Indian Journal of Engineering & Materials SciencesVol. 5, August 1998, pp.202-21 0

Biological conversion of sulfide to elemental sulfur

P F Henshaw, 1K Bewtra & N Biswas

Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario,Canada N9B 3P4

Received 21 February 1997

The use of a biological reactor (bioreactor) as a means of removing hydrogen sulfide from wastewater and converting it to elemental sulfur has several advantages over conventional physical/chemicaltreatment processes. A suspended-growth continuous-flow stirred-tank bioreactor utilizing the greensulfur bacterium Chlorobium limicola was successfully operated at five different sulfide loading ratesand three different hydraulic retention times. In all but one of the five experiments, the rate of con-sumption of the sulfide was equal to its loading rate. The separation of elemental sulfur from the biore-actor contents is essential to realize its value as a chemical industry feedstock. Separations of elementalsulfur by plain settling, settling at elevated pH, filtration and centrifuging were tested at bench scale us-ing the contents of several batch bioreactors. Under plain settling, elemental sulfur and bacteria wereremoved from suspension to the same degree. Raising the pH to 8.6 or 8.8 resulted in some of the sulfuror bacteria settlingindependently of the sulfur-bacteria floes. Filtration was found to give conflictingresults with different batches of bacteria. Centrifugation resulted in the best separation between ele-mental sulfur and bacteria; 90% of the elemental sulfur and 29% of the bacteria could be removed fromsuspension.

Hydrogen sulfide (H2S) gas is highly toxic andmalodorous '. In water, sulfide (S2-) has an oxygendemand/ of 2 mole Oyrnole S2- and thus wouldconsume oxygen and have an adverse effect onaquatic life if discharged into the environment."Sour" water is produced in petroleum refinerieswherever process water comes in contact with gasstreams containing hydrogen sulfide. In Canada,refinery waste waters containing sulfide must meetthe Federal Refinery Effluent Regulations andGuidelines. Currently the upper limit is 0.3 kgS2-1l 000 m ' of oil refined/day for refineries thatcommenced operations on or after Nov.l, 1973.Refineries that were operating before that date aresubject to the guidelines of 0.6 kg S2-/l000 rrr' ofoil refined/day'. In most petroleum refineries, ele-mental sulfur is recovered from sour water bysteam stripping followed by one of many catalyticsulfur recovery processes". Elemental sulfur, So, isnontoxic and is used as a feedstock for the chemi-cal, fertilizer and materials manufacturing indus-tries'. Conventional chemical processes for sulfurrecovery from sour water and gas are expensivebecause .of the need to replace poisoned catalysts,contaminated reactor liquids and corroded reaction

vessels". The use of a biological process is poten-tially less expensive because it acts at low tem-peratures and pressures, generates its own catalyst(bacteria), and can remove very low concentrationsof sulfide. The latter point is especially importantif one considers that new environmental regula-tions tend to be more stringent than their predeces-sors, which may result in the addition of succes-sive treatment stages to conventional processes. Abiological process is being developed to removeH2S from sour water streams. While dissolved inwater, H2S is less likely to be an environmental orsafety hazard than H2S gas.

Chlorobium limicola (ATCC 17092) is a natu-rally occurring green sulfur bacterium (GSB) ca-pable of oxidizing S2- to So. The SO is producedattached to the outside of the cells. This photo-synthetic bacterium requires CO2, inorganic nutri-ents and sulfide for growth and is strictly anaero-bic. Cork?" demonstrated the utility of C. limicolain producing SO from H2S where the sulfide fedinto the photosynthetic reactor was in the gasphase. The removal of S2- in the liquid phase byphotosynthetic bioprocesses has been demon-strated in some studies, but SOwas either not quan-

HENSHA W et al.: BIOLOGICAL CONVERSION OF SULFIDE TO ELEMENTAL SULFUR 203

tified or not produced+". These authors have meas-ured the production of elemental sulfur from sul-fide by GSB in batch9

