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Pilot-Scale Biological Sulfide Oxidation Process for Treating Effluent from Rayon Industry
Maung Myo Kyaw and Prof. Ajit P.Annachhatre Environmental Engineering and Management, School of Environment, Resources and Development, Asian Institute of Technology, Thailand, May 2008

Abstract This investigation was involved the design of pilot scale biological sulfide oxidation concentric draught-tube internal-loop airlift reactor based on the available laboratory scale experiment. This pilot scale biological sulfide oxidation airlift reactor was fabricated and installed at Thai Rayon Industry. The sulfide oxidation airlift bioreactor which contained mixed cultured of sulfide oxidizing bacteria oxidized sulfide to elemental sulfur and sulfate. The purpose of this experiment was to investigate the optimum oxygen supply for maximum sulfide removal and sulfur recovery as well as sulfide loading rate. In this research, the concentric draft-tube internal-loop bioreactor was used because of its good mixing property. Thus a high recirculation inside the reactor was achieved and most of sulfide was converted to elemental sulfur which was settled down in the secondary sedimentation tank. The biological sulfide oxidation was investigated under the optimal operating conditions for maximizing sulfide removal and sulfur production during the steady state conditions. The airlift bioreactor was operated under optimum pH (7.5-8) and ambient temperature (30-40 ºC). The flow rate and HRT were maintained 600 L/day and 4 hrs respectively. The sulfide loading rate steadily increased up to 4.38 kg HS-/m3-day by increasing the influent sulfide concentrations up to 730 mg/L. The DO concentrations were maintained less than 0.2 mg/L during the whole process. In addition, cost benefit analysis (CBA) was compared base on economic and environmental aspects between conventional treatment and developed technology.

Keywords:

sulfide oxidation, sulfur recovery, airlift bioreactor, sulfide loading rate, dissolved oxygen concentration

Introduction The anaerobic treatment of wastewaters containing partially oxidized sulfur components such as thiosulfate, sulfite, or sulfate, causes the reduction of these compounds to sulfide. Such types of wastewaters are produced from different industries like the petrochemical, the photographic processing, and the pulp-paper industries and those wastewaters are called “sour water” (Kuenen, 1975) which was originated to describe a waste contaminated with sulfide. Sour water streams in the refineries are generated from sour stream condensates from distillation, thermal or hydrogen cracking operations, and product heating process (Berne and Cordonnier, 1995). Sulfide oxidation under sufficient conditions of oxygen or nitrate, the oxidation of sulfide can proceed spontaneously. Biological oxidation of sulfide can be proceeds under aerobic, anoxic and anaerobic conditions. In the presence of oxygen or nitrate, the colorless sulfur bacteria oxidize sulfide to sulfur or sulfate. Anaerobic sulfide oxidation is performed by the green and purple phototrophic bacteria, using the sulfide electrons for the reduction and assimilation of CO2 and gaining energy from the sunlight. Hydrogen sulfide is toxic to aquatic animal life in very low concentrations. The threshold limit value for fresh or salt water fish is 0.5 ppm. Sulfide in wastewater is also corrosive and has a very unpleasant odor and sulfide has a high oxygen demand of 2 mol O2/mol S2resulting in depletion of oxygen where sour wastewater is discharged (Kobayashi et al., 1982). Biological treatment for sulfate rich wastewater consists of two main processes namely sulfate reduction and sulfide oxidation, which take place in separate reactors. First, the sulfate is converted into sulfide in sulfate reduction process and some of this sulfide is used for heavy metal precipitation. The metals precipitate as metal sulfides, which are separately removed. It is possible to selectively recover different metals from the contaminated water based on the principle of different solubility of metal sulfides at different pH. This way it is possible to selectively recover, for example, Cu2+, Pb 2+ and Zn2+ with compared to precipitation of metals with lime, much lower effluent concentrations can be achieved (ppb level) by using biotechnology due to the much lower solubility of metal sulfides (Buisman., et al., 1999). SO42- + 4H2 Metal + sulfide SRB

H2S + 4H2O metal-sulfide↓

Eq. 1 Eq. 2

Remaining sulfide is partially oxidized and converted to settleable oxidized sulfur cake which is easily to remove in process. Other advantages are no chemical addition required and can be reused for the raw material of sulfuric acid or it can be used as fertilizer. The reuse of metal sulfides depends on the type of metals. If for example only Zn2+ is precipitated, it can be recovered in zinc refineries. In case of H2S removal from wastewater, the influent sulfide is oxidized into sulfur and hydroxide and this alkalinity forming is reused from the effluent. The most two important biological conversions of an aerobic sulfide removal system are outlined as follow.

