Application of a 2-step process for the biological treatment of sulfidic spent caustics

Potential of biological sulphur recovery from thiosulphate by haloalkaliphilic Thioalkalivibrio denitrificans

The aim of this study was to investigate the potential for elemental sulphur recovery from sulphurous solutions under aerobic and anoxic conditions by haloalkalophilic Thioalkalivibrio denitrificans at 0.8-19.6 g S₂O₃^2--S L^-1 and 0.2-0.58 g NO₂ L^-1, respectively. The experiments were conducted as batch assays with haloalkaline (pH 10 and ≥ 14 g Na⁺ L^-1) thiosulphate solution. Aerobically, the highest biotransformation rate of thiosulphate obtained was 0.03 h^-1 at 8.5 g L S₂O₃^2--S. Based on Monod model, the maximum substrate utilisation rate (q_m) was 0.024 h^-1 with half saturation constant (K_s) 0.42 g S₂O₃^2--S L^-1 at initial [S₂O₃^2--S] of 14 g L^-1. S⁰ accumulated at [S₂O₃^2--S] ≥ 1.5 g L^-1 (10% yield at initial 9.5 g S₂O₃^2--S L^-1) and the highest S⁰ yield estimated with the model was 61% with initial [S₂O₃^2--S] of 16.5 g L^-1. Anoxically, the maximum nitrite removal rate based on Monod modelling was 0.011 h^-1 with K_s= 0.84 g NO₂^- L^-1. Aerobically and anoxically the maximum specific growth rates (µ_m) were 0.046 and 0.022 h^-1, respectively. In summary, high-rate aerobic biotransformation kinetics of thiosulphate were demonstrated, whereas the rates were slower and no S⁰ accumulated under anoxic conditions. Thus, future developments of biotechnical applications for the recovery of S⁰ from haloalkaline streams from the process industry should focus on aerobic treatment.HighlightsHaloalkaline S₂O₃^2- biotransformations kinetics by Thioalkalivibrio denitrificansAerobic thiosulphate-S bioconversion up to 0.024 h^-1 with K_s = 0.42 g S₂O₃^2--S L^-110% S⁰ yield with initial 9.5 g S₂O₃^2--S L^-1 in aerobic conditionAnoxic NO₂ removal up to 0.01 h^-1 with K_s = 0.84 g NO₂^- L^-1.

A novel sulfur-driven autotrophic denitrification coupled with bio-cathode system for bioelectricity generation and groundwater remediation

This study explored the feasibility of treating wastewater using sulfur-driven autotrophic denitrification (SAD) coupled with the bio-cathode of microbial fuel cell (MFC), focusing on simultaneous bioelectricity generation, denitrification, and desulphurization. A maximum output voltage of 360 mV was obtained with a power generation cycle of 25 h when simulated wastewater with 100.0 mg/L of each NO₃^--N and S^2--S was employed as the influent in the SAD-BMFC. Compared with solo SAD or MFC, SAD-BMFC obtained a higher NO₃^--N removal rate (E_{12 h} = 87.7%, E_{24 h} = 100%), and less NO₂^--N accumulation. S^2--S of the influent was almost completely removed, oxidized to S⁰-S (88.6-90.2 mg/L) and SO₄^2--S (9.8-11.4 mg/L). The reaction system achieved self-balance of acidity-alkalinity (pH 7.05-7.35). The SAD process was the main pathway for NO₃^--N removal (80.2%) and a smaller proportion of electrons came from the bio-cathode. This study effectively combined SAD with a bio-cathode system for simultaneous energy harvest and bio-enhanced remediation of groundwater contaminated by both NO₃^--N and S^2--S.

Oxytetracycline co-metabolism with denitrification/desulfurization in SRB mediated system

Aquaculture wastewater contained a high remnant of oxytetracycline (OTC) and nitrate. In this study, OTC co-metabolized with denitrification/desulfurization was investigated in terms of kinetic analysis, pathway, microbial communities and produces analysis in sulfate-reducing bacteria (SRB) mediated system. Long-term acclimatization with sulfate (300 mg-S/L) could markedly accelerate the removed rate of OTC from 0.9 to 1.4 mg/g-SS/d, with the kinetic constants increasing from 0.2760 to 0.5232 d^-1, mainly via enzymes including adenosine-5'-phos-phosulfate reductase and cytochrome P450, and non-enzymatic process related to intermediates (adenosine-5'-phos-phosulfate and S⁰). Furthermore, OTC was likely detoxified by SRB enriched sludge mainly via hydrolysis, dehydration, oxidation and reduction. The denitrification process would postpone the OTC degradation via outcompeting electron donors with the desulfurization process. Redundancy analysis suggested that sulfur-oxidizing bacteria (Acidithiobacillus, Ochrobactrum) were highly related to OTC degradation processes. This study provides deep insight and a new opportunity for the treatment of aquaculture wastewater containing OTC, sulfate and nitrate by SRB sludge.

Comparative analysis of microbial communities from different full-scale haloalkaline biodesulfurization systems

Abstract

In biodesulfurization (BD) at haloalkaline and dO₂-limited conditions, sulfide-oxidizing bacteria (SOB) effectively convert sulfide into elemental sulfur that can be used in agriculture as a fertilizer and fungicide. Here we show which bacteria are present in this biotechnological process. 16S rRNA gene amplicon sequencing of biomass from ten reactors sampled in 2018 indicated the presence of 444 bacterial Amplicon Sequence Variants (ASVs). A core microbiome represented by 30 ASVs was found in all ten reactors, with Thioalkalivibrio sulfidiphilus as the most dominant species. The majority of these ASVs are phylogenetically related to bacteria previously identified in haloalkaline BD processes and in natural haloalkaline ecosystems. The source and composition of the feed gas had a great impact on the microbial community composition followed by alkalinity, sulfate, and thiosulfate concentrations. The halophilic SOB of the genus Guyparkeria (formerly known as Halothiobacillus) and heterotrophic SOB of the genus Halomonas were identified as potential indicator organisms of sulfate and thiosulfate accumulation in the BD process.