•lo and continuous-flow bio-

reactors II •

A schematic of a continuous-flow sulfide re-moval/sulfur recovery system employing a sus-pended-growth bioreactor is shown in Fig. 1. Sourwater flows into the bioreactor wherein the sulfideis converted to elemental sulfur. It is desirable tophysically remove elemental sulfur from the biore-actor for three reasons. Firstly, SOhas a commer-cial value which can be realized only if it is sepa-rated and purified. Secondly, SOin the bioreactorblocks the available light needed by these photo-synthetic bacteria. Finally, SOis an alternate elec-tron donor to S2- in GSB and leaving it in the bio-reactor may result in further oxidation of the ele--mental sulfur to sulfate, decreasing the yield of SOfrom the process. After the elemental sulfur is re-moved from the reactor effluent stream, most ofthe bacteria can be separated from the waste waterfor recirculation back into the reactor. The devel-opment of the S2- removal/SO recovery system hasproceeded along two tracks. First, the characteris-tics of a continuous-flow bioreactor employingGSB to remove sulfide have been studied at abench scale. Second, this paper describes severalseparation techniques that have been tested at abench scale to separate SOfrom the contents of abatch biological reactor. The techniques evaluatedwere: settling, settling at elevated pH, filtration,and centrifugation.

Separation of the elemental sulfur from thebacteria and waste water by gravity settling wouldbe the least expensive separation alternative.Cork" reported that elemental sulfur settled frombioreactor contents within 24 h. Kim and Chang"

WQstewater

8IIJIEACHIIBacteriQ

WQsteWQter.H1Sc •••

Fig. I-Schematic of proposed bioreactor system

successfully used a settling unit to remove some SOfrom the recycle stream of a gas-fed bioreactor,claiming it removed 80 to 90% of the elementalsulfur. Jar tests on the GSB/sulfur mixture grownin batch reactors revealed that raising the pH re-sulted in flocculation and increased settling of thebacteria and SO(Henshaw P F, unpublished data).The use of a centrifuge to produce a settling forceseveral times the force of gravity was suggested asa means of removing elemental sulfur from sus-pension".

Microphotographs of green sulfur bacteria andadjacent elemental sulfur have revealed that thediameters of the SOgranules are greater than theshort dimension of the ovoid GSB cells IS • AverageSOparticle and GSB sizes of 9.4 urn and 1.1 urn,respectively, were reported in a culture of C. limi-cola", A membrane or fibre filter might utilize thissize difference to remove elemental sulfur fromsuspension by sieving action.

Materials and MethodsMethods of analysis

Sulfide-- The methylene blue colorimetricmethod was used 17.

Elemental sulfur-For settling tests of sulfur inwater, about 70 mL of 95% ethanol was added toan aqueous sample of less than 20 mL. The mix-ture was refluxed for two hours and made up to250 mL with 95% ethanol, centrifuged, and theoptical density measured at 264 nm against 95%ethanol. In settling tests where sulfur and bacteriawere present, the colorimetric method was usedwith 'the addition of mercuric chloride". In filtra-tion and centrifugation experiments, aqueous sam-

£' 19 .pIes were extracted into chloroform pnor toquantification by high-performance liquid chro-matography (HPLC)20.

Bacteria--Bacteriochlorophyll (bchl) was ex-tracted into methanol and its absorbance wasmeasured". The concentration ofbchl was taken asan indicator of the bacterial concentration. A 1.00mL sample was transferred by disposable pipetteinto 10.00 mL of ACS grade methanol in a centri-fuge tube. The tube was capped, swirled by handfor one minute, and centrifuged at 1,400 g's(gravities) for 10 min. The absorbance of the su-pernatant was measured at 670 nm against an ACSgrade methanol blank.

