Article

# Performance evaluation of sulfide-based autotrophic denitrification for petrochemical industry wastewater

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## Abstract

Oil refineries generate wastewater pollutants such as sulfide and nitrate from gasoline sweetening and other concomitant processes, respectively. However, the concentration of biodegradable organic substrate in the industrial wastewater is insufficient for the complete denitrification process. Thus, this study investigated sulfide-based denitrification treatment for real sulfide-rich industrial wastewater, with a low C/N molar ratio, produced in the oil refinery. The performance of two continuous laboratory packed-bed reactors (PBRs) packed with different biomass carriers was evaluated. PBRs were denoted as R1 (polyester type carriers) and R2n (polyester type carriers coated with nanofibers) and operated at 24 °C with the fixed S/N molar ratio at 0.28, respectively. The maximum sulfide and nitrate-nitrogen applied loading rate was 0.17 kg/(m³⋅d) and 0.25 kg/(m³⋅d), respectively. Results showed nitrate removal efficiency was 94.8 % and 95.3 % in R1 and R2n, respectively. Sulfide was almost completely removed in both PBRs, achieving the sulfide removal efficiency of 99 % throughout the entire experiment. The R2n achieved a higher specific volumetric denitrification rate then R1 since nanofiber carriers enhanced bacterial attachment. This study indicated great potential for further application in petrochemical wastewater treatment.

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The characteristics of reaction between S0 and NO2--N or NO3--N in the sulfur autotrophic denitrification (SADN) process were studied using S0 as an electron donor and NO2--N and NO3--N as electron acceptors. The effect of changes in pH and temperature on the processes of NO2--N and NO3--N reduction were also studied to identify the optimum control parameters for strengthening the preference of S0 on NO3--N; thus, achieving the efficient accumulation of NO2--N. The results showed that the affinity of S0 for NO3--N was considerably higher than that for NO2--N. The optimum pH values for the reductions of NO2--N and NO3--N were 7.0 and 8.5, respectively, and both optimum temperatures were 35 °C. By controlling different pH, the NO3--N conversion efficiency reached 90%, at which time the accumulation of NO2--N was more than 95%. Microbial community analysis showed that Thiobacillus, Sulfurimonas, and Thioahalobacter were the main genera in the S0-SADN process.
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Anoxic thiosulfate (S2O3²⁻) oxidation using autotrophic denitrification by a mixed culture of nitrate reducing, sulfur oxidizing bacteria (NR-SOB) was studied in a fluidized bed reactor (FBR). The long-term performance of the FBR was evaluated for 306 days at three nitrogen-to-sulfur (N/S) molar ratios (0.5, 0.3 and 0.1) and a hydraulic retention time (HRT) of 5 h. S2O3²⁻ removal efficiencies >99% were obtained at a N/S ratio of 0.5 and a S2O3²⁻ and nitrate (NO3⁻) loading rate of 820 (±84) mg S-S2O3²⁻ L⁻¹ d⁻¹ and 173 (±10) mg N-NO3⁻ L⁻¹ d⁻¹, respectively. The S2O3²⁻ removal efficiency decreased to 76% and 26% at N/S ratios of 0.3 and 0.1, respectively, and recovered to 80% within 3 days after increasing the N/S ratio from 0.1 back to 0.5. The highest observed half-saturation (Ks) and inhibition (KI) constants of the biofilm-grown NR-SOB obtained from batch cultivations were 172 and 800 mg S-S2O3²⁻ L⁻¹, respectively. Thiobacilus denitrificans was the dominant microorganism in the FBR. Artificial neural network modeling successfully predicted S2O3²⁻ and NO3⁻ removal efficiencies and SO4²⁻ production in the FBR. Additionally, results from the sensitivity analysis showed that the effluent pH was the most influential parameter affecting the S2O3²⁻ removal efficiency.
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Sulfur reduction is a promising alternative to sulfate reduction as it can generate sulfide at a low cost for the precipitation of heavy metals or autotrophic denitrification in wastewater treatment. However, the extremely low water solubility of elemental sulfur limits its bioavailability and results in a low sulfur-reduction rate. Polysulfide, which is naturally generated through reactions between sulfur and sulfide, can enhance the bioavailability of sulfur and thus contribute to high-rate sulfur reduction. Based on this principle, a laboratory-scale sulfur-reducing bioreactor was designed in this study for wastewater treatment. After 164 days of operation, the sulfide production rate (SPR) in the bioreactor reached 126 mg S/L-h, which is significantly higher than those of other sulfate-reducing systems. Moreover, dissolved zero-valent sulfur (referred to as polysulfide) was detected in the sulfur-reducing reactor when the organics were completely depleted, indicating that polysulfide can form naturally and be readily reduced to sulfide in the bioreactor. We found that the produced sulfide promoted the formation of more polysulfide, which enabled a self-accelerating chain reaction of sulfur reduction via polysulfide. This stimulation effect was further validated by the seven-hour batch tests. In the batch test without sulfide addition initially, a continuous increase in the hourly SPR was observed with increasing sulfide concentration. Furthermore, in the batch tests with the addition of 50 to 200 mg S/L sulfide at the beginning, the average SPR in the first three hours increased with elevating initial sulfide concentration due to more polysulfide formation and reduction. However, high sulfide concentration (> 250 mg S/L) hindered the continuous increase in SPR. Additionally, when polysulfide formation was prevented through the addition of Fe2+, the SPR dropped by 97.6% compared to that in the presence of polysulfide. This validates the key role of polysulfide in the high-rate sulfur reduction process. Overall, the findings suggest that high-rate sulfur reduction can be achieved for autotrophic denitrification or heavy-metal removal in wastewater treatment.
