obtained as 0.14 1/h, 1.17 mg/L, 0.52 g_x/g_s and 0.26 g_s/g_x h, respectively.

Biological sulfur oxidation in wastewater treatment: A review of emerging opportunities

Sulfide prevails in both industrial and municipal waste streams and is one of the most troublesome issues with waste handling. Various technologies and strategies have been developed and used to deal with sulfide for decades, among which biological means make up a considerable portion due to their low operation requirements and flexibility. Sulfur bacteria play a vital role in these biotechnologies. In this article, conventional biological approaches dealing with sulfide and functional microorganisms are systematically reviewed. Linking the sulfur cycle with other nutrient cycles such as nitrogen or phosphorous, and with continued focus of waste remediation by sulfur bacteria, has led to emerging biotechnologies. Furthermore, opportunities for energy harvest and resource recovery based on sulfur bacteria are also discussed. The electroactivity of sulfur bacteria indicates a broad perspective of sulfur-based bioelectrochemical systems in terms of bioelectricity production and bioelectrochemical synthesis. The considerable PHA accumulation, high yield and anoxygenic growth conditions in certain phototrophic sulfur bacteria could provide an interesting alternative for bioplastic production. In this review, new merits of biological sulfide oxidation from a traditional environmental management perspective as well as a waste to resource perspective are presented along with their potential applications.

H2S biotreatment with sulfide-oxidizing heterotrophic bacteria

Many industrial activities produce H₂S, which is toxic at high levels and odorous at even very low levels. Chemolithotrophic sulfur-oxidizing bacteria are often used in its remediation. Recently, we have reported that many heterotrophic bacteria can use sulfide:quinone oxidoreductase and persulfide dioxygenase to oxidize H₂S to thiosulfate and sulfite. These bacteria may also potentially be used in H₂S biotreatment. Here we report how various heterotrophic bacteria with these enzymes were cultured with organic compounds and the cells were able to rapidly oxidize H₂S to zero-valence sulfur and thiosulfate, causing no apparent acidification. Some also converted the produced thiosulfate to tetrathionate. The rates of sulfide oxidation by some of the tested bacteria in suspension, ranging from 8 to 50 µmol min⁻¹ g⁻¹ of cell dry weight at pH 7.4, sufficient for H₂S biotreatment. The immobilized bacteria removed H₂S as efficiently as the bacteria in suspension, and the inclusion of Fe₃O₄ nanoparticles during immobilization resulted in increased efficiency for sulfide removal, in part due to chemical oxidation H₂S by Fe₃O₄. Thus, heterotrophic bacteria may be used for H₂S biotreatment under aerobic conditions.

Mathematical modeling of simultaneous carbon-nitrogen-sulfur removal from industrial wastewater

A mathematical model of carbon, nitrogen and sulfur removal (C-N-S) from industrial wastewater was constructed considering the interactions of sulfate-reducing bacteria (SRB), sulfide-oxidizing bacteria (SOB), nitrate-reducing bacteria (NRB), facultative bacteria (FB), and methane producing archaea (MPA). For the kinetic network, the bioconversion of C-N by heterotrophic denitrifiers (NO₃^-→NO₂^-→N₂), and that of C-S by SRB (SO₄^2-→S^2-) and SOB (S^2-→S⁰) was proposed and calibrated based on batch experimental data. The model closely predicted the profiles of nitrate, nitrite, sulfate, sulfide, lactate, acetate, methane and oxygen under both anaerobic and micro-aerobic conditions. The best-fit kinetic parameters had small 95% confidence regions with mean values approximately at the center. The model was further validated using independent data sets generated under different operating conditions. This work was the first successful mathematical modeling of simultaneous C-N-S removal from industrial wastewater and more importantly, the proposed model was proven feasible to simulate other relevant processes, such as sulfate-reducing, sulfide-oxidizing process (SR-SO) and denitrifying sulfide removal (DSR) process. The model developed is expected to enhance our ability to predict the treatment of carbon-nitrogen-sulfur contaminated industrial wastewater.