204 INDIAN 1. ENG. MATER. SCt, AUGUST 1998

Growth of bacteriaIn inorganic growth medium" was used for the

culture tubes and experimental reactors. The bicar-bonate and sulfide solutions were combined andfilter-sterilized directly into the autoclaved culturetubes or batch reactor containing sterile mineralsalts solution. The content of a culture tube wasused to inoculate alL round-bottom flask whichwas, in turn used to inoculate the content of a 15 LNew Brunswick Scientific Co. model F-14 fer-menter as previously described".

Continuous-flow stirred tank reactorIn the continuous-flow bioreactor experiments,

the components of the growth medium" were splitamongst several influent streams. In Runs 3, 4 and5 there were three influent streams: deionized,water, a concentrated nutrient solution and a sul-fide stock solution (Fig. 2). In Trials 9 and 10,there were only two feeds: a nutrient medium and asulfide stock solution. Apart from the subculturingtubes and 1 L batch reactor, the reactors andequipment were not sterile.

A New Brunswick Scientific Co. model F-14fermenter was used as a continuous reactor. Thereactor was wrapped in a metallized Mylar filmand two 105 mm high by 110 mm wide windowswere cut, one above the other, into the film so thatthe top of the upper window was at the water levelof the constant temperature (30°C) water bath. Asecond reactor was used as a submerged lightsource by removing the stir paddles and clamping

'--r----------...., r--------------,'I -11 GAS [lJTLET 08: I ~T~Et~-~ I: ft~.

CO2 III '! III I I ••uel~, I ,•.., w

Ii: :r--·-·-·1~~IQ.~.1I II! _ .

r-;7.-==rt=~ill~I~I~'L E~r::U£NT i.o nI I .

.nEFFLl.O/TREACTOR

iN: =. Nor-nolly (Io~ed Valve.!...~:~~~'lonO:7tVO'vt- - - "' Gas (ondui t_._.- • Control line

Fig. 2--Schematic of continuous stirred-tank reactor as usedin runs 3, 4 and 5

two Philips IR 175 Watt R-PAR bulbs inside thevessel directed into the windows of the first reac-tor. Both reactors were mounted in a New Bruns-wick Scientific Co. model FS-314 fermenter driveassembly. This resulted in an average irradiance of258 W/m2 over both windows. The bioreactor stir-ring speed was approximately 200 rpm. Masterflex(Cole-Palmer Co. ,Chicago) variable speed pumpswith solid state speed controls and standard pumpheads were used for all influent and effluent reac-tor streams. The liquid level in the reactor was keptconstant by a level probe inside the 'reactor whichactuated the effluent pump. The pH inside the re-actor was kept between 6.8 and 7.2 by an Omega(Omega Engineering Co., Stamford, CT) PHP-166Chemical Metering Pump controlled by an OmegaPHM 55 pH Regulator connected to an Ingold (In-gold Electrodes Inc., Wilmington, MA) 465-35-K9combined glass electrode inserted into the reactor.

Feed pumps were adjusted by timing the revo-lutions until the proper speed (typically 2 to 10rpm) was achieved. For Runs 3, 4 and 5, the con-centration of the sulfide stock solution was in-creased in successive Runs while the HRT washeld constant (Table 1). Throughout each Run, aconstant sulfide loading rate was maintained re-gardless of the condition in the reactor. In Trials 9and 10, HRTs higher than those in Runs 3, 4, and 5were used and the sulfide loading was adjusted soas to have a low but constant sulfide concentrationin the reactor.