Key points

• Biodesulfurization (BD) reactors share a core microbiome

• The source and composition of the feed gas affects the microbial composition in the BD reactors

• Guyparkeria and Halomonas indicate high concentrations of sulfate and thiosulfate in the BD process

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11771-y.

Recent advances in microbial capture of hydrogen sulfide from sour gas via sulfur-oxidizing bacteria

Biological desulfurization offers several remarkably environmental advantages of operation at ambient temperature and atmospheric pressure, no demand of toxic chemicals as well as the formation of biologically re-usable sulfur (S0), which has attracted increasing attention compared to conventionally physicochemical approaches in removing hydrogen sulfide from sour gas. However, the low biomass of SOB, the acidification of process solution, the recovery of SOB, and the selectivity of bio-S0 limit its industrial application. Therefore, more efforts should be made in the improvement of the BDS process for its industrial application via different research perspectives. This review summarized the recent research advances in the microbial capture of hydrogen sulfide from sour gas based on strain modification, absorption enhancement, and bioreactor modification. Several efficient solutions to limitations for the BDS process were proposed, which paved the way for the future development of BDS industrialization.

A New Thioalkalivibrio sp. Strain Isolated from Petroleum-Contaminated Brackish Estuary Sediments: A New Candidate for Bio-Based Application for Sulfide Oxidation in Halo-Alkaline Conditions

A new halo-alkaline sulfur-oxidising bacterial strain was isolated from brackish estuary sediments contaminated by total petroleum hydrocarbon. The isolate was classified as a new strain of Thioalkalivibrio sulfidiphilus sp., showing a higher capability of adaptation to pH and a higher optimal sodium concentration for growth, when compared to Thioalkalivibrio sulfidiphilus sp. HL-EbGr7, type strain of the species. The strain was capable to grow in saline concentrations up to 1.5 M Na+ and pH up to 10. The genome of the new isolate was sequenced and annotated. The comparison with the genome of Thioalkalivibrio sulfidiphilus sp. HL-EbGr7 showed a duplication of an operon encoding for a putative primary sodium extruding pump and the presence of a sodium/proton antiporter with optimal efficiency at halo-alkaline conditions. The new strain was able to oxidize sulfide at halo-alkaline conditions at the rate of 1 mmol/mg-N/h, suitable for industrial applications dedicated to the recovery of alkaline scrubber for H2S emission absorption and abatement.

Characteristics and Treatment of Wastewater from the Mercaptan Oxidation Process: A Comprehensive Review

Sulfur compounds are removed from petroleum by the addition of sodium hydroxide at a very high concentration. As a result, a residue called spent soda or spent caustic is generated, being extremely aggressive to the environment. In this work, the chemical properties of this residue are described in detail. The sodium hydroxide remains that have not reacted, sulfur compounds, and organic matter are the primary pollutants reported. Additionally, the main characteristics of the methods of treatment used to reduce them are described. This review comes from comprehensive and updated research and bibliographic analysis about the investigation on the topic. The advantages and disadvantages of the different treatment methods are highlighted. We established some criteria to set out when assessing the application of each one of these treatments is considered.

Sulfide induced phosphate release from iron phosphates and its potential for phosphate recovery

Sulfide is frequently suggested as a tool to release and recover phosphate from iron phosphate rich waste streams, such as sewage sludge, although systematic studies on mechanisms and efficiencies are missing. Batch experiments were conducted with different synthetic iron phosphates (purchased Fe(III)P, Fe(III)P synthesized in the lab and vivianite, Fe(II)₃(PO₄)₂*8H₂O), various sewage sludges (with different molar Fe:P ratios) and sewage sludge ash. When sulfide was added to synthetic iron phosphates (molar Fe:S = 1), phosphate release was completed within 1 h with a maximum release of 92% (vivianite), 60% (purchased Fe(III)P) and 76% (synthesized Fe(III)P). In the latter experiment, rebinding of phosphate to Fe(II) decreased net phosphate release to 56%. Prior to the re-precipitation, phosphate release was very efficient (P released/S input) because it was driven by Fe(III) reduction and not by, more sulfide demanding, FeS_x formation. This was confirmed in low dose sulfide experiments without significant FeS_x formation. Phosphate release from vivianite was very efficient because sulfide reacts directly (1:1) with Fe(II) to form FeS_x, without Fe(III) reduction. At the same time vivianite-Fe(II) is as efficient as Fe(III) in binding phosphate. From digested sewage sludge, sulfide dissolved maximally 30% of all phosphate, from the sludge with the highest iron content which was not as high as suggested in earlier studies. Sludge dewaterability (capillary suction test, 0.13 ± 0.015 g²(s²m⁴)^-1) dropped significantly after sulfide addition (0.06 ± 0.004 g²(s²m⁴)^-1). Insignificant net phosphate release (1.5%) was observed from sewage sludge ash. Overall, sulfide can be a useful tool to release and recover phosphate bound to iron from sewage sludge. Drawbacks -deterioration of the dewaterability and a net phosphate release that is lower than expected-need to be investigated.

