Download Biological conversion of sulfide to elemental sulfur

Indian Journal of Engineering & Materials Sciences
Vol. 5, August 1998, pp.202-21 0
Biological conversion of sulfide to elemental sulfur
P F Henshaw, 1 K 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 waste
water and converting it to elemental sulfur has several advantages over conventional physical/chemical
treatment processes. A suspended-growth continuous-flow stirred-tank bioreactor utilizing the green
sulfur bacterium Chlorobium limicola was successfully operated at five different sulfide loading rates
and three different hydraulic retention times. In all but one of the five experiments, the rate of consumption of the sulfide was equal to its loading rate. The separation of elemental sulfur from the bioreactor contents is essential to realize its value as a chemical industry feedstock. Separations of elemental
sulfur by plain settling, settling at elevated pH, filtration and centrifuging were tested at bench scale using the contents of several batch bioreactors. Under plain settling, elemental sulfur and bacteria were
removed from suspension to the same degree. Raising the pH to 8.6 or 8.8 resulted in some of the sulfur
or bacteria settlingindependently of the sulfur-bacteria floes. Filtration was found to give conflicting
results with different batches of bacteria. Centrifugation resulted in the best separation between elemental sulfur and bacteria; 90% of the elemental sulfur and 29% of the bacteria could be removed from
suspension.
Hydrogen sulfide (H2S) gas is highly toxic and
malodorous '. In water, sulfide (S2-) has an oxygen
demand/ of 2 mole Oyrnole S2- and thus would
consume oxygen and have an adverse effect on
aquatic life if discharged into the environment.
"Sour" water is produced in petroleum refineries
wherever process water comes in contact with gas
streams containing hydrogen sulfide. In Canada,
refinery waste waters containing sulfide must meet
the Federal Refinery Effluent Regulations and
Guidelines. Currently the upper limit is 0.3 kg
S2-1l 000 m ' of oil refined/day for refineries that
commenced operations on or after Nov.l, 1973.
Refineries that were operating before that date are
subject to the guidelines of 0.6 kg S2-/l000 rrr' of
oil refined/day'. In most petroleum refineries, elemental sulfur is recovered from sour water by
steam stripping followed by one of many catalytic
sulfur recovery processes". Elemental sulfur, So, is
nontoxic and is used as a feedstock for the chemical, fertilizer and materials manufacturing industries'. Conventional chemical processes for sulfur
recovery from sour water and gas are expensive
because .of the need to replace poisoned catalysts,
contaminated reactor liquids and corroded reaction
vessels". The use of a biological process is potentially less expensive because it acts at low temperatures and pressures, generates its own catalyst
(bacteria), and can remove very low concentrations
of sulfide. The latter point is especially important
if one considers that new environmental regulations tend to be more stringent than their predecessors, which may result in the addition of successive treatment stages to conventional processes. A
biological process is being developed to remove
H2S from sour water streams. While dissolved in
water, H2S is less likely to be an environmental or
safety hazard than H2S gas.
Chlorobium limicola (A TCC 17092) is a naturally occurring green sulfur bacterium (GSB) capable of oxidizing S2- to So. The SO is produced
attached to the outside of the cells. This photosynthetic bacterium requires CO2, inorganic nutrients and sulfide for growth and is strictly anaerobic. Cork?" demonstrated the utility of C. limicola
in producing SO from H2S where the sulfide fed
into the photosynthetic reactor was in the gas
phase. The removal of S2- in the liquid phase by
photosynthetic
bioprocesses
has been demonstrated in some studies, but SOwas either not quan-
HENSHA W et al.: BIOLOGICAL
CONVERSION OF SULFIDE TO ELEMENTAL
tified or not produced+". These authors have measured the production of elemental sulfur from sulfide by GSB in batch9•lo and continuous-flow bioreactors II •
A schematic of a continuous-flow sulfide removal/sulfur recovery system employing a suspended-growth bioreactor is shown in Fig. 1. Sour
water flows into the bioreactor wherein the sulfide
is converted to elemental sulfur. It is desirable to
physically remove elemental sulfur from the bioreactor for three reasons. Firstly, SOhas a commercial value which can be realized only if it is separated and purified. Secondly, SO in the bioreactor
blocks the available light needed by these photosynthetic bacteria. Finally, SOis an alternate electron donor to S2- in GSB and leaving it in the bioreactor may result in further oxidation of the ele-mental sulfur to sulfate, decreasing the yield of SO
from the process. After the elemental sulfur is removed from the reactor effluent stream, most of
the bacteria can be separated from the waste water
for recirculation back into the reactor. The development of the S2- removal/SO recovery system has
proceeded along two tracks. First, the characteristics of a continuous-flow bioreactor employing
GSB to remove sulfide have been studied at a
bench scale. Second, this paper describes several
separation techniques that have been tested at a
bench scale to separate SO from the contents of a
batch biological reactor. The techniques evaluated
were: settling, settling at elevated pH, filtration,
and centrifugation.