Effect of pH on elemental sulfur conversion and microbial communities by autotrophic simultaneous desulfurization and denitrification

pH has an important influence on the elemental sulfur accumulated in an autotrophic simultaneous desulfurization and denitrification process. The influent nitrate to sulfide (N/S) mole ratio was set to 0.5, 0.67, 1.0, 1.33 and 2.0 with a 200 mg/L sulfide concentration. The effect of pH on elemental sulfur conversion and microbial communities was studied. Sulfide removal was achieved to the extent of 98% under near-neutral and weak base conditions after 24 h of reaction. The conversion rate of elemental sulfur was 29.41% under the near-neutral condition. The weak base condition led to greater formation of sulfate, and the nitrate used by the microorganisms was transformed mainly to N2 with a removal rate of 96%. Increasing the retention time from 24 to 48 h caused the removal rate of nitrate increased from 63.58% to 90% under the near-neutral condition. Sulfurovum sp. was the functioning bacterial species, and bands 1 and 2 represent different species of Sulfurovum sp. in the system according to the PCR-DGGE analysis of the microbial community structure. The functional bacteria represented by band 1 produced mainly sulfate, but the functional bacteria represented by band 2 produced mainly elemental sulfur.

Characterization of a newly isolated strain Pseudomonas sp. C27 for sulfide oxidation: Reaction kinetics and stoichiometry

Sulfide biooxidation by the novel sulfide-oxidizing bacteria Pseudomonas sp. C27, which could perform autotrophic and heterotrophic denitrification in mixotrophic medium, was studied in batch and continuous systems. Pseudomonas sp. C27 was able to oxidize sulfide at concentrations as high as 17.66 mM. Sulfide biooxidation occurred in two distinct stages, one resulting in the formation of sulfur with nitrate reduction to nitrite, followed by thiosulfate formation with nitrite reduction to N₂. The composition of end-products was greatly impacted by the ratio of sulfide to nitrate initial concentrations. At a ratio of 0.23, thiosulfate represented 100% of the reaction products, while only 30% with a ratio of 1.17. In the continuous bioreactor, complete removal of sulfide was observed at sulfide concentration as high as 9.38 mM. Overall sulfide removal efficiency decreased continuously upon further increases in influent sulfide concentrations. Based on the experimental data kinetic parameter values were determined. The value of maximum specific growth rate, half saturation constant, decay coefficient, maintenance coefficient and yield were to be 0.11 h⁻¹, 0.68 mM sulfide, 0.11 h⁻¹, 0.21 mg sulfide/mg biomass h and 0.43 mg biomass/mg sulfide, respectively, which were close to or comparable with those reported in literature by other researches.

Aerobic desulfurization of biogas by acidic biotrickling filtration in a randomly packed reactor

Biotrickling filters for biogas desulfurization still must prove their stability and robustness in the long run under extreme conditions. Long-term desulfurization of high loads of H2S under acidic pH was studied in a lab-scale aerobic biotrickling filter packed with metallic Pall rings. Reference operating conditions at steady-state corresponded to an empty bed residence time (EBRT) of 130s, H2S loading rate of 52gS-H2Sm(-3)h(-1) and pH 2.50-2.75. The EBRT reduction showed that the critical EBRT was 75s and the maximum EC 100gS-H2Sm(-3)h(-1). Stepwise increases of the inlet H2S concentration up to 10,000 ppmv lead to a maximum EC of 220gS-H2Sm(-3)h(-1). The H2S removal profile along the filter bed indicated that the first third of the filter bed was responsible for 70-80% of the total H2S removal. The oxidation rate of solid sulfur accumulated inside the bioreactor during periodical H2S starvation episodes was verified under acidic operating conditions. The performance under acidic pH was comparable to that under neutral pH in terms of H2S removal capacity. However, bioleaching of the metallic packing used as support and chemical precipitation of sulfide/sulfur salts occurred.