2HS- + O2 2HS- + 4O2

SOB

2S0 + 2OH2SO42- + 2H+

∆Gº = -210.81 kJ/mol Eq. 3

SOB

∆Gº = -796.48 kJ/mol Eq. 4

And other researcher, Gonzalez et al., 2005, found that thiosulfate was converted to elemental sulfur under limited oxidation and converted to thiosulfate under excess amount of oxygen. S2O32- + ½ O2 SOB S0 + SO42-

∆Gº = -231 kJ/mol

Eq. 5

S2O32- + 2O2 + H2O

SOB

2SO42- + 2H+ ∆Gº = -739 kJ/mol

Eq. 6

Objectives of study The objectives of this study were to design pilot scale sulfide oxidation air lift reactor based on data from laboratory investigation and to start up the biological sulfide oxidation process in airlift reactor by using acclimatized seed sludge and then, to optimize the operating parameters such as sulfide loading rate, dissolved oxygen concentration and pH to get the maximum sulfur recovery.

Materials and Methods The wastewater from spinning process in Thai Rayon Industry was containing high concentration of sulfate and metal zinc. In sulfate reducing gaslift reactor, sulfate was reduced by sulfate reducing bacteria (SRB) by using hydrogen gas as an electron donor and sulfate reduced to sulfide. One portion of these sulfides was used for Zn removal and the remaining sulfide was fed to bottom part of the concentric draught tube internal loop airlift reactor for sulfide oxidation process and recovered sulfide as sulfur by using sulfur oxidation bacteria (SOB) in airlift reactor, which can be further used as raw material for sulfuric acid production. The Pilot scale sulfide oxidation airlift reactor was operated under ambient temperature to treat the sulfide rich wastewater (effluent from sulfate reduction gaslift reactor). The waste stream was continuously fed to the bottom part of the airlift reactor and air was sparged at the bottom of the draft tube and creating air bubble up to top of the draft tube by using air diffuser. The wastewater and air were completely mixed through the draft tube and to create the airlift condition. Because of different gas holdup between upper and bottom parts of the reactor, the liquid circulation was well promoted and the biomass and wastewater were completely mixed throughout the reactor working volume while DO concentration increased through the draft tube from the bottom to the top by aeration and decreased along the down comer from the top to the bottom due to oxygen consumption of activated sludge.

The DO concentration inside the reactor was controlled to favor the partial oxidation of sulfide to elemental sulfur by the group of colorless sulfur oxidizing bacteria. The operation was carried out to obtain optimum operating conditions to get the maximum sulfur recovery. The biologically produced sulfur was settled down to the bottom part of the reactor by gravitationally and removed from sludge drain out pipe. The airflow meter was set up to control the air fed into the reactor. The pH was maintained within 7.5 to 8 by adding hydrochloric acid. Sodium bicarbonate used as carbon source was provided for sulfur oxidizing bacteria and the nutrient solution was supplied for the growth of bacteria. The sedimentation tank was provided in the downstream of the airlift reactor to recover the biologically produced sulfur. The process diagram for pilot scale sulfide oxidation airlift reactor is illustrated in Figure 1.
Air