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In order to achieve the biological removal of nitrate of groundwater, the denitrification properties of an environmental strain of Thiobacillus denitrificans were studied at a pilot scale. This strain was previously described and showed good denitrification abilities at low temperatures. Its mean nitrate removal velocity was twice as great at 10°C, than at 20°C. A pilot plant in fixed bed conditions, filled with a pouzzolane / Neutralg® mixture was used for this purpose. The feed solution was tap water to which only nitrate and thiosulphate were added; the temperature was 10°C. An optimum volumetric nitrate loading rate of 1.5 kg N-NO3-. m-3 d-1 was determined for which 100 % denitrification yield was reached. The maximum volumetric nitrate loading rate of the plant was evaluated. The influence of the operating conditions such as residence time and nitrate concentrations were investigated as well as the efficiency of the plant by studying molar ratios between the removed and released products.
Article
Anoxic H2S oxidation under denitrifying conditions produced sulphur and sulphate in almost equal proportions by an isolated Thiobacillus denitrificans. Under nitrate reducing conditions the rate of sulphide oxidation was approximately 0.9g sulphide/g biomass h. Nitrate was reduced to nitrite and accumulated during sulphide oxidation. Above 100mg nitrite/l, the sulphide oxidation rate declined and at 500mg/l it was totally arrested. The optimum pH for the anoxic sulphide oxidation was around 7.5. Concentrations of sulphate 1500mg/l and acetate 400mg/l had no effect on anoxic sulphide oxidation.
Article
Biooxidation of sulphide under denitrifying conditions is a key process in control of souring in oil reservoirs and in treatment of gas and liquids contaminated with sulphide and nitrate. In this work, biooxidation of sulphide was studied using a representative culture originated from an oil reservoir. Effects of sulphide concentration, sulphide to nitrate molar ratio, and loading rates of sulphide and nitrate on their removal rates and composition of the end products were investigated. In the batch system sulphide removal rate passed through a maximum as sulphide concentration was increased from 2.1 to 16.3 mM, with the highest rate (2.06 mM h−1) observed with 10.7 mM sulphide. Nitrate removal was coupled to sulphide oxidation and the highest removal rate was 1.05 mM h−1. In the continuous bioreactors fed with 10 and 5, 15 and 7.5, and 20 and 10 mM sulphide and nitrate, cell wash-out occurred as dilution rate was increased above 0.15, 0.13 and 0.08 h−1, respectively. Prior to cell wash-out linear increases in sulphide and nitrate removal rates were observed as loading rate was increased. The highest sulphide and nitrate removal rates of 2.0 and 0.92 mM h−1 were obtained in the bioreactor fed with 15 mM sulphide and 7.5 mM nitrate at loading rates of 2.1 and 0.93 mM h−1, respectively. Short residence times and high sulphide to nitrate ratios promoted the formation of sulphur, a desired end product for ex situ treatment of contaminated streams. Combination of long residence times and low sulphide to nitrate ratios, which favours formation of sulphate, is the suitable strategy for in situ removal of H2S from oil reservoirs.
Article
The denitrifying sulfide removal (DSR) process with bio-granules comprising both heterotrophic and autotrophic denitrifiers can simultaneously convert nitrate, sulfide and acetate into di-nitrogen gas, elementary sulfur and carbon dioxide, respectively, at high loading rates. This study determines the reaction rate of sulfide oxidized into sulfur, as well as the reduction of nitrate to nitrite, would be enhanced under a micro-aerobic condition. The presence of limited oxygen mitigated the inhibition effects of sulfide on denitrifier activities, and enhanced the performance of DSR granules. The advantages and disadvantages of applying the micro-aerobic condition to the DSR process are discussed.