Bioconversion of high concentrations of hydrogen sulfide to elemental sulfur in airlift bioreactor

Several bioreactor systems are used for biological treatment of hydrogen sulfide. Among these, airlift bioreactors are promising for the bioconversion of hydrogen sulfide into elemental sulfur. The performance of airlift bioreactors is not adequately understood, particularly when directly fed with hydrogen sulfide gas. The objective of this paper is to investigate the performance of an airlift bioreactor fed with high concentrations of H2S with special emphasis on the effect of pH in combination with other factors such as H2S loading rate, oxygen availability, and sulfide accumulation. H2S inlet concentrations between 1,008 ppm and 31,215 ppm were applied and elimination capacities up to 113 g H2S m(-3) h(-1) were achieved in the airlift bioreactor under investigation at a pH range 6.5-8.5. Acidic pH values reduced the elimination capacity. Elemental sulfur recovery up to 95% was achieved under oxygen limited conditions (DO < 0.2 mg/L) and at higher pH values. The sulfur oxidizing bacteria in the bioreactor tolerated accumulated dissolved sulfide concentrations >500 mg/L at pH values 8.0-8.5, and near 100% removal efficiency was achieved. Overall, the resident microorganisms in the studied airlift bioreactor favored pH values in the alkaline range. The bioreactor performance in terms of elimination capacity and sulfur recovery was better at pH range 8-8.5.

Biogas biodesulfurization in an anoxic biotrickling filter packed with open-pore polyurethane foam

Biogas biodesulfurization by an anoxic biotrickling filter packed with open pore polyurethane foam at the laboratory scale (packed volume 2.4L) has been studied. The biotrickling system was operated for 620 days with biogas supplied continuously and two nitrate feeding regimes were tested (manual and programmed). Biomass immobilization was carried out under the manual nitrate feeding regime and a study was then carried out on the effects on removal efficiency of the following parameters: nitrate source, H2S inlet load, nitrate concentration, sulfate accumulation, temperature, pH and trickling liquid velocity. The effect of increased H2S inlet load was studied under the programmed nitrate feeding regime. The results show that a removal efficiency of 99% can be obtained when working under the following conditions: inlet loads below 130gSm(-3)h(-1), a programmed nitrate feeding system, temperature of 30°C, sulfate concentration below 33gL(-1), a pH between 7.3 and 7.5, and a trickling liquid velocity higher than 4.6mh(-1).

Operational aspects, pH transition and microbial shifts of a H2S desulfurizing biotrickling filter with random packing material

Pall rings, a common random packing material, were used in the biotrickling filtration of biogas with high H2S. Assessment of 600d of operation covered the reactor start-up, the operation at neutral pH and the transition from neutral to acid pH. During the start-up period, operational parameters such as the aeration rate and the trickling liquid velocity were optimized. During the steady-state operation at neutral pH, the performance of the random packing material was investigated by reducing the gas contact time at both constant and increasing H2S loads. The random packing material showed similar elimination capacities and removal efficiencies in comparison with previous studies with a structured packing material, indicating that Pall rings are suitable for biogas desulfurization in biotrickling filters. The diversity of Eubacteria and the structure of the community were investigated before and after the pH transition using the bacterial tag-encoded FLX amplicon pyrosequencing. The pH transition to acid pH drastically reduced the microbial diversity and produced a progressive specialization of the sulfur-oxidizing bacteria community without any detrimental effect on the overall desulfurizing capacity of the reactor. During acidic pH operation, a persistent accumulation of elemental sulfur was found.

Sulfate-reduction, sulfide-oxidation and elemental sulfur bioreduction process: modeling and experimental validation

This study describes the sulfate-reducing (SR) and sulfide-oxidizing (SO) process using Monod-type model with best-fit model parameters both being reported and estimated. The molar ratio of oxygen to sulfide (ROS) significantly affects the kinetics of the SR+SO process. The S(0) is produced by SO step but is later consumed by sulfur-reducing bacteria to lead to "rebound" in sulfide concentration. The model correlated well all experimental data in the present SR+SO tests and the validity of this approach was confirmed by independent sulfur bioreduction tests in four denitrifying sulfide removal (DSR) systems. Modeling results confirm that the ratio of oxygen to sulfide is a key factor for controlling S(0) formation and its bioreduction. Overlooking S(0) bioreduction step would overestimate the yield of S(0).