Airlift Reactor
HCl Nutrient + NaHCO3

Settler
Effluent

Tap Water Synthetic Wastewater

`

Sulfur Sludge

Feeding Tank

Air Flow Meter

Feeding Pump Sulfur Sludge

Air

Figure 1: Process Flowchart for Sulfide Oxidation in Airlift Reactor

Internal-loop airlift reactor configuration The fiber reinforced plastic (FRP) reactor with working volume of 100 L and total volume of 246 L (excluded settling zone) is provided. The 65 mm-inner diameter acrylic draft tube with the length of 2710 mm is concentrically inserted into the 217 mm-inner diameter reactor. Air is supplied from elevation tank and controlled by airflow meter. The air is sparged thoroughly through inside the reactor by using air diffuser at the bottom of the draft tube. Air was creating air bubble up to top of riser. The bottom clearance (50 mm) is

sufficiently provided for recirculation of wastewater from downcomer. The settling zone is provided at the bottom part of the reactor to settle biologically produced sulfur that was removed weekly at the bottom port. The design parameter of sulfide oxidation airlift reactor is mentioned in table 1. Table 1: Design Parameter of Sulfide Oxidation Internal-loop Airlift Reactor Parameter Values (Volumetric Sulfide Loading Rate)design 6 kgHS-/m3-day Flow Rate (Qavg) 0.6 m3/day Flow rate (Qmax) 1 m3/day Reactor Working Volume 0.1 m3 Total Volume 0.247 m3 Draft Tube Length 2.71 m Draft Tube Diameter (Dd) 0.065 m Column Diameter (DCi) 0.217 m Column Height 2.59 m Upper Column Diameter 0.67 m Upper Column Height 0.42 m Baffle diameter 0.217 m Baffle height 0.55 m Settling Basin Height 0.132 m Maximum air flow rate 10 L/min

Preparation for start up Biomass: The reactor was started up by using activated sludge from Thai Rayon Industry in Thailand. The active sludge was acclimatized by feeding with sulfide-containing synthesis wastewater (Na2S.3H2O) and mixed with NaHCO3 (carbon source) in the ratio of 4.309: 1(mol S2-/mol Carbon) in an open tank. Wastewater: The synthesis wastewater (Na2S.3H2O) was provided to biological sulfide oxidation airlift reactor. NaHCO3 was used as a carbon source in a ratio of 4.309: 1 (mol S2-/mol Carbon). The concentric hydrochloric acid (38% W/W) was added to synthetic wastewater tank to adjust the optimum pH. Nutrient Solution: The nutrient solution composed with NH4Cl and KH2PO4 was provided with the C: N: P ratio of 100: 5: 1 (by weight) mentioned in Table 2; Table 2: Chemical Composition for biological sulfide oxidation process Composition Quantity Feed concentration for start up Up to 200 mg HS/L Feed concentration for steady state Up to 730 mg HS/L NaHCO3 4.309: 1 (mol S2-/mol Carbon) C:N:P 100:5:1 Adjust pH 7.5-8 with 38% HCl (W/W)

Laboratory analytical methods Sulfide and Sulfate: Sulfide and sulfate were measured by Standard Methods for Examination of Water and Wastewater, 20 th Edition, 1998 (APHA-AWWA-WEF, 1998) Sulfide: Sulfide was analyzed by iodometric method, Method LWI-5001/ETM 112 for water, wastewater analysis according to standard method, 1998. Sulfate: Sulfate was analyzed by Turbidimetric method by method 4500 and by using HACH-2100P Turbidimeter. Sulfite: Sulfite was analyzed by Thai Rayon Laboratory Standard Method. The brief description for this method was mentioned in appendix A. Thiosulfate: Thiosulfate was analyzed by iodometric method, Method LWI-5003/ETM 304 (Scrubber Liquor Analysis). TSS and VSS: TSS and VSS were measured by solid determination methods, method numbers 2540 B and 2540 E mentioned in Standard Methods for Examination of Water and Wastewater, 20th Edition, 1998 (APHA-AWWA-WEF, 1998). Sulfur Mass Balance in Sulfur Oxidation Process: The amount of biologically produced sulfur (S0) can be calculated by subtracting the produced amount of S2-- sulfur, SO42--sulfur, S2O32--sulfur, SO32--sulfur from the sulfide conversion. Concentration of sulfate resulting from the addition of nutrient solution into the reactor was subtracted from the total sulfate concentration determined in the reactors. The mass balance for sulfur production is expressed as following equation: (HS--S + SO4 2—S + S203 2--S+ (HS--S+ SO4 2--S+ S20 3 2--S+ SO32--S)out = SO32--S) in *Qin * Qout + S0 S0 * 100 % Sulfur Production = (HS--S + SO4 2--S + S20 3 2--S+ SO32--S) in *Qin
2-