SettlingA clear acrylic settling column was used, 90

mm in diameter and approximately 2200 mm tallwith four sampling ports, 600 mm apart starting ata depth 130 to 300 mm below the liquid surface.The settling test was performed on: elemental sul-fur in water, bacteria in reactor contents and bacte-ria and elemental sulfur in reactor contents. Fortesting the settling of bacteria alone, GSB weregrown in a 15 L batch reactor and the settling testwas performed several days after the sulfide hadbeen completely consumed by the bacteria. In testswhere bacteria and SO were required, sulfide wasadded to the batch reactor one day prior to the col-umn test, and the test was performed as close aspossible to the time when sulfide was completelyconsumed, thus ensuring that elemental sulfurwould be at its highest concentration. During the

ItU"SItA \V ct al.: BIOLOGICAL CONVERSION OF SULFIDE TO ELEMENTAL SULFUR 205

Table I-Variables in continuous reactor operation

.Experiment Vol. Our. [S=]; Load. HRT pH [S=]",

L h mg/L rng/L h h adj. mg/L

Run 3 13.7 316 90 2.1 45 b 1340Run 4 13.7 383 190 4.4 44 b 2760Run 5 13.7 283 260 5.6 46 a 4040Trial 9 12.0 1680 550 3.2 173 a 2730Trial 10 12.0 1220 260 2.7 99 a 2440

Vol.= reactor liquid volume, Our.= duration of experiment, [S];= effective inlet sulfide concentration, Load = average reactorsulfide loading, pH adj.= acid (a) or base (b) used to adjust pH in reactor, [S=]sss=sulfide stock solution concentration.

tests, the content of the column was stirred for ap-proximately five minutes, then allowed to settle.Samples were collected from all of the samplingports at various time intervals. The samples wereanalyzed for elemental sulfur and/or bacteriochlo-rophyll. The data were analyzed according to themethod of Ramalho'".

Settling at Elevated pHIn this series of tests, a clear acrylic settling col-

umn 102 mm in diameter and 1830 mm tall wasemployed. The sampling ports were 355 mm apart,and the column was covered with aluminum foil toprevent the creation of density currents caused byheating of the column contents by light. GSB weregrown in a batch bioreactor until the sulfide hadbeen consumed by the elemental sulphur. One dayprior to the test, sulfide was added to the batch re-actor so that the SOconcentration would be at itspeak. At this point, the contents of the batch reac-tor were pumped into the column and 700 to 1000mL of I M NaOH was added while stirring tobring the pH up to the desir~d level. In some cases,additional deionized water needed to be added tothe column so that there would be a sufficient liq-uid level above the upper sampling port throughoutthe test. The column was slowly stirred for twominutes to encourage flocculation, after which aseries of samples was taken at all ports to deter-mine the starting bacteria and elemental sulfurconcentration throughout the column. Additionalsamples were taken 20-25 and 60-70 min after theinitial sample. The first 15 mL from each port wasdrained and discarded prior to collecting each 40-mL sample. After collection, the 40 mL sampleswere stirred with a magnetic stirrer while aliquotswere withdrawn for the elemental sulfur (5 to 10

mL) and bacteriochlorophyll (1 mL) assays. Thedata were analyzed graphically".

FiltrationSulfide was added to the bioreactor approxi-

mately one day prior to the test to allow the bacte-ria to produce elemental sulfur. Samples weretaken for elemental sulfur and bacteriochlorophyllfrom a stirred 50 mL grab from the batch bioreac-tor. The sample was then filtered under vacuumthrough a 4.7 mm diameter membrane filter ofpore size 0.45, 0.8, 1.2, 3.0, 5.0 or 8.0 urn. Pre-liminary tests included filtration through glass fi-bre filters with effective pore sizes of 1.2, 1.5 or2.5 urn. Samples for elemental sulfur and bacterio-chlorophyll analysis were taken from the filtrate.The elemental sulfur samples were analyzed byHPLC. The reduction in elemental sulfur and bac-teriochlorophyll concentrations were then calcu-lated. The experiment was repeated with othersamples of bioreactor contents that were: blendedin a commercial blender (Osterizer Galaxie- Ten),adjusted to pH 8.8 with 1 M NaOH or diluted withdeionized water.