Enhanced elementary sulfur recovery in integrated sulfate-reducing, sulfur-producing rector under micro-aerobic condition

Biological treatment of sulfate-laden wastewater consists of two separate reactors to reduce sulfate to sulfide by sulfate-reducing bacteria (SRB) and to oxidize sulfide to sulfur (S(0)) by sulfide oxidation bacteria (SOB). To have SRB+SOB in a single reactor faced difficulty of low S(0) conversion. This study for the first time revealed that dissolved oxygen (DO) level can be used to manipulate SRB+SOB reactions in a single reactor. This work demonstrated successful operation of an integrated SRB+SOB reactor under micro-aerobic condition. At DO = 0.10-0.12 mg l(-1), since the activities of SOB were enhanced by limited oxygen, the removal efficiency for sulfate reached 81.5% and the recovery of S(0) peaked at 71.8%, higher than those reported in literature. At increased DO, chemical oxidation of sulfide with molecular oxygen competed with SOB so conversion of S(0) started to decline. At DO>0.30 mg l(-1) activities of SRB were inhibited, leading to failure of the SRB+SOB reactor.

Hexavalent chromium reduction in a sulfur reducing packed-bed bioreactor

The most commonly used approach for the detoxification of hazardous industrial effluents and wastewaters containing Cr(VI) is its reduction to the much less toxic and immobile form of Cr(III). This study investigates the cleanup of Cr(VI) containing wastewaters using elemental sulfur as electron acceptor, for the production of hydrogen sulfide that induces Cr(VI) reduction. An elemental sulfur reducing packed-bed bioreactor was operated at 28-30°C for more than 250 days under varying influent Cr(VI) concentrations (5.0-50.0 mg/L) and hydraulic retention times (HRTs, 0.36-1.0 day). Ethanol or acetate (1000 mg/L COD) was used as carbon source and electron donor. The degree of COD oxidation varied between 30% and 85%, depending on the operating conditions and the type of organic carbon source. The oxidation of organic matter was coupled with the production of hydrogen sulfide, which reached a maximum concentration of 750 mg/L. The biologically produced hydrogen sulfide reduced Cr(VI) chemically to Cr(III) that precipitated in the reactor. Reduction of Cr(VI) and removal efficiency of total chromium always exceeded 97% and 85%, respectively, implying that the reduced chromium was retained in the bioreactor. This study showed that sulfur can be used as an electron acceptor to produce hydrogen sulfide that induces efficient reduction and immobilization of Cr(VI), thus enabling decontamination of Cr(VI) polluted wastewaters.

HS-SPME/GC-MS analysis of volatile and semi-volatile organic compounds emitted from municipal sewage sludge

The aim of the research involved identification and semi-quantitative determination of unknown volatile and semi-volatile organic compounds emitted to air by sewage sludge formed in the process of municipal wastewater treatment in a sewage treatment plant. Samples taken directly after completion of the technological process as well as the sludge stored on the premise of the sewage treatment plant were analyzed. A simple method using off-line headspace solid-phase microextraction combined with gas chromatography-mass spectrometry has been proposed for extraction and detection of organic pollutants. For reliable identification of compounds, combination of two independent parameters: mass spectra and linear temperature programmed retention indices were employed. Over 170 compounds of different structure were identified including aliphatic and aromatic hydrocarbons, alcohols, esters, carbonyls, as well as sulfur, nitrogen, and chlorine containing compounds. The prevailing substances included: ethyl ether, n-hexane, p-xylene, o-xylene, mesitylene, m-ethylbenzene, limonene, n-decane, n-undecane, and n-dodecane. A few compounds such as methanetiol, dimethyl polisulfide, octaatomic sulfur, phthalic anhydride, and indoles were identified in the sludge for the first time.