Recirculation ratio regulates denitrifying sulfide removal and elemental sulfur recovery by altering sludge characteristics and microbial community composition in an EGSB reactor

Expanded granular sludge blanket (EGSB) is regarded as a promising reactor to carry out denitrifying sulfide removal (DSR) and elemental sulfur (S⁰) recovery. Although the recirculation ratio is an essential parameter for EGSB reactors, how it impacts the DSR process remains poorly understood. Here, three lab-scale DSR-EGSB reactors were established with the different recirculation ratios (3:1, 6:1 and 9:1) to evaluate the corresponding variations in pollutant removal, S⁰ recovery, anaerobic granular sludge (AGS) characteristics and microbial community composition. It was found that an intermediate recirculation ratio (6:1) could facilitate long-term reactor stability. Adequate recirculation ratio could enhance S⁰ recovery, but an excessive recirculation ratio (9:1) was likely to cause AGS fragmentation and biomass loss. The S⁰ desorbed more from sludge at higher recirculation ratios, probably due to the enhanced hydraulic disturbance caused by the increased recirculation ratios. At the low recirculation ratio (3:1), S⁰ accumulation as inorganic suspended solids in AGS led to a decrease in VSS/TSS ratio and mass transfer efficiency. Although typical denitrifying and sulfide-oxidizing bacteria (e.g., Azoarcus, Thauera and Arcobacter) were predominant in all conditions, facultative and heterotrophic functional bacteria (e.g., Azoarcus and Thauera) were more adaptable to higher recirculation ratios than autotrophs (e.g., Arcobacter, Thiobacillus and Vulcanibacillus), which was conducive to the formation of bacterial aggregates to response to the increased recirculation ratio. The study revealed recirculation ratio regulation significantly impacted the DSR-EGSB reactor performance by altering AGS characteristics and microbial community composition, which provides a novel strategy to improve DSR performance and S⁰ recovery.

Continuous sulfur biotransformation in an anaerobic-anoxic sequential batch reactor involving sulfate reduction and denitrifying sulfide oxidization

The pathways and intermediates of continuous sulfur biotransformation in an anaerobic and anoxic sequential batch reactor (AA-SBR) involving sulfate reduction (SR) and denitrifying sulfide oxidization (DSO) were investigated. In the anoxic phase, DSO occurred in two sequential steps, the oxidation of sulfide (S^2-) to elemental sulfur (S⁰) and the oxidation of S⁰ to sulfate (SO₄^2-). The oxidation rate of S^2- to S⁰ was 3.31 times faster than that of S⁰ to SO₄^2-, resulting in the accumulation of S⁰ as a desired intermediate under S^2--S/NO₃^--N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S⁰ suspended in the effluent, about 40% of S⁰ retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S⁰ was subsequently oxidized to SO₄^2- under S^2--S/NO₃^--N ratio of 0.5:1. In anaerobic, S⁰ coexist with SO₄^2- (in fresh wastewater) were simultaneously reduced to S^2-, and the reduction rate of SO₄^2- to S^2- was 3.17 times faster than that of S⁰ to S^2-, resulting in a higher production of S⁰ in subsequent anoxic phase. Microbial community analysis indicated that SO₄^2-/S⁰-reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S^2-/S⁰-oxidizing bacteria (e.g. Paracoccus and Thermothrix) co-participated in continuous sulfur biotransformation in the AA-SBR. A conceptual model was established to describe these main processes and key intermediates. The research offers a new insight into the reaction processes optimization for S⁰ recovery and simultaneous removal of SO₄^2- and NO₃^- in an AA-SBR.

Development and validation of a physiologically based kinetic model for starting up and operation of the biological gas desulfurization process under haloalkaline conditions

Hydrogen sulfide is a toxic and corrosive gas that must be removed from gaseous hydrocarbon streams prior to combustion. This paper describes a gas biodesulfurization process where sulfur-oxidizing bacteria (SOB) facilitate sulfide conversion to both sulfur and sulfate. In order to optimize the formation of sulfur, it is crucial to understand the relations between the SOB microbial composition, kinetics of biological and abiotic sulfide oxidation and the effects on the biodesulfurization process efficiency. Hence, a physiologically based kinetic model was developed for four different inocula. The resulting model can be used as a tool to evaluate biodesulfurization process performance. The model relies on a ratio of two key enzymes involved in the sulfide oxidation process, i.e., flavocytochrome c and sulfide-quinone oxidoreductase (FCC and SQR). The model was calibrated by measuring biological sulfide oxidation rates for different inocula obtained from four full-scale biodesulfurization installations fed with gases from various industries. Experimentally obtained biological sulfide oxidation rates showed dissimilarities between the tested biomasses which could be explained by assuming distinctions in the key-enzyme ratios. Hence, we introduce a new model parameter α to whereby α describes the ratio between the relative expression levels of FCC and SQR enzymes. Our experiments show that sulfur production is the highest at low α values.