Article
In this paper we describe an alternative flow-chart for full treatment of wastewaters rich in organic substrates, ammonia (or organic nitrogen), and sulfate, such as those generated in fish cannery industries. Biogas generated during anaerobic pretreatment of these wastewaters is rich in hydrogen sulfide that needs to be removed to enable application of the biogas. Nitrogen elimination is traditionally achieved by subsequent nitrification and denitrification of the effluent of the anaerobic reactor. Alternatively, the hydrogen sulfide in the biogas can be applied as an electron donor in an autotrophic post-denitrification step. In order to study whether sufficient hydrogen sulfide containing biogas for denitrification was produced in the anaerobic reactor, the biogas composition as a function of the anaerobic reactor-pH was estimated based on a typical wastewater composition and chemical equilibrium equations. It is demonstrated that typical sulfate and nitrogen concentrations in fish cannery wastewater are highly appropriate for application of autotrophic post-denitrification. A literature review furthermore suggested that the kinetic parameters for autotrophic denitrification by Thiobacillus denitrificans represent no bottleneck for its application. Initial experimental studies in fixed-film reactors were conducted with sodium sulfide and nitrate as an electron donor-acceptor couple. The results revealed that only moderate volumetric treatment capacities (< 1 g-NO3- N l(-1) day(-1)) could be achieved. Mass balances suggested that incomplete sulfide oxidation to elemental sulfur occurred, limiting biomass retention and the treatment capacity of the reactor. Future research should clarify the questions concerning product formation from sulfide oxidation.
Article
Biotechnology can be used to assess the well being of ecosystems, transform pollutants into benign substances, generate biodegradable materials from renewable sources, and develop environmentally safe manufacturing and disposal processes. Simultaneous elimination of sulfide and nitrite from synthetic wastewaters was investigated using a bioreactor. A laboratory scale anoxic sulfide-oxidizing (ASO) reactor was operated for 135 days to evaluate the potential for volumetric loading rates, effect of hydraulic retention time (HRT) and substrate concentration on the process performance. The maximal sulfide and nitrite removal rates were achieved to be 13.82 and 16.311 kg/(m3 day), respectively, at 0.10 day HRT. The process can endure high sulfide concentrations, as the sulfide removal percentage always remained higher than 88.97% with influent concentration up to 1920 mg/L. Incomplete sulfide oxidation took place due to lower consumed nitrite to sulfide ratios of 0.93. It also tolerated high nitrite concentration up to 2265.25mg/L. The potential achieved by decreasing HRT at fixed substrate concentration is higher than that by increasing substrate concentration at fixed HRT. The process can bear short HRT of 0.10 day but careful operation is needed. Nitrite conversion was more sensitive to HRT than sulfide conversion when HRT was decreased from 1.50 to 0.08 day. Stoichiometric analyses and results of batch experiments show that major part of sulfide (89-90%) was reduced by nitrite while some autooxidation (10-11%) was resulted from presence of small quantities of dissolved oxygen in the influent wastewater. There was ammonia amassing in considerably high amounts in the bioreactor when the influent nitrite concentration reached above 2265.25mg/L. High ammonia concentrations (200-550 mg/L) in the bioreactor contributed towards the overall inhibition of the process. Present biotechnology exhibits practical value with a high potential for simultaneous removal of nitrite and sulfide from concentrated wastewaters at shorter HRT.
Article
An autotrophic denitrification process using reduced sulfur compounds (thiosulfate and sulfide) as electron donor in an activated sludge system is proposed as an efficient and cost effective alternative to conventional heterotrophic denitrification for inorganic (or with low C/N ratio) wastewaters and for simultaneous removal of sulfide or thiosulfate and nitrate. A suspended culture of sulfur-utilizing denitrifying bacteria was fast and efficiently established by bio-augmentation of activated sludge with Thiobacillus denitrificans. The stoichiometry of the process and the key factors, i.e. N/S ratio, that enable combined sulfide and nitrogen removal, were determined. An optimum N/S ratio of 1 (100% nitrate removal without nitrite formation and low thiosulfate concentrations in the effluent) has been obtained during reactor operation with thiosulfate at a nitrate loading rate (NLR) of 17.18 mmol N L(-1) d(-1). Complete nitrate and sulfide removal was achieved during reactor operation with sulfide at a NLR of 7.96 mmol N L(-1) d(-1) and at N/S ratio between 0.8 and 0.9, with oxidation of sulfide to sulfate. Complete nitrate removal while working at nitrate limiting conditions could be achieved by sulfide oxidation with low amounts of oxygen present in the influent, which kept the sulfide concentration below inhibitory levels.
Article
Present investigation deals with the effect of sulfide to nitrate (S/N) molar ratio on the simultaneous anaerobic sulfide and nitrate removal on capacity, stability and selectivity of the process. The volumetric sulfide-sulfur and nitrate-nitrogen removal rates at molar S/N ratio of 5:2 were 4.86 kg(m(3)d)(-1) and 0.99 kg(m(3)d)(-1), respectively, which were higher than those at S/N molar ratios of 5:5 and 5:8. Moreover, the fluctuations in the effluent at S/N ratio of 5:2 were less than those at the other two tested ratios. During the operation, the ratio of converted sulfide to converted nitrate tended to approach 5:2. The selectivity for elemental sulfur and dinitrogen was improved when the S/N molar ratio was set at 5:2 rather than 5:5 or 5:8. The process became unstable if the influent sulfide surpassed its critical concentration. The electron balance between reactants was also analyzed for different S/N molar ratios.