Sulfidogenic biotreatment of synthetic acid mine drainage and sulfide oxidation in anaerobic baffled reactor

The treatment of synthetic acid mine drainage (AMD) water (pH 3.0-6.5) containing sulfate (3.0-3.5 g L(-1)) and various metals (Co, Cu, Fe, Mn, Ni, and Zn) was studied in an ethanol-fed sulfate-reducing 4-compartment anaerobic baffled reactor (ABR) at 32°C. The reactor was operated for 160 days at different chemical oxygen demand (COD)/sulfate ratios, hydraulic retention times (HRT), pH, and metal concentrations to study the robustness of the process. The last compartment of the reactor was aerated at different rates to study the bio-oxidation of sulfide to elemental sulfur. The highest sulfate reduction efficiency (88%) was obtained with a feed sulfate concentration of 3.5 g L(-1), COD/sulfate mass ratio of 0.737, feed pH of 3.0 and HRT of 2 days without aeration in the 4th compartment. The corresponding COD removal efficiency was about 92%. The alkalinity produced in the sulfidogenic ethanol oxidation neutralized the acidic mine water from pH 3.0-4.5 to pH 7.0-8.0. Effluent soluble and total heavy metal concentrations were substantially reduced with removal efficiencies generally higher than 99%, except for Mn (25-77%). Limited aeration in the 4th compartment of ABR promoted incomplete oxidation of sulfide to elemental sulfur rather than complete oxidation to sulfate. Depending on the aeration rate and HRT, 32-74% of produced sulfide was oxidized to elemental sulfur. This study demonstrates that by optimizing operating conditions, sulfate reduction, metal removal, alkalinity generation, and excess sulfide oxidation can be achieved in a single ABR treating AMD.

Performance of a sulfide-oxidizing, sulfur-producing membrane biofilm reactor treating sulfide-containing bioreactor effluent

Sulfide-containing waste streams are generated in mining, petrochemical plants, tanneries, viscose rayon manufacture, and the gasification of coal. Colorless sulfur bacteria can oxidize sulfide to elemental sulfur (S°), which can be recovered, when oxygen is their electron acceptor. This study evaluated sulfide oxidation and S° recovery in an oxygen-based membrane biofilm reactor (MBfR) treating the effluent from a sulfidogenic anaerobic baffled reactor. Sulfide oxidation efficiency (37-99%) and S° recovery (64-89% of oxidized sulfide) could be controlled by manipulating the sulfide loading, oxygen pressure to the fibers, and hydraulic retention time (HRT). For example, too-low oxygen pressure decreased S° recovery due to decreased sulfide oxidation, but too-high oxygen pressure lowered S° recovery due to its oxidation to sulfate. Most importantly, high sulfide oxidation (>98%) and conversion to S° (>75%) could be achieved together when the sulfide loading was less than 1.7 mol/m²·d and the O₂ pressure was sufficient to give an O₂ flux of at least 1.5 mol/m²·d. However, higher sulfide loading could be compensated by a higher O₂ pressure, and the best performance occurred when the sulfide loading was high (2 molS/m²·d), the O₂ pressure was high (∼1 atm), and the HRT was short (1.9 h). Membrane fouling caused a low O₂ flux, which led to low sulfide-oxidation efficiency, but fouling could be reversed by mild acid washing.

Modeling denitrifying sulfide removal process using artificial neural networks

The denitrifying sulfide removal (DSR) process has complex interactions between autotrophic and heterotrophic denitrifers; thus, constructing a detailed mechanistic model and proper control architecture is difficult. Artificial neural networks (ANNs) are capable of inferring the complex relationships between input and output process variables without a detailed characterization of the mechanisms governing the process. This work presents a novel ANN that accurately predicts the steady-state performance of an expended granular sludge bed (EGSB)-DSR bioreactor for nitrite denitrification and the complete DSR process. The proposed ANN shows that at a threshold hydraulic retention time (HRT)<7h, influent sulfide concentration markedly affects reactor performance.