Response of the reactor performance and microbial community to a shift of ISDD process from micro-aerobic to anoxic condition

Micro-aerobic condition has proven to be effective in enhancing sulfide oxidation to elemental sulfur (S⁰) during integrated simultaneous desulfurization and denitrification process (ISDD). In this study we investigated and compared the performance and microbial community of ISDD process operating under initially anoxic, then micro-aerobic and finally switch back to anoxic condition. For all the three tested scenarios, comparable bioreactor performance in terms of sulfate (95.0 ± 4.4%, 90.6 ± 3.8%, 89.8 ± 3.5%) and nitrate (∼100%) removal was achieved. However, a shift of ISDD bioreactor from micro-aerobic to anoxic environment clearly increased the S⁰ production (30.6%), relative to that at initial anoxic condition (14.2%). Further anoxic bioreactor operation with different influent nitrate concentrations also obtained satisfactory performance particularly in terms of S⁰ production. Microbial community analysis results showed that functional microorganisms selectively enriched at micro-aerobic condition, particularly sulfide-oxidizing bacteria (SOB), could also function well and enhance S⁰ production when bioreactor switching from micro-aerobic to anoxic environment. We proposed that micro-aerobic strategy could function as a bio-selector and provide a new idea in functional microorganisms selectively enrichment for wastewater treatment.

Investigation of transient forms of sulfur during biological treatment of spent caustic

In the present study, the production of various transient forms of sulfur during biological oxidation of sulfidic spent caustics under haloalkaline conditions in a stirred tank bioreactor is investigated. Also, the effects of abiotic aeration (chemical oxidation), dissolved oxygen (DO) concentration and sodium concentration on forms of sulfur during biological treatment are demonstrated. Thioalkalivibrio versutus strain was used for sulfide oxidation in spent caustic (SC). The aeration had an important effect on sulfide oxidation and its final products. At DO concentrations above 2 mg l^-1, majority of sulfide was oxidized to sulfate. Maximum sulfide removal efficiency (%R) and yield of sulfate production [Formula: see text] was obtained in Na⁺ concentration ranging from 0.6 to 2 M. Abiotic aeration, which is the most important factor of production of thiosulfate, resulted in the formation of an undesired product-polysulfide. However, abiotic aeration can be used as a pretreatment to biological treatment. In the bioreactor the removal efficiency was obtained as 82.7% and various forms of sulfur such as polysulfide, biosulfur, thiosulfate and sulfate was observed during biological treatment of SC.

Enhanced performance of denitrifying sulfide removal process at high carbon to nitrogen ratios under micro-aerobic condition

The success of denitrifying sulfide removal (DSR) processes, which simultaneously degrade sulfide, nitrate and organic carbon in the same reactor, counts on synergetic growths of autotrophic and heterotrophic denitrifiers. Feeding wastewaters at high C/N ratio would stimulate overgrowth of heterotrophic bacteria in the DSR reactor so deteriorating the growth of autotrophic denitrifiers. The DSR tests at C/N=1.26:1, 2:1 or 3:1 and S/N =5:6 or 5:8 under anaerobic (control) or micro-aerobic conditions were conducted. Anaerobic DSR process has <50% sulfide removal with no elemental sulfur transformation. Under micro-aerobic condition to remove <5% sulfide by chemical oxidation pathway, 100% sulfide removal is achieved by the DSR consortia. Continuous-flow tests under micro-aerobic condition have 70% sulfide removal and 55% elemental sulfur recovery. Trace oxygen enhances activity of sulfide-oxidizing, nitrate-reducing bacteria to accommodate properly the wastewater with high C/N ratios.

Pyrosequencing reveals microbial community dynamics in integrated simultaneous desulfurization and denitrification process at different influent nitrate concentrations

Integrated simultaneous desulfurization and denitrification (ISDD) process has proven to be feasible for the coremoval of sulfate, nitrate, and chemical oxygen demand (COD). In this study, we aimed to reveal the microbial community dynamics in the ISDD process with different influent nitrate (NO₃^-) concentrations. For all tested scenarios, full denitrification was accomplished while sulfate removal efficiency decreased along with increased influent NO₃^- concentrations. The proportion of S⁰ to influent SO₄^2- maintained a low level (5.6-17.0%) regardless of the increased influent NO₃^- concentrations. Microbial community analysis results showed that higher influent NO₃^- concentrations affected the microbial community structure greatly. Phyla Proteobacteria, Spirochaetae, Firmicutes, Synergistetes, and Chloroflexi dominated in all the community compositions, of which Proteobacteria exhibited a clear difference among eight microbial samples. Members of δ-Proteobacteria, with 16S rRNA gene sequences related to Desulfobulbus, were clearly decreased at influent NO₃^- = 3000 and 3500 mg/L, suggesting an inhibitory effect of NO₃^- on sulfate reduction. In contrast, as influent NO₃^- concentration increased, microbial community was notably enriched in γ-Proteobacteria and ε-Proteobacteria, which revealed the enrichment of 16S rRNA gene sequences related to Pseudomonas (γ-Proteobacteria), and Arcobacteria and Sulfurospirillum (ε-Proteobacteria).

Elemental sulfur recovery and spatial distribution of functional bacteria and expressed genes under different carbon/nitrate/sulfide loadings in up-flow anaerobic sludge blanket reactors

To characterize the impact of influent loading on elemental sulfur (S⁰) recovery during the denitrifying and sulfide oxidation process, three identical, lab-scale UASB reactors (30cm in length) were established in parallel under different influent acetate/nitrate/sulfide loadings, and the reactor performance and functional community structure were investigated. The highest S⁰ recovery was achieved at 77.9% when the acetate/nitrate/sulfide loading was set to 1.9/1.6/0.7kgd^-1m^-3. Under this condition, the genera Thauera, Sulfurimonas, and Azoarcus were predominant at 0-30, 0-10 and 20-30cm, respectively; meanwhile, the sqr gene was highly expressed at 0-30cm. However, as the influent loading was halved and doubled, S⁰ recovery was decreased to 27.9% and 45.1%, respectively. As the loading was halved, the bacterial distribution became heterogeneous, and certain autotrophic sulfide oxidation genera, such as Thiobacillus, dominated, especially at 20-30cm. As the loading doubled, the bacterial distribution was relatively homogeneous with Thauera and Azoarcus being predominant, and the nirK and sox genes were highly expressed. The study verified the importance of influent loading to regulate S⁰ recovery, which could be achieved as Thauera and Sulfurimonas dominated. An influent loading that was too low or too high gave rise to insufficient oxidation or over-oxidation of the sulfide and low S⁰ recovery performance.