Separation of the elemental sulfur from the
bacteria and waste water by gravity settling would
be the least expensive separation alternative.
Cork" reported that elemental sulfur settled from
bioreactor contents within 24 h. Kim and Chang"
WQstewater
8IIJIEACHII
BacteriQ
WQsteWQter.
H1Sc •••
Fig. I-Schematic
of proposed bioreactor system
SULFUR
203
successfully used a settling unit to remove some SO
from the recycle stream of a gas-fed bioreactor,
claiming it removed 80 to 90% of the elemental
sulfur. Jar tests on the GSB/sulfur mixture grown
in batch reactors revealed that raising the pH resulted in flocculation and increased settling of the
bacteria and SO(Henshaw P F, unpublished data).
The use of a centrifuge to produce a settling force
several times the force of gravity was suggested as
a means of removing elemental sulfur from suspension".
Microphotographs of green sulfur bacteria and
adjacent elemental sulfur have revealed that the
diameters of the SO granules are greater than the
short dimension of the ovoid GSB cells IS • Average
SOparticle and GSB sizes of 9.4 urn and 1.1 urn,
respectively, were reported in a culture of C. limicola", A membrane or fibre filter might utilize this
size difference to remove elemental sulfur from
suspension by sieving action.
Materials and Methods
Methods of analysis
Sulfide-- The methylene
blue
colorimetric
method was used 17.
Elemental sulfur-For settling tests of sulfur in
water, about 70 mL of 95% ethanol was added to
an aqueous sample of less than 20 mL. The mixture was refluxed for two hours and made up to
250 mL with 95% ethanol, centrifuged, and the
optical density measured at 264 nm against 95%
ethanol. In settling tests where sulfur and bacteria
were present, the colorimetric method was used
with 'the addition of mercuric chloride". In filtration and centrifugation experiments, aqueous sam£'
19
.
pIes were extracted into chloroform
pnor to
quantification by high-performance
liquid chromatography (HPLC)20.
Bacteria--Bacteriochlorophyll (bchl) was extracted into methanol and its absorbance was
measured". The concentration ofbchl was taken as
an indicator of the bacterial concentration. A 1.00
mL sample was transferred by disposable pipette
into 10.00 mL of ACS grade methanol in a centrifuge tube. The tube was capped, swirled by hand
for one minute, and centrifuged at 1,400 g's
(gravities) for 10 min. The absorbance of the supernatant was measured at 670 nm against an ACS
grade methanol blank.
204
INDIAN 1. ENG. MATER. SCt, AUGUST 1998
Growth of bacteria
two Philips IR 175 Watt R-PAR bulbs inside the
vessel directed into the windows of the first reactor. Both reactors were mounted in a New Brunswick Scientific Co. model FS-314 fermenter drive
assembly. This resulted in an average irradiance of
258 W/m2 over both windows. The bioreactor stirsalts solution. The content of a culture tube was ring speed was approximately 200 rpm. Masterflex
used to inoculate alL round-bottom flask which (Cole-Palmer Co. ,Chicago) variable speed pumps
was, in turn used to inoculate the content of a 15 L with solid state speed controls and standard pump
New Brunswick Scientific Co. model F-14 fer- heads were used for all influent and effluent reacmenter as previously described".