Sulfur formation and recovery in a thiosulfate-oxidizing bioreactor

This work describes the design and performance of a thiosulfate-oxidizing bioreactor that allowed high elemental sulfur production and recovery efficiency. The reactor system, referred to as a Supernatant-Recycling Settler Bioreactor (SRSB), consisted of a cylindrical upflow reactor and a separate aeration vessel. The reactor was equipped with an internal settler and packing material (structured corrugated PVC sheets) to facilitate both cell retention and the settling of the formed elemental sulfur. The supernatant from the reactor was continuously recirculated through the aerator. An inlet thiosulfate concentration of 100 mmol l(-1) was used. The reactor system was fed with 89 mmol l(-1) d(-1) thiosulfate reaching 98 to 100% thiosulfate conversion with an elemental sulfur yield of 77%. Ninety-three percent of the produced sulfur was harvested from the bottom of the reactor as sulfur sludge. The dry sulfur sludge contained 87% elemental sulfur. The inclusion of an internal settler and packing material in the reactor system resulted in an effective retention of sulfur and biomass inside the bioreactor, preventing the oxidation of thiosulfate and elemental sulfur to sulfate in the aerator and, therefore, improving the efficiency of elemental sulfur formation and recovery.

Biological treatment of toxic petroleum spent caustic in fluidized bed bioreactor using immobilized cells of Thiobacillus RAI01

In the present studies, newly isolated Thiobacillus sp was used for the treatment of synthetic spent sulfide caustic in a laboratory-scale fluidized bed bioreactor. The sulfide oxidation was tested using Ca-alginate immobilized Thiobacillus sp. Initially, response surface methodology was applied for the optimization of four parameters to check the sulfide oxidation efficiency in batch mode. Further, reactor was operated in continuous mode for 51 days at different sulfide loading rates and retention times to test the sulfide oxidation and sulfate and thiosulfate formation. Sulfide conversions in the range of 90-98% were obtained at almost all sulfide loading rates and hydraulic retention times. However, increased loading rates resulted in lower sulfide oxidation capacity. All the experiments were conducted at constant pH of around 6 and temperature of 30 +/- 5 degrees C.

Effect of pH on anoxic sulfide oxidizing reactor performance

The effects of pH on the performance of anoxic sulfide oxidizing (ASO) reactor were evaluated. Performance was investigated under various operational conditions at influent pH range of 4-11. At the influent pH of 7-7.5 during loading tests and HRT tests, the sulfide oxidation was partial. In general, the amount of sulfate formed decreased with the increasing sulfide and nitrite loadings. The bacterial communities in ASO reactors were more sensitive to acidic pH compared with alkaline pH, as nitrite and sulfide removal rates dropped significantly when exposed to acidic pH 3. High dissolved bisulfide ions, nitrite and excess of sulfate (>300 mg/L) might have inhibited the sulfide oxidation under highly acidic and alkaline conditions in the ASO reactor. Based on sulfide and nitrite removal efficiencies, the ASO reactor can be operated in a wide range of pH, i.e. 5-11.

Biological sweetening of energy gases mimics in biotrickling filters

Removal of hydrogen sulfide from waste and energy-rich gases is required, not only because of environmental health and safety reasons, but also because of operational reasons if such gases have to be used for energy generation. A biotrickling filter for the removal of ultra-high concentrations of H2S from oxygen-poor gases is proposed and studied in this work. Two laboratory-scale biotrickling filters were used to study the startup period and to determine the long-term performance of the gas sweetening process. The inlet H2S concentration ranged from 900 to 12000ppmv and two different packing materials were investigated. There was no toxicity effect observed even at a the highest H2S concentration, and maximum elimination capacities of 280 and 250g H2Sm(-3)h(-1) were obtained at gas contact times of 167 and 180s, respectively. Elemental sulfur and sulfate were found to be the most abundant end-products of the biological oxidation of sulfide when operated under microaerophilic conditions. The biotrickling filter was able to quickly recover its nominal performance after different load increases and system shutdowns simulating field operation. The results reported here show that biotreatment can be an interesting alternative to conventional gas sweetening systems normally used for such applications.

Microbial conversion of sulfur dioxide in flue gas to sulfide using bulk drug industry wastewater as an organic source by mixed cultures of sulfate reducing bacteria

Mixed cultures of sulfate reducing bacteria (SRB) were isolated from anaerobic cultures and enriched with SRB media. Studies on batch and continuous reactors for the removal of SO(2) with bulk drug industry wastewater as an organic source using isolated mixed cultures of SRB revealed that isolation and enrichment methodology adopted in the present study were apt to suppress the undesirable growth of anaerobic bacteria other than SRB. Studies on anaerobic reactors showed that process was sustainable at COD/S ratio of 2.2 and above with optimum sulfur loading rate (SLR) of 5.46kgS/(m(3)day), organic loading rate (OLR) of 12.63kg COD/(m(3)day) and at hydraulic residence time (HRT) of 8h. Free sulfide (FS) concentration in the range of 300-390mgFS/l was found to be inhibitory to mixed cultures of SRB used in the present studies.