Bioaugmentation of activated sludge with elemental sulfur producing strain Thiopseudomonas denitrificans X2 against nitrate shock load

The sulfide and nitrogen compounds in wastewaters are toxic and cause a serious environmental problem. Thiopseudomonas denitrificans X2, which is the type species of a novel genus Thiopseudomonas was used for bioaugmentation. It oxidized sulfide and acetate with nitrate, and generated elemental sulfur that could be recovered as resource. The generation rate of elemental sulfur was enhanced significantly by the bioaugmentation under the condition of excessive nitrate feeding. The inoculums survived and worked actively in the activated sludge system as the dominant population. Thiopseudomonas denitrificans X2 could be applied to wastewater treatment and resource recovery simultaneously.

Study on Haloalkaliphilic Sulfur-Oxidizing Bacterium for Thiosulfate Removal in Treatment of Sulfidic Spent Caustic

Due to the disadvantages of physiochemical methods for sulfidic spent caustic treatment, attentions are drawn to the environmental-friendly biotreatments including sulfur-oxidizing halo-alkaliphiles. Thioalkalivibrio versutus DSM 13738 was grown at alkaline (pH10) autotrophic medium with sodium carbonate/bicarbonate as the sole source of carbon and amended with sodium thiosulfate as the electron and energy source. The effect of various parameters including temperature (25-40 °C), pH (8-11), NaCl concentration (0.5-5 % w/v) and sodium thiosulfate concentrations (100-750 mM) was evaluated on bacterial growth and thiosulfate removal. This strain could eliminate sodium thiosulfate at very high concentrations up to 750 mM. The results showed that the highest specific growth rate was pH 9.5 and thiosulfate removal of Thioalkalivibrio versutus occurred at pH 10.5. The optimum salt concentration for thiosulfate removal was 2.5 % w/v and 5 % NaCl and specific growth rate elevated 2.5% w/v. It was also specified that this strain thrives occurred in 37 °C and at 35 and 37 °C higher removal of thiosulfate. Following chemical oxidation of sulfide to thiosulfate, application of Thioalkalivibrio versutus could be promising for spent caustic treatment. Since thiosulfate is utilized as an energy source, highest removal efficiency occurred at marginally different conditions compared to optimal growth.

Study on Haloalkaliphilic Sulfur-Oxidizing Bacterium for Thiosulfate Removal in Treatment of Sulfidic Spent Caustic

Due to the disadvantages of physiochemical methods for sulfidic spent caustic treatment, attentions are drawn to the environmental-friendly biotreatments including sulfur-oxidizing halo-alkaliphiles.Thioalkalivibrio versutusDSM 13738 was grown at alkaline (pH10) autotrophic medium with sodium carbonate/bicarbonate as the sole source of carbon and amended with sodium thiosulfate as the electron and energy source. The effect of various parameters including temperature (25-40 °C), pH (8-11), NaCl concentration (0.5-5 % w/v) and sodium thiosulfate concentrations (100-750 mM) was evaluated on bacterial growth and thiosulfate removal. This strain could eliminate sodium thiosulfate at very high concentrations up to 750 mM. The results showed that the highest specific growth rate was pH 9.5 and thiosulfate removal ofThioalkalivibrio versutusoccurred at pH 10.5. The optimum salt concentration for thiosulfate removal was 2.5 % w/v and 5 % NaCl and specific growth rate elevated 2.5% w/v. It was also specified that this strain thrives occurred in 37 °C and at 35 and 37 °C higher removal of thiosulfate. Following chemical oxidation of sulfide to thiosulfate, application ofThioalkalivibrio versutuscould be promising for spent caustic treatment. Since thiosulfate is utilized as an energy source, highest removal efficiency occurred at marginally different conditions compared to optimal growth.

Electrochemical sulfide removal and caustic recovery from spent caustic streams

Spent caustic streams (SCS) are produced during alkaline scrubbing of sulfide containing sour gases. Conventional methods mainly involve considerable chemical dosing or energy expenditures entailing high cost but limited benefits. Here we propose an electrochemical treatment approach involving anodic sulfide oxidation preferentially to sulfur coupled to cathodic caustic recovery using a two-compartment electrochemical system. Batch experiments showed sulfide removal efficiencies of 84 ± 4% with concomitant 57 ± 4% efficient caustic production in the catholyte at a final concentration of 6.4 ± 0.1 wt% NaOH (1.6 M) at an applied current density of 100 A m(-2). Subsequent long-term continuous experiments showed that stable cell voltages (i.e. 2.7 ± 0.1 V) as well as constant sulfide removal efficiencies of 67 ± 5% at a loading rate of 47 g(S) L(-1) h(-1) were achieved over a period of 77 days. Caustic was produced at industrially relevant strengths for scrubbing (i.e. 5.1 ± 0.9 wt% NaOH) at current efficiencies of 96 ± 2%. Current density between 0 and 200 A m(-2) and sulfide loading rates of 50-200 g(S) L(-1) d(-1) were tested. The higher the current density the more oxidized the sulfur species produced and the higher the sulfide oxidation. On the contrary, high loading rate resulted in a reduction of sulfide oxidation efficiency. The results obtained in this study together with engineering calculations show that the proposed process could represent a cost-effective approach for sodium and sulfur recovery from SCS.