Eq. 7

Eq. 8

Where; HS--Sin = influent sulfide concentration (mg HS-/L) HS -Sout = effluent sulfide concentration (mg HS-/L) SO42--Sin = influent sulfate concentration (mg SO42-/L) 2SO4 -Sout = effluent sulfate concentration (mg SO42-/L) 2SO3 -Sin = influent sulfite concentration (mg SO32-/L) 2SO3 -Sout = effluent sulfite concentration (mg SO32-/L) S2O32--Sin = influent thiosulfate concentration (mg S2O32-/L) 2SO3 -Sout= effluent sulfite concentration (mg SO3 2-/L) S0 = biologically produced sulfur (g/day) Qin = influent flow rate (L/day) Qout = effluent flow rate (L/day)

Result and Discussion This chapter discusses on the result of pilot scale investigation of biological sulfide oxidation process in airlift reactor which conducted to sulfate reduction process in Thai Rayon Industry. The pilot plant was continuously operated for 72 days and the experiments were investigated into 2 main parts: Start up and steady state operation of the biological sulfide oxidation process by using synthesis wastewater mixed with industrial wastewater, and Steady state operation of biological sulfide oxidation process by using synthesis wastewater mixed with tap water.

Pilot scale biological sulfide oxidation process For biological sulfide oxidizing process, the main important biological conversions of an aerobic sulfide removal system are outlined as following: (Buisman et al., 1990) 2HS- + O2 2HS- + 4O2 2S0 + 2OH- ∆Gº = -210.81 kJ/mol 2SO42- + 2H+ ∆Gº = -796.48 kJ/mol

Eq. 9 Eq. 10

At dissolved oxygen concentration below 0.1 mg/L (oxygen limiting condition), sulfur is the major end product of the sulfide oxidation (Eq. 4.1), while sulfate is formed under excess oxygen circumstances (Eq. 4.2). Since elemental sulfur is easier to be removed and sulfate formation leads to four-fold higher oxygen consumption and consequently a higher energy demand, the formation of elemental sulfur is preferable (Janssen et al., 1995). Apart from the biological sulfide oxidation, the chemical oxidation of sulfide has to be accounted as well. In case of highly loaded bioreactors, not all sulfides would be converted into sulfur due to a limitation in biological activity. The chemical auto-oxidation of sulfide to thiosulfate becomes relatively important as indicated below (Chen & Morris et al., 1972): 2HS- + 2O2 H2O + S2O32- ∆Gº = -387.34 kJ/mol

Eq. 11

And other researcher, Gonzalez et al., 2005, found that thiosulfate was converted to elemental sulfur under limited oxidation and converted to thiosulfate under excess amount of oxygen (Eq. 4.4 and Eq.4.5). S2O32- + ½ O2 SOB S0 + SO42-

∆Gº = -231 kJ/mol

Eq. 12

S2O32- + 2O2 + H2O

SOB

2SO42- + 2H+ ∆Gº = -739 kJ/mol

Eq. 13

Reactor startup After 10 days of cultivation, 40 L (40 % capacity of reactor) of sludge which contained VSS concentration of 4,000 mg/l was filled in the airlift bioreactor with 60 L tap water. And the sulfide containing synthesis wastewater was mixed with effluent from sulfate reducing process in feeding tank and then fed into the reactor with the flow rate of 600 L/d. The sulfide influent concentrations were steadily increased from 50 mg/L to 100 mg/L (loading rate up to 0.67 kg HS-/m3-day) and influent flow rate was maintained 600 L/d during startup period for 15 days and the hydraulic retention time (HRT) was 4 hrs. The temperature was between the ranges of 25 to 40 ºC (ambient temperature) and pH was maintained 7.5 to 8 by adding with concentrated HCl (38 % W/W). . The nutrient solution (NH4Cl and KH2PO4) was provided as the ratio of C: N: P = 100: 25: 5 to growth of bacteria and NaHCO3 was supplied the ratio of 4 g of Na2S/g of NaHCO3. Both of nutrient solution and NaHCO3 were prepared in a same tank. During the start up period, DO concentrations were maintained less than 0.1 mg/L by providing the air flow rate between 0.3 to 0.5 L/min and controlled by an air-flow controller. The operation parameter of sulfide oxidation airlift reactor at start up condition was shown in table 3. Table 3: Operation parameter at start up condition Parameter Amount 3 Flow rate 0.6 m /day Feed concentration 50 to 100 mg HS-/L Volumetric sulfide loading rate Up to 0.67 kg HS-/m3.day HRT 4 hrs DO ≤ 0.1 mg/L Air flow rate 0.3 to 0.5 L/min pH 7.5-8.5 Temperature 25-40 ºC (Ambient temperature) Start up period 15 days In startup period, the sulfide removal efficiencies were between the ranges of 80% to 96% and effluent pH was slightly lower than influent pH and acidity was generated (Figure: 2). In this period, most of the influent sulfide was converted to sulfate and sulfur was not produced and sulfate was the major end product.