tor streams. The liquid level in the reactor was kept
constant by a level probe inside the 'reactor which
Continuous-flow stirred tank reactor
actuated the effluent pump. The pH inside the reIn the continuous-flow bioreactor experiments,
actor was kept between 6.8 and 7.2 by an Omega
the components of the growth medium" were split
(Omega Engineering Co., Stamford, CT) PHP-166
amongst several influent streams. In Runs 3, 4 and
Chemical Metering Pump controlled by an Omega
5 , there were three influent streams: deionized
PHM 55 pH Regulator connected to an Ingold (Inwater, a concentrated nutrient solution and a sulgold Electrodes Inc., Wilmington, MA) 465-35-K9
fide stock solution (Fig. 2). In Trials 9 and 10,
combined glass electrode inserted into the reactor.
there were only two feeds: a nutrient medium and a
Feed pumps were adjusted by timing the revosulfide stock solution. Apart from the subculturing
lutions until the proper speed (typically 2 to 10
tubes and 1 L batch reactor, the reactors and
rpm) was achieved. For Runs 3, 4 and 5, the conequipment were not sterile.
centration of the sulfide stock solution was inA New Brunswick Scientific Co. model F-14
creased in successive Runs while the HRT was
fermenter was used as a continuous reactor. The
held constant (Table 1). Throughout each Run, a
reactor was wrapped in a metallized Mylar film
constant sulfide loading rate was maintained reand two 105 mm high by 110 mm wide windows
gardless of the condition in the reactor. In Trials 9
were cut, one above the other, into the film so that
and 10, HRTs higher than those in Runs 3, 4, and 5
the top of the upper window was at the water level
were used and the sulfide loading was adjusted so
of the constant temperature (30°C) water bath. A
as to have a low but constant sulfide concentration
second reactor was used as a submerged light
in the reactor.
source by removing the stir paddles and clamping
In inorganic growth medium" was used for the
culture tubes and experimental reactors. The bicarbonate and sulfide solutions were combined and
filter-sterilized directly into the autoclaved culture
tubes or batch reactor containing sterile mineral
:8
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GAS [lJTLET
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(Io~ed Valve
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• Control line
Fig. 2--Schematic of continuous stirred-tank reactor as used
in runs 3, 4 and 5
Settling
A clear acrylic settling column was used, 90
mm in diameter and approximately 2200 mm tall
with four sampling ports, 600 mm apart starting at
a depth 130 to 300 mm below the liquid surface.
The settling test was performed on: elemental sulfur in water, bacteria in reactor contents and bacteria and elemental sulfur in reactor contents. For
testing the settling of bacteria alone, GSB were
grown in a 15 L batch reactor and the settling test
was performed several days after the sulfide had
been completely consumed by the bacteria. In tests
where bacteria and SO were required, sulfide was
added to the batch reactor one day prior to the column test, and the test was performed as close as
possible to the time when sulfide was completely
consumed, thus ensuring that elemental sulfur
would be at its highest concentration. During the
ItU"SItA \V ct al.: BIOLOGICAL
CONVERSION
Table I-Variables
Experiment
Run
Run
Run
Trial
Trial
OF SULFIDE TO ELEMENTAL
in continuous reactor operation
Vol.
Our.
[S=];
Load.
HRT
L
h
mg/L
rng/L h
h
13.7
13.7
13.7
12.0
12.0
316
383
283
1680
1220
90
190
260
550
260
2.1
4.4
5.6
3.2
2.7
45
44
46
173
99
3
4
5
9
10
205
SULFUR
.
pH
adj.
[S=]",
mg/L
b
1340
2760
4040
2730
2440
b
a
a
a
Vol.= reactor liquid volume, Our.= duration of experiment, [S];= effective inlet sulfide concentration, Load = average reactor
sulfide 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 approximately five minutes, then allowed to settle.
Samples were collected from all of the sampling
ports at various time intervals. The samples were
analyzed for elemental sulfur and/or bacteriochlorophyll. The data were analyzed according to the
method of Ramalho'".