Sulfide-oxidizing bacteria establishment in an innovative microaerobic reactor with an internal silicone membrane for sulfur recovery from wastewater

A novel bioreactor, employing a silicone membrane for microaeration, was studied for partial sulfide oxidation to elemental sulfur. The objective of this study was to assess the feasibility of using an internal silicone membrane reactor (ISMR) to treat dissolved sulfide and to characterize its microbial community. The ISMR is an effective system to eliminate sulfide produced in anaerobic reactors. Sulfide removal efficiencies reached 96 % in a combined anaerobic/microaerobic reactor and significant sulfate production did not occur. The oxygen transfer was strongly influenced by air pressure and flow. Pyrosequencing analysis indicated various sulfide-oxidizing bacteria (SOB) affiliated to the species Acidithiobacillus thiooxidans, Sulfuricurvum kujiense and Pseudomonas stutzeri attached to the membrane and also indicated similarity between the biomass deposited on the membrane wall and the biomass drawn from the material support, supported the establishment of SOB in an anaerobic sludge under microaerobic conditions. Furthermore, these results showed that the reactor configuration can develop SOB under microaerobic conditions and can improve and reestablish the sulfide conversion to 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.

Efficient regulation of elemental sulfur recovery through optimizing working height of upflow anaerobic sludge blanket reactor during denitrifying sulfide removal process

In this study, two lab-scale UASB reactors were established to testify S(0) recovery efficiency, and one of which (M-UASB) was improved from the previous T-UASB by shortening reactor height once S(2-) over oxidation was observed. After the height was shortened from 60 to 30cm, S(0) recovery rate was improved from 7.4% to 78.8%, and while, complete removal of acetate, nitrate and S(2-) was simultaneously maintained. Meanwhile, bacterial community distribution was homogenous throughout the reactor, with denitrifying sulfide oxidization bacteria predominant, such as Thauera and Azoarcus spp., indicating the optimized condition for S(0) recovery. The effective control of working height/volume in reactors plays important roles for the efficient regulation of S(0) recovery during DSR process.

Effect of dissolved oxygen on elemental sulfur generation in sulfide and nitrate removal process: characterization, pathway, and microbial community analysis

Microaerobic bioreactor treatment for enriched sulfide and nitrate has been demonstrated as an effective strategy to improve the efficiencies of elemental sulfur (S(0)) generation, sulfide oxidation, and nitrate reduction. However, there is little detailed information for the effect and mechanism of dissolved oxygen (DO) on the variations of microbial community in sulfur generation, sulfide oxidation, and nitrate reduction systems. Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) was employed to evaluate the variations of microbial community structures in a sulfide oxidation and nitrate reduction reactor under different DO conditions (DO 0-0.7 mg · L(-1)). Experimental results revealed that the activity of sulfide-oxidizing bacteria (SOB) and nitrate-reducing bacteria (NRB) could be greatly stimulated in 0.1-0.3 mg-DO · L(-1). However, when the DO concentration was further elevated to more than 0.5 mg · L(-1), the abundance of NRB was markedly decreased, while the heterotrophic microorganisms, especially carbon degradation species, were enriched. The reaction pathways for sulfide and nitrate removal under microaerobic conditions were also deduced by combining batch experiments with functional species analysis. It was likely that the oxidation of sulfide to sulfur could be performed by both aerobic heterotrophic SOB and sulfur-based autotrophic denitrification bacteria with oxygen and nitrate as terminal electron acceptor, respectively. The nitrate could be reduced to nitrite by both autotrophic and heterotrophic denitrification, and then the generated nitrite could be completely converted to nitrogen gas via heterotrophic denitrification. This study provides new insights into the impacts of microaerobic conditions on the microbial community functional structures of sulfide-oxidizing, nitrate-reducing, and sulfur-producing bioreactors, which revealing the potential linkage between functional microbial communities and reactor performance.

Enhanced elementary sulfur recovery with sequential sulfate-reducing, denitrifying sulfide-oxidizing processes in a cylindrical-type anaerobic baffled reactor

Simultaneous removal of COD, SO4(2-) and NO3(-) and recovery of elemental sulfur (S(0)) were evaluated in a four-compartment anaerobic baffled reactor (ABR) with separated functional units of sulfate reduction (SR) and denitrifying sulfide removal (DSR). Optimal SO4(2-)-S/NO3(-)-N ratio was evaluated as 5:5, with a substantial improvement of S(0) recovery maintained at 79.1%, one of the highest level ever reported; meanwhile, removal rates of COD, SO4(2-) and NO3(-) were approached at 71.9%, 92.9% and 98.6%, respectively. Nitrate served as a key factor to control the shift of SR and DSR related populations, with the possible involvement of Thauera sp. during SR and Sulfurovum sp. or Acidiferrobacter sp. during DSR, respectively. DsrB and aprA genes were the most abundant during SR and DSR processes, respectively. Cylindrical-type ABR with the improved elemental sulfur recovery was recommended to deal with sulfate and nitrate-laden wastewater under the optimized SO4(2-)/NO3(-) ratio.