Steady state operation The sulfide removal efficiency for the biological sulfide oxidation process in airlift bioreactor was investigated by using synthesis wastewater mixed with industrial wastewater. Firstly, metal zinc was removed from industrial wastewater because of its toxicity to microorganism. So some of the sulfide containing synthesis wastewater (100 L/day) from feeding tank was recycled to mixing tank and zinc was precipitated as zinc sulfide in primary sedimentation tank and zinc concentration was reduced to less than 3 mg/L after precipitation. The steady state operation parameters for using synthesis wastewater mixed with industrial wastewater were mention in table 4.

Table 4: The steady state operation parameter for Phase I Parameter Amount Flow rate 0.6 m3/day Feed concentration 100 to 710 mg HS-/L Volumetric sulfide loading rate Up to 4.26 kg HS-/m3.day HRT 4 hrs DO ≤ 0.2 mg/L Air flow rate 1 to 4 L/min pH 7.5-8.5 Temperature 25-40 ºC (Ambient temperature) Steady state period 22 days Carbon source (NaHCO3) 4.309: 1 (mol S2-/mol Carbon) Nutrient solution (C:N:P) 100:5:1 Adjust pH 7.5-8 with 38% HCl (W/W) The sulfide loading rates were steadily increased from 0.77 kg HS-/m3-day to 4.26 kg HS/m3-day by increasing the influent concentration from 112 mg/L to 710 mg/L and the flow rate and HRT were maintained 600 L/day and 4 hrs respectively. DO concentration were maintained less than 0.2 mg/L by adjust the air flow rate during this period (Figure 3). After 17 days running, the effluent pH was slightly higher than influent pH and alkalinity was started to generated and sulfur was started to produce. After 45 days running, when sulfide loading rate was over 2.5 kg HS-/m3-day, effluent pH were sharply higher than influent pH (Figure 2).
Inf pH 14 Influence & Effluence pH 13 12 11 10 9 8 7 6 0 5 10 15 20 25 30 35 Days 40 45 50 55 60 65 70 Eff pH Temp: 45 40 35 Temperature 30 25 20 15 10 5 0

Figure 2: Temperature and pH monitoring for sulfide oxidation process

DO 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Days DO concentration (mg/l)

Figure 3: DO monitoring for sulfide oxidation process

Inf Sulfide Inf & Eff Sulfide Concentration (mg/l)
800 700 600 500 400 300 200 100 0 0 5 10 15 20 25 30 35

Eff Sulfide

Phase I

Phase II

40

45

50

55

60

65

70

Days

Figure 4: Influent and Effluent Sulfide Concentration on Phase I & II Operations During 30 days operation, the influent sulfide concentration started from 80.5 mg/L and then steadily increased up to 700 mg/l and the flow rate was maintained 600 L/day. And the sulfide removal efficiency was between the ranges of 87% to 100% during the operation period (Figure 5). During the first 10 days reactor operation, the sulfide removal efficiencies were the range between 95% to 100% and the volumetric sulfide loading rate was steadily increased up to 2.73 kg HS-/m3-day and the sulfur was produced up to 579 g/day. After this period, the TSS concentration was very high (11.5 g/L) because of the biologically produced sulfur inside the reactor, and these sulfur particles were interfered to oxygen respiration to microorganism and the sulfide removal efficiencies were decreased and also sulfur production declined to 480.5 g/day. So some amount of sludge (10 L of sludge) was removed from the reactor after 10 days operation.