Co-treatment of hydrogen sulfide and methanol in a single-stage biotrickling filter under acidic conditions

Biofiltration of waste gases is cost-effective and environment-friendly compared to the conventional techniques for treating large flow rates of gas streams with low concentrations of pollutants. Pulp and paper industry off-gases usually contain reduced sulfur compounds, such as hydrogen sulfide and a wide range of volatile organic compounds (VOCs), e.g., methanol. It is desirable to eliminate both of these groups of compounds. Since the co-treatment of inorganic sulfur compounds and VOCs in biotrickling filters is a relatively unexplored area, the simultaneous biotreatment of H2S and methanol as the model VOC was investigated. The results showed that, after adaptation, the elimination capacity of methanol could reach around 236 g m(-3) h(-1) with the simultaneous complete removal (100%) of 12 ppm H2S when the empty bed residence time is 24 s. The pH of the system was around 2. Methanol removal was hardly affected by the presence of hydrogen sulfide, despite the low pH. Conversely, the presence of the VOC in the waste gas reduced the efficiency of H2S biodegradation. The maximal methanol removal decreased somewhat when increasing the gas flow rate. This is the first report on the degradation of methanol at such low pH in a biotrickling filter and on the co-treatment of H2S and VOCs under such conditions.

Degradation of methanethiol by methylotrophic methanogenic archaea in a lab-scale upflow anaerobic sludge blanket reactor

In a lab-scale upflow anaerobic sludge blanket reactor inoculated with granular sludge from a full-scale wastewater treatment plant treating paper mill wastewater, methanethiol (MT) was degraded at 30 degrees C to H2S, CO2, and CH4. At a hydraulic retention time of 9 h, a maximum influent concentration of 6 mM MT was applied, corresponding to a volumetric loading rate of 16.5 mmol liter-1 day-1. The archaeal community within the reactor was characterized by anaerobic culturing and denaturing gradient gel electrophoresis analysis, cloning, and sequencing of 16S rRNA genes and quantitative PCR. Initially, MT-fermenting methanogenic archaea related to members of the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by Methanomethylovorans hollandica, which was detected in aggregates but not inside the granules that originated from the inoculum, the microbial composition of which remained fairly unchanged. Possibly other species within the Methanosarcinacaea also contributed to the fermentation of MT, but they were not enriched by serial dilution in liquid media. The archaeal community within the granules, which was dominated by Methanobacterium beijingense, did not change substantially during the reactor operation. Some of the species related to Methanomethylovorans hollandica were enriched by serial dilutions, but their growth rates were very low. Interestingly, the enrichments could be sustained only in the presence of MT and did not utilize any of the other typical substrates for methylotrophic methanogens, such as methanol, methyl amine, or dimethylsulfide.

Online oxygen control for sulfide oxidation in anaerobic treatment of high-sulfate wastewater

A new technique for sulfide control was investigated in an upflow-anaerobic filter (UAF) treating high-strength, sulfate-rich wastewater. The technique used periodic oxygen injection using oxidation-reduction potential (ORP) as a controlling parameter to regulate oxygen injection. The UAF was operated at a constant influent total-organic carbon of 6740 mg/L but with different influent sulfates of 1000, 3000, and 6000 mg/L. At 1000 and 3000 mg/L influent sulfates, the produced sulfide did not impose any inhibition to methane-producing bacteria (MPB). However, at 6000 mg/L influent sulfate, the produced dissolved sulfide of 804 mg S/L (free sulfide = 280 mg S/L) severely inhibited the methanogenesis, but not the sulfidogenesis. Upon oxygen injection at elevated ORP of -265 mV, sulfides were almost completely eliminated with a concomitant improvement in methane yield by 46%. If oxygenation was excessive because of an oversetting of ORP, the excess oxygen could be used rapidly by facultative heterotrophs, thereby protecting the MPB from oxygen stress. Regarding online sulfide oxidation, it was found that the biogas and injected oxygen needed to pass through an aqueous layer containing trace metals, which were found to have a significant catalytic effect on abiotic sulfide oxidation.