Regeneration of elemental sulfur in a simultaneous sulfide and nitrate removal reactor under different dissolved oxygen conditions

A continuous reactor in microaerobic conditions was adopted for sulfide-oxidizing, nitrate-reducing and elemental sulfur (S(0)) regenerating, simultaneously. The results showed that appropriate dissolved oxygen (DO) enhanced S(0) regeneration efficiency, sulfide oxidation efficiency, and nitrate reduction efficiency. When the DO concentration was 0.1-0.3 mg L(-1), the microaerobic bioreactor simultaneously converted 8.16 kg-Sm(-3)d(-1) of sulfide to S(0) and 2.48 kg-Nm(-3)d(-1) of nitrate to nitrogen with the sulfide and nitrate removal efficiency of 100% and 90% respectively. Compared with anaerobic sulfide and nitrate removal process previously reported, the loading sulfide was higher and more S(0) was generated during the operation in microaerobic reactor. Analysis using the 16S rDNA gene clone library revealed that Azoarcus, Thauera, Paracoccus, Sulfurospirillum, Arcobacter and Clostridium were the dominant microorganisms in the sulfide and nitrate removal system.

Bioreactor performance and functional gene analysis of microbial community in a limited-oxygen fed bioreactor for co-reduction of sulfate and nitrate with high organic input

Limited-oxygen mediated synergistic relationships between sulfate-reducing bacteria (SRB), nitrate-reducing bacteria (NRB) and sulfide-oxidizing bacteria (SOB, including nitrate-reducing, sulfide-oxidizing bacteria NR-SOB) were predicted to simultaneously remove contaminants of nitrate, sulfate and high COD, and eliminate sulfide generation. A lab-scale experiment was conducted to examine the impact of limited oxygen on these oxy-anions degradation, sulfide oxidation and associated microbial functional responses. In all scenarios tested, the reduction of both nitrate and sulfate was almost complete. When limited-oxygen was fed into bioreactors, S(0) formation was significantly improved up to ∼ 70%. GeoChip 4.0, a functional gene microarray, was used to determine the microbial gene diversity and functional potential for nitrate and sulfate reduction, and sulfide oxidation. The diversity of the microbial community in bioreactors was increased with the feeding of limited oxygen. Whereas the intensities of the functional genes involved in sulfate reduction did not show a significant difference, the abundance of the detected denitrification genes decreased in limited oxygen samples. More importantly, sulfide-oxidizing bacteria may alter their populations/genes in response to limited oxygen potentially to function more effectively in sulfide oxidation, especially to elemental sulfur. The genes fccA/fccB from nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB), such as Paracoccus denitrificans, Thiobacillus denitrificans, Beggiatoa sp., Thiomicrospira sp., and Thioalkalivibrio sp., were more abundant under limited-oxygen condition.

GeoChip-based analysis of the microbial community functional structures in simultaneous desulfurization and denitrification process

The elemental sulfur (S°) recovery was evaluated in the presence of nitrate in two development models of simultaneous desulfurization and denitrification (SDD) process. At the loading rates of 0.9 kg S/(m³·day) for sulfide and 0.4 kg N/(m³·day) for nitrate, S° conversion rate was 91.1% in denitrifying sulfide removal (DSR) model which was higher than in integrated simultaneous desulfurization and denitrification (ISDD) model (25.6%). A comprehensive analysis of functional diversity, structure and metabolic potential of microbial communities was examined in two models by using functional gene array (GeoChip 2.0). GeoChip data indicated that diversity indices, community structure, and abundance of functional genes were distinct between two models. Diversity indices (Simpson's diversity index (1/D) and Shannon-Weaver index (H')) of all detected genes showed that with elevated influent loading rate, the functional diversity decreased in ISDD model but increased in DSR model. In contrast to ISDD model, the overall abundance of dsr genes was lower in DSR model, while some functional genes targeting from nitrate-reducing sulfide-oxidizing bacteria (NR-SOB), such as Thiobacillus denitrificans, Sulfurimonas denitrificans, and Paracoccus pantotrophus were more abundant in DSR model which were highly associated with the change of S(0) conversion rate obtained in two models. The results obtained in this study provide additional insights into the microbial metabolic mechanisms involved in ISDD and DSR models, which in turn will improve the overall performance of SDD process.

Microbial community functional structure in response to micro-aerobic conditions in sulfate-reducing sulfur-producing bioreactor

Limited oxygen supply to anaerobic wastewater treatment systems had been demonstrated as an effective strategy to improve elemental sulfur (S(0)) recovery, coupling sulfate reduction and sulfide oxidation. However, little is known about the impact of dissolved oxygen (DO) on the microbial functional structures in these systems. We used a high throughput tool (GeoChip) to evaluate the microbial community structures in a biological desulfurization reactor under micro-aerobic conditions (DO: 0.02-0.33 mg/L). The results indicated that the microbial community functional compositions and structures were dramatically altered with elevated DO levels. The abundances of dsrA/B genes involved in sulfate reduction processes significantly decreased (p < 0.05, LSD test) at relatively high DO concentration (DO: 0.33 mg/L). The abundances of sox and fccA/B genes involved in sulfur/sulfide oxidation processes significantly increased (p < 0.05, LSD test) in low DO concentration conditions (DO: 0.09 mg/L) and then gradually decreased with continuously elevated DO levels. Their abundances coincided with the change of sulfate removal efficiencies and elemental sulfur (S(0)) conversion efficiencies in the bioreactor. In addition, the abundance of carbon degradation genes increased with the raising of DO levels, showing that the heterotrophic microorganisms (e.g., fermentative microorganisms) were thriving under micro-aerobic condition. This study provides new insights into the impacts of micro-aerobic conditions on the microbial functional structure of sulfate-reducing sulfur-producing bioreactors, and revealed the potential linkage between functional microbial communities and reactor performance.