5 Volumetric Sulfide Loading Rate (kg HS-/m3-d) 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 Days VSLR % Removal Efficiency 40 45 50 55 60 65 70

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% % Removal Efficiency

Figure 5: Sulfide Loading Rate & Sulfide Removal Efficiency monitoring on Phase I & Phase II Operations During the operation, when the sulfide loading rates were lower than 3 kg HS-/ m3-day, sulfur productions were 70% to 80% and sulfate productions were 20% to 30%. When sulfide loading rate was higher than 3 kg HS-/ m3-day, sulfur productions were increased up to 90 % and sulfate production were decreased below 20%. So from this experiment, at high sulfide loading rate, the sulfur is the major end product. And also other researchers, Annachhatre and Suktrakoolvait, 2001, found that more sulfur was produced when sulfide loading rate was started to raise from 1.3 kg HS-/ m3-day.

% sulfur production 100% 90% %S ulfur & Sulfate production 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 5 10 15 Days

% Sulfate production

VSLR 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

20

25

30

Figure 6: The sulfide loading rate and % Sulfur & Sulfate Production from influent sulfur species monitoring for Phase II operation

During the Phase II operation, the influent sulfide concentrations were steadily increased up to 730 mg/L and the influent sulfate concentrations were less than 30 mg/L. The effluent sulfide concentrations were less than 50 mg/L and the effluent sulfate concentrations were between 200 mg/L to 400 mg/L during the process. After 15 days period, although the sulfide loading rates were raised, the effluent sulfate concentrations were declined below 350 mg/L. That was meant that, when the high sulfide loading rate, the sulfur was the major end product. In Phase II operation, at low sulfide loading rate and DO concentration less than 0.2 mg/L, most of the sulfide was converted to sulfate and sulfur production was low during the first 5 days operation. When sulfide loading rate was higher than 2.5 kg HS-/ m3-day and DO concentration still maintained less than 0.2 mg/L, though the sulfur production was sharply high, sulfate formations in effluent were lower than the low sulfide loading rate operation. According to the stoichiometry equations of sulfide and thiosulfate aerobic oxidation reactions, oxygen requirement for sulfur production was fourfold less than the oxygen requirement for sulfate formation. So oxygen was the key parameter for sulfur recovery and maintains the optimum DO concentration was also essential. In this research, DO concentrations for both Phase I and Phase II operations were carefully maintained the optimum condition (DO < 0.2 mg/L). From this experiment, when DO concentration was less than 0.15 mg/L, the sulfur was the main product and when DO concentrations were maintained around 0.1 mg/L, the maximum sulfur productions were occurred. When DO concentrations were higher than 0.15 mg/L, the sulfur formations were significantly high.

Conclusion This study was shown that the biological sulfide oxidation process in the airlift reactor was more satisfactory for sulfide removal efficiency and elemental sulfur production than the conventional methods. The sulfide oxidation process was strongly depended on the parameters of volumetric sulfide loading rate, DO concentration, pH and temperature. The airlift bioreactor was operated under optimum pH (7.5-8) and ambient temperature (30-40 ºC) and influent wastewater flow rate and HRT were maintained 600 L/day and 4 hrs respectively. The DO concentrations were maintained less than 0.2 mg/L during the process and the sulfide loading rate steadily increased up to 4.38 kg HS-/m3-day. Under steady state condition, the elemental sulfur was successfully produced and which is heavier than the sludge and easily to settle inside the reactor. Based on the experiment results investigated from this research, the following conclusions are given as follow: 1. Under the oxygen limited conditions (DO less than 0.2 mg/L), the sulfide removal efficiencies were between 88% to 100% and elemental sulfur were produced 70 % to 90% of influent sulfur species. 2. At the low sulfide loading rate (0.48-1.25 kg HS -/m3-day), sulfate was the major end product and when sulfide loading rate higher than 1.25 kg HS-/m3-day, the elemental sulfur production gradually increased. 3. When sulfide loading rate was higher than 3 kg HS-/m3-day and DO concentration less than 0.15 mg/L (the optimum DO was around 0.1 mg/L), the sulfur was the major end product (80% to 90% sulfur conversion) and the sulfate conversions were reduced below 20% of influent sulfur species.