CentrifugationSulfide was added to the batch reactor approxi-

mately one day prior to the test to allow the bacte-ria to produce elemental sulfur for the test. Sam-ples were taken for elemental sulfur and bacterio-chlorophyll from a stirred 50 mL grab from thebatch bioreactor. The cap on the sample was thensealed, and the sample centrifuged at 500 or 800RPM (55.9 or 143.1 g's) in an International Centri-fuge Centra-8 centrifuge with a #269 rotor. Centri-fuge times were 3, 6, 9, 12, 16, 17 or 30 min andlater, confirmatory tests were done for 10 min.

206 INDIAN J. ENG. MATER. SCI., AUGUST 1998

These times were exclusive of a two minute accel-eration period for the centrifuge to obtain the setspeed. The experiments were also conducted onseveral samples blended in a commercial blender.

Results and Discussion

Continuous-flow stirred tank reactorFig. 3 shows that up to a sulfide loading of 5

mglLh, the rate of sulfide consumption was equalto the rate of sulfide loading. In other words, all ofthe sulfide input to the reactor was consumed. Theproduct of the consumption was SOin all cases ex-cept Run 3, where the under-loaded reactor pro-duced SO/-. Run 5, where the sulfide loading wasabove 6 mg/Lh, can be characterized as over-loaded in terms of sulfide. One would have ex-pected the consumption/loading curve to level offwhen the maximum loading had been reached. Infact, the reactor "failed" in Run 5 and S2- con-sumption decreased, thus allowing sulfide to ac-cumulate in the bioreactor. Had the experimentbeen allowed to continue, the sulfide concentrationwould have reached the point of toxicity to thebacteria. Previous researchers" found that sulfide

5 0 Run 3

• Run 4V Run 5.•. Triol 90 Trial 10

n1

4

coiiE:l•.Cou.,"0

3

:lIf)

2

-o•"0

'"

I R2 = 0.961 (e.eluding Run 5) Io LL~~~~~~~~~~~~~~~

o 2 3 5 6 7

Sulfide Looding Role (mg/h·L) Overflow Role (m/s)

o ~~~~~~~~~-L~~~~~~0.0000 0.0005 0.0010 0.0015 0.0020 0.00254

consumption equaled S2- loading up to and in-cluding a loading of 64 mg/Lh. Therefore, the re-actor sulfide loading at which overloading occursdepends upon the reactor configuration.

The rate of S2- consumption was not found to berelated to the sulfide or elemental sulfur concen-tration in the experiments in this study.

SettlingWhen allowed to settle independently, GSB and

elemental sulfur settle at different rates (Fig. 4).According to the curves for independent settling,SOwould be completely removed from aIm col-umn after 27.8 min (overflow rate = 0.6 mrnls),whereas only 18% of the bacteria would be re-moved in this time. According to the curve forbacterial settling, the maximum removal of bacte-ria, even with an infinite settling time, would beabout 40%. When settled together, the removal ofSOin the presence of GSB and the removal of GSBin the presence of SOwere intermediate betweenthe individual removal curves of SOand bacteria. Inaddition, the removal curves for SO were similarand coincided at an overflow rate of 0.66 mrnlsindicating that the two solids settled together.

80

70

o Elemenlal Sulfur alone• Bacteria aloneV Elemental Sulfur in the

presence of bacteria.•• Bacteria in the presence

of elemental sulfur

o>oE••Q:

50

40

30

20

10

Fig. 4--R.emoval of elemental sulfur and bacteria by plainFig. 3-The effect of sulfide loading on sulfide consumption settling

HENSHA W et af.: BIOLOGICAL CONVERSION OF SULFIDE TO ELEMENTAL SULFUR 207

Forty-four per cent of both the SOand GSB wereremoved from a I m depth of column after 26 min.The reported elemental sulfur removal values inthe combined experiment were lower than the ac-tual SOremoval, since SOwas measured at only onedepth in the column. These results show that dur-ing simple settling, the association between bacte-ria and SO remains and therefore this techniquecannot be used to separate these two types of sus-pended solids.