Fine-tuning key parameters of an integrated reactor system for the simultaneous removal of COD, sulfate and ammonium and elemental sulfur reclamation

In this paper, we proposed an integrated reactor system for simultaneous removal of COD, sulfate and ammonium (integrated C-S-N removal system) and investigated the key parameters of the system for a high level of elemental sulfur (S(0)) production. The system consisted of 4 main units: sulfate reduction and organic carbon removal (SR-CR), autotrophic and heterotrophic denitrifying sulfide removal (A&H-DSR), sulfur reclamation (SR), and aerated filter for aerobic nitrification (AN). In the system, the effects of key operational parameters on production of elemental sulfur were investigated, including hydraulic retention time (HRT) of each unit, sulfide/nitrate (S(2-)-S/NO3(-)-N) ratios, reflux ratios between the A&H-DSR and AN units, and loading rates of chemical oxygen demand (COD), sulfate and ammonium. Physico-chemical characteristics of biosulfur were studied for acquiring efficient S(0) recovery. The experiments successfully explored the optimum parameters for each unit and demonstrated 98% COD, 98% sulfate and 78% nitrogen removal efficiency. The optimum HRTs for SR-CR, A&H-DSR and AN were 12h, 3h and 3h, respectively. The reflux ratio of 3 could provide adequate S(2-)-S/NO3(-)-N ratio (approximately 1:1) to the A&H-DSR unit for obtaining maximum sulfur production. In this system, the maximum production of S(0) reached 90%, but only 60% S(0) was reclaimed from effluent. The S(0) that adhered to the outer layer of granules was deposited in the bottom of the A&H-DSR unit. Finally, the microbial community structure of the corresponding unit at different operational stage were analyzed by 16S rRNA gene based high throughput Illumina MiSeq sequencing and the potential function of dominant species were discussed.

Integrated simultaneous desulfurization and denitrification (ISDD) process at various COD/sulfate ratios

The integrated simultaneous desulfurization and denitrification (ISDD) is a novel treatment process to handle sulfate and nitrate-laden wastewaters of high loadings. This study experimentally explored the effect of COD/SO4(2-) ratio on the performance of ISDD process, particularly considering the complex interactions between sulfate-reducing bacteria (SRB), heterotrophic denitrifiers (hNRB) and autotrophic denitrifiers (aNRB). There existed an optimal COD/SO4(2-) ratio (=1.5:1 in the present study) to reach 100% SO4(2-) and NO3(-) removals and 42.6% S(0) recovery. At COD/SO4(2-)=1.5:1, the functional strains could form granules with high retention in the ISDD reactor. The microbial community analysis identified the SRB, hNRB and aNRB in the studied system, whose shifts correlated well with the noted ISDD performance change at different COD/SO4(2-) ratio. Interactions between different groups of bacteria and the possible strategy to enhance the ISDD performance were discussed.

Simultaneous removal of sulfide, nitrate and acetate under denitrifying sulfide removal condition: modeling and experimental validation

Simultaneous removal of sulfide (S(2-)), nitrate (NO3(-)) and acetate (Ac(-)) under denitrifying sulfide removal process (DSR) is a novel biological wastewater treatment process. This work developed a mathematical model to describe the kinetic behavior of sulfur-nitrogen-carbon and interactions between autotrophic denitrifiers and heterotrophic denitrifiers. The kinetic parameters of the model were estimated via data fitting considering the effects of initial S(2-) concentration, S(2-)/NO3(-)-N ratio and Ac(-)-C/NO3(-)-N ratio. Simulation supported that the heterotrophic denitratation step (NO3(-) reduction to NO2(-)) was inhibited by S(2-) compared with the denitritation step (NO2(-) reduction to N2). Also, the S(2-) oxidation by autotrophic denitrifiers was shown two times lower in rate with NO2(-) as electron acceptor than that with NO3(-) as electron acceptor. NO3(-) reduction by autotrophic denitrifiers occurs 3-10 times slower when S(0) participates as final electron donor compared to the S(2-)-driven pathway. Model simulation on continuous-flow DSR reactor suggested that the adjustment of hydraulic retention time is an efficient way to make the reactor tolerating high S(2-) loadings. The proposed model properly described the kinetic behaviors of DSR processes over wide parametric ranges and which can offer engineers with basis to optimize bioreactor operation to improve the treatment capacity.

A physiologically based kinetic model for bacterial sulfide oxidation

In the biotechnological process for hydrogen sulfide removal from gas streams, a variety of oxidation products can be formed. Under natron-alkaline conditions, sulfide is oxidized by haloalkaliphilic sulfide oxidizing bacteria via flavocytochrome c oxidoreductase. From previous studies, it was concluded that the oxidation-reduction state of cytochrome c is a direct measure for the bacterial end-product formation. Given this physiological feature, incorporation of the oxidation state of cytochrome c in a mathematical model for the bacterial oxidation kinetics will yield a physiologically based model structure. This paper presents a physiologically based model, describing the dynamic formation of the various end-products in the biodesulfurization process. It consists of three elements: 1) Michaelis-Menten kinetics combined with 2) a cytochrome c driven mechanism describing 3) the rate determining enzymes of the respiratory system of haloalkaliphilic sulfide oxidizing bacteria. The proposed model is successfully validated against independent data obtained from biological respiration tests and bench scale gas-lift reactor experiments. The results demonstrate that the model is a powerful tool to describe product formation for haloalkaliphilic biomass under dynamic conditions. The model predicts a maximum S⁰ formation of about 98 mol%. A future challenge is the optimization of this bioprocess by improving the dissolved oxygen control strategy and reactor design.

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.