Oxygen transfer and consumption in a thiosulfate oxidizing bioreactor with sulfur production

AIMS

To evaluate the contribution of oxygen transfer and consumption in a sulfoxidizing system to increase the elemental sulfur yield from thiosulfate oxidation.

METHODS AND RESULTS

A 10 l thiosulfate oxidizing bioreactor with suspended cells operating under microaerophilic conditions and a separated aerator with a variable volume of 0.8--1.7 l were operated with a consortium containing mainly Thiobacillus sp. that oxidizes several sulfide species to elemental sulfur and sulfate. From the gas-liquid oxygen balance, the k(L)a was estimated under different operation conditions. A k(L)a of around 200 h(-1) favoured elemental sulfur production and can serve as scale-up criterion. It was further shown that more than 50% of the oxygen fed to the system was consumed in the aerator.

CONCLUSIONS

The performance of the sulfoxidizing system can be improved by controlling oxygen transfer.

SIGNIFICANCE AND IMPACT OF THE STUDY

The proposed method for the k(L)a determination was based on the oxygen balance, which incorporates the oxygen concentrations measured in the liquid in steady state, reducing the interference of the response time in the traditional non-steady state methods. This approach can be used to optimize reactors where microaerophilic conditions are desirable.

Treatment of sulphide containing wastewater with sulphur recovery in a novel reverse fluidized loop reactor (RFLR)

A novel aerobic bioreactor, the reverse fluidized loop reactor (RFLR) was tested for recovering sulphur from aqueous sulphide in this study. The RFLR contained buoyant carrier particles on which chemolithotrophic sulphide oxidizing bacteria formed a biofilm, which oxidized sulphide to sulphur and separated from the aqueous phase. The redox potential of the RFLR was regulated to control the oxygenation of sulphide for sulphur production. The RFLR was operated without any pH control and under various controlled pHs. The sulphide removal and nature of products formed under various sulphide loading rates and pH were examined. Under pH uncontrolled state, 95% of sulphur was recovered up to 11 kg sulphide/m3 d. The maximum sulphide loading supplied to the reactor was 30 kg sulphide/m3 d at pH 8, of which 90% was completely oxidized and 65% recovered as sulphur. The decline in bacterial sulphide oxidation leads to chemical oxidation of sulphide and subsequent accumulation of intermediary products such as thiosulphate and polysulphide in the reactor.

Hydrogen sulfide oxidation by a microbial consortium in a recirculation reactor system: sulfur formation under oxygen limitation and removal of phenols

Wastewater from petroleum refining may contain a number of undesirable contaminants including sulfides, phenolic compounds, and ammonia. The concentrations of these compounds must be reduced to acceptable levels before discharge. Sulfur formation and the effect of selected phenolic compounds on the sulfide oxidation were studied in autotrophic aerobic cultures. A recirculation reactor system was implemented to improve the elemental sulfur recovery. The relation between oxygen and sulfide was determined calculating the O2/S2- loading rates (Q(O2)/Q(S)2- = Rmt), which adequately defined the operation conditions to control the sulfide oxidation. Sulfur-producing steady states were achieved at Rmt ranging from 0.5 to 1.5. The maximum sulfur formation occurred at Rmt of 0.5 where 85% of the total sulfur added to the reactor as sulfide was transformed to elemental sulfur and 90% of it was recovered from the bottom of the reactor. Sulfide was completely oxidized to sulfate (Rmt of 2) in a stirred tank reactor, even when a mixture of phenolic compounds was present in the medium. Microcosm experiments showed that carbon dioxide production increased in the presence of the phenols, suggesting that these compounds were oxidized and that they may have been used as carbon and energy source by heterotrophic microorganisms present in the consortium.