4. The biologically produced sulfur was accumulated inside the reactor during the steady state operation and which interfered the sulfur production and sulfide removal process. So this biosulfur inside the reactor should be removed regularly during operation. 5. Finally, the biological sulfide oxidation in the concentric draught tube internal loop airlift bioreactor showed the satisfactory results for sulfide removal and sulfur recovery. In addition, the sulfate reduction process followed by sulfide oxidation process is very suitable treatment for sulfate rich wastewater and which is a beneficial choice for environmental aspect as well as economic aspect.

List of Abbreviations °C dc dD dO DO eFRP g ∆G° h H hr kJ Kg L m mg min mm mol Q r s SOB SRB T TDS TSS V Zn Degree Celsius Column diameter or equivalent hydraulic diameter Draught-tube diameter Diameter of sparger hole Dissolved oxygen concentration Electron Fiber reinforced plastic Gram Gibb’s free energy Overall reactor height Henry law constant Hour Kilojole Kilogram Liter Meter Milligram Minute Millimeter Mole Volumetric flow rate Rate of reaction Second Sulfide oxidizing bacteria Sulfate reducing bacteria Temperature Total dissolved solid Total suspended solid Volume Zinc

References Annachhatre A.P.1; Suktrakoolvait S.1, Biological Sulfide Oxidation in a Fluidized Bed Reactor, Environmental Technology, Volume 22, Number 6, 1 June 2001 , pp. 661672(12) Publisher: Selper Ltd. Buisman, C.N.J. (1989). Biotechnological sulfide removal with oxygen. PhD thesis, Wageningen Agricultural University, Wageningen, The Netherlands. Buisman, C.N.J., Geraats, B.G., Ijspeert, P., and Lettinga, G. (1990). Optimization of sulfur production in biotechnological sulfide removing reactor. Biotech. Bioeng 35, 50-56. Buisman, C.J.N., Geraats, B.G., Ijspeert, P., and Lettinga, G., 1990a. Optimization of Sulfur Production in a Biotechnological Sulfide-Removing Reactor. Biotech. Bioeng, 35:50-56. Buisman C.J.N. *, S.H.J. Vellinga**, G.H.R. Janssen* and H. Dijkman*, (1999). Biological sulfide production for metal recovery, *PAQUES, B.V., P.O. Box 52, 8560 AB Balk, Netherlands, http://www.paques.nl/?pid=101 Chen, K.Y. & Morris, J.C. (1972). Kinetic of oxidation of aqueous sulfide by O2. Environ Sci Technol, 6, 529-537. Chisti, M.Y. (1989). Airlift Bioreactors, Elsevier Science Publishers Ltd., New York. Chisti, M.Y., and Moo-Young, M., (1987a), in Bioreactor and Biotransformations, Elsevier (London), pp. 335-349 Gonzalez. A, Alcantara. S, Razo-Flores. E, and Revah. S., 2005, Oxygen Transfer and Consumption in a Thiosulfate Oxidizing Bioreactor with Sulfur Production, The Society for applied Microbiology, letters in Applied Microbiology, 41, 141-146 Jensen, A.B., Webb, C. (1995). Treatment of H2S containing gases: a review ofmicrobiological alternatives. Enzyme Microbiol Technology. 17, 2-10. Janssen, A.J.H., Ma, S.C., Lens, P., and Lettinga, G. 1997. Performance of a SulfideOxidizing Expanded-Bed Reactor Supplied with Dissolved Oxygen. Biotech. Bioeng. 53: 32-40. Kuenen, J.G. (1975). Colorless Sulfur Bacteria and their Role in the Sulfur Cycle. Plant Soil, 43, 49-76. Lee, C. & Sublette, K.L. (1993). Microbial Treatment of Sulfide-Laden Water. Wat Res 27(5), 839-846. Standard Methods for the Examination of Water and Wastewater, 20th Edition, 1998. American Public Health Association, 1015 Fifteenth Street, NW, Washington, DC 20005-2605

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