Settling at elevated pHThe settling rates of bacteria and elemental sul-

fur were increased by elevating the pH. At pH 8.8,the removal rate of bacteria at an overflow rate of0.6 mmls was 50% (Fig. 5) as compared to 43% atneutral pH (Fig. 4). More importantly though, therate of removal of SO was different from that ofGSB, being 75% at an overflow rate of 0.6 mm/s.The difference in removal rates between elementalsulfur and bacteria is even more pronounced at pH8.6 (Fig. 5) but the overall settling rates are lowerthan at pH 8.8. In fact, the removals of SO andbacteria at pH 8.6 at an overflow rate of 0.6 mm/swere 37 and 14% respectively. For So, this removal

100r-~--~~----~~==~======~pH = 8.8

V £Iemental Sulfur•• Bacteria

pH = 8.6o £Iemental Sulfur• Bacteria

90

80

70

..-, 60~'-'0 50>0E•0:: 40

30

20

10

00.0000 0.0005

Overflow Rote (m/s)

Fig. 5-Removal of elemental sulfur and bacteria by settlingat elevated pH

0.0010

is essentially equal to the value at neutral pH in thepresence of bacteria. For bacteria, this value isroughly equal to that for bacteria settling alone atneutral pH. From these experiments, it can be con-cluded that the association between the bacteriaand SOis weakened by raising the pH. At a pH of8.8, elemental sulfur was released from the sulfur-bacteria complexes to settle independently. At pH8.6, bacteria were released to settle at their nor-mally slow rate.

Separation of bacteria and elemental sulfur canbe achieved by this method. The best separationoccurred at pH 8.6 and an overflow rate of 0.22mmls when 70% of the elemental sulfur was set-tled from a mixed suspension compared to 27% ofthe GSB. The viability of the bacteria after settlingat a pH of 8.6 was not tested.

FiltrationThe results of preliminary tests, where a clean

separation of elemental sulfur from the bioreactorcontents was achieved, were not reproducible inthe confirmatory tests. In preliminary tests, all ofthe SObut only 30% of the GSB were trapped on a5 urn pore size membrane filter (Table 2). Simi-larly, 100% of the elemental sulfur but only 50%of the bacteria were removed by a glass fibre filterthat retained particles larger than 1.2 urn. Laterstudies using membranes with pore sizes rangingfrom 1.2 to 8.0 urn resulted in the removal of allsuspended material at all pore sizes. The greaterremoval in the latter study was attributed to longerGSB chains as a consequence of the longer bacte-ria retention time in the reactor. Thus, the batch-to-batch variation in bacterial characteristics pre-cludes filtration as a reliable method of separatingbacteria and elemental sulfur.

Exploratory studies looked into the possibilityof diluting, blending or raising the pH of the reac-tor contents prior to separation by filtration. Noneof these techniques resulted in differential separa-tion of elemental sulfur from the reaction mixture(Table 2).

CentrifugationIn preliminary tests, the use of a centrifuge to

produce a strong separating force, was shown toaccomplish a good separation of 90: 10 (% SOre-moved: % GSB removed) from the contents of abatch bioreactor. Further tests (Fig. 6) achieved

208 INDIAN J. ENG. MATER. SCI., AUGUST 1998

Table 2---Removal of elemental sulfur and bacteria by filtration

Pore size~m

Treatment ofmixture Reduction on filtration (%)

Preliminary TestsBacteria Elemental sulfur

Confirmatory TestsBacteria Elemental sulfur

0.45 regular 105 1090.8 regular 99 1171.2· regular 0 68

33 9028 9135 92

1.2 regular 773963

1.5. regular 0 602.5· regular 68 1153.0· regular 20 793.0 regular 101

99102

5.0 regular II 12551 9956 9945 100

10110593

5.0 blended 9898

8.0 regular 9295

8.0 regular 9695

8.0 pH=8.82 9594

8.0 diluted 9197

• effective retention

100100100

100100100

100100100100100100100100100100100100100

similar results, in which the elemental sulfur re-moval approached 100% as the centrifuging timeexceeded 20 min. Conversely, even after pro-longed centrifuging, the bacterial removal fromsuspension was only 35-40%. There is more scatterin the bacteria removal data than the SOremovaldata. Presumably, unaccounted-for characteristicsof the bacteria batches which affected the removalof bacteria were expressed. Good separation of90:29 was achieved when the sample was spun at143 gravities for 9 min. Centrifuging at lowerspeeds (56 gravities) resulted in less removal(65:26) of both elemental sulfur and bacteria fromsuspension and the difference in removal percent-ages was less.

Extreme agitation of the sample in a householdblender to break the SO-bacteria association prior tocentrifuging gave similar results to those reportedfor unblended elemental sulfur removal, but thevariability in bacteria removal after blending wasmuch greater than in the unblended case. Theseresults are not shown because the GSB removalvariability precluded assigning a statistically-basedcurve to the bacterial data, as was done for the un-blended samples.

Even with centrifuging, the low elemental sulfurconcentrations used in these experiments resultedin low SOconcentrations in the pellet. To illustratethis, consider that the average elemental sulfurconcentration in these experiments was 19 mg/L. If .

HENSHAW et al.: BIOLOGICAL CONVERSION OF SULFIDE TO ELEMENTAL SULFUR 209

100

,--90 ,,,,,

0 Sulfur. BOOrpm,80 9' " • Bchl. 800 rpm

:6.- 6.- Sulfur. 500 rpm, ••• Bchl. 500 rpm

70, .t:.. - 800 rpmP- ._-- 500 rpm

,,60 , •••,

t? , ••• ••• •~ ,0 , ...

D 50 ~...

> •0

E , •• ,'"

,.0 . •••~. ~-.---.- ..-.-.- ... .-30 .•. t

••••20

• •••10 .•. •••

00 10 20 30 .0

Time. t (min)

Fig. ~Removal of elemental sulfur and bacteria by centrifu-gation

90% of the SOwas removed from aIL sample,17 mg would end up in the pellet. The average bchlconcentration was 39 mg/L. If 29% of this wasremoved from aIL sample, 11 mg bchl would bein the pellet after centrifuging. Assuming 3% ofthe VSS in the biomass is bchl!', the VSS in thepellet would be 11 mg/0.03 = 370 mg. Therefore,the percentage of elemental sulfur in the pelletwould be about 100(17/(17+370» = 4% on a drymass basis,

ConclusionsA continuous stirred tank bioreactor success-

fully removed dissolved sulfide at loadings of upto 5 mg S2-/L h. At higher loadings the bioreactorfailed and sulfide accumulated within the reactor.

Four methods of separating elemental sulfurfrom a suspension of SOand GSB were tested on abench scale: settling, settling at elevated pH, fil-tration and centrifuging. Of these, centrifuging wasthe most consistent and gave the highest differencein removal of SOand GSB from the bioreactor sus-pension. Even so, the SO content in the sulfur-enriched fraction after separation was low, due tothe low SO concentration in the bioreactor. The

poor test performance of some of these methods ofseparation may have been due to the inconsistentnature and low concentration of the bacteria. Thetests described were all conducted on bacteriagrown in batch reactors. It is hoped in the futurethat the most promising of these techniques can berepeated on the effluent from a continuous reactor.

AcknowledgmentThis research was supported by funding from

Imperial Oil Canada and the Natural Sciences andEngineering Research Council of Canada. The ex-periments on the separation of elemental sulfurwere conducted under the supervision of theauthors by Yvette Ly, David Diemer and BrianDantas for their senior year engineering projects inthe Department of Civil and Environmental Engi-neering at the University of Windsor.

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