Settling at Elevated pH
In this series of tests, a clear acrylic settling column 102 mm in diameter and 1830 mm tall was
employed. The sampling ports were 355 mm apart,
and the column was covered with aluminum foil to
prevent the creation of density currents caused by
heating of the column contents by light. GSB were
grown in a batch bioreactor until the sulfide had
been consumed by the elemental sulphur. One day
prior to the test, sulfide was added to the batch reactor so that the SOconcentration would be at its
peak. At this point, the contents of the batch reactor were pumped into the column and 700 to 1000
mL of I M NaOH was added while stirring to
bring the pH up to the desir~d level. In some cases,
additional deionized water needed to be added to
the column so that there would be a sufficient liquid level above the upper sampling port throughout
the test. The column was slowly stirred for two
minutes to encourage flocculation, after which a
series of samples was taken at all ports to determine the starting bacteria and elemental sulfur
concentration throughout the column. Additional
samples were taken 20-25 and 60-70 min after the
initial sample. The first 15 mL from each port was
drained and discarded prior to collecting each 40mL sample. After collection, the 40 mL samples
were stirred with a magnetic stirrer while aliquots
were withdrawn for the elemental sulfur (5 to 10
mL) and bacteriochlorophyll
(1 mL) assays. The
data were analyzed graphically".
Filtration
Sulfide was added to the bioreactor approximately one day prior to the test to allow the bacteria to produce elemental sulfur. Samples were
taken for elemental sulfur and bacteriochlorophyll
from a stirred 50 mL grab from the batch bioreactor. The sample was then filtered under vacuum
through a 4.7 mm diameter membrane filter of
pore size 0.45, 0.8, 1.2, 3.0, 5.0 or 8.0 urn. Preliminary tests included filtration through glass fibre filters with effective pore sizes of 1.2, 1.5 or
2.5 urn. Samples for elemental sulfur and bacteriochlorophyll analysis were taken from the filtrate.
The elemental sulfur samples were analyzed by
HPLC. The reduction in elemental sulfur and bacteriochlorophyll concentrations were then calculated. The experiment was repeated with other
samples of bioreactor contents that were: blended
in a commercial blender (Osterizer Galaxie- Ten),
adjusted to pH 8.8 with 1 M NaOH or diluted with
deionized water.
Centrifugation
Sulfide was added to the batch reactor approximately one day prior to the test to allow the bacteria to produce elemental sulfur for the test. Samples were taken for elemental sulfur and bacteriochlorophyll from a stirred 50 mL grab from the
batch bioreactor. The cap on the sample was then
sealed, and the sample centrifuged at 500 or 800
RPM (55.9 or 143.1 g's) in an International Centrifuge Centra-8 centrifuge with a #269 rotor. Centrifuge times were 3, 6, 9, 12, 16, 17 or 30 min and
later, confirmatory tests were done for 10 min.
206
INDIAN J. ENG. MATER. SCI., AUGUST 1998
These times were exclusive of a two minute acceleration period for the centrifuge to obtain the set
speed. The experiments were also conducted on
several samples blended in a commercial blender.
consumption equaled S2- loading up to and including a loading of 64 mg/Lh. Therefore, the reactor sulfide loading at which overloading occurs
depends upon the reactor configuration.
The rate of S2- consumption was not found to be
Results and Discussion
related to the sulfide or elemental sulfur concentration in the experiments in this study.
Continuous-flow stirred tank reactor
Fig. 3 shows that up to a sulfide loading of 5
mglLh, the rate of sulfide consumption was equal
to the rate of sulfide loading. In other words, all of
the sulfide input to the reactor was consumed. The
product of the consumption was SOin all cases except Run 3, where the under-loaded reactor produced SO/-. Run 5, where the sulfide loading was
above 6 mg/Lh, can be characterized as overloaded in terms of sulfide. One would have expected the consumption/loading curve to level off
when the maximum loading had been reached. In
fact, the reactor "failed" in Run 5 and S2- consumption decreased, thus allowing sulfide to accumulate in the bioreactor. Had the experiment
been allowed to continue, the sulfide concentration
would have reached the point of toxicity to the
bacteria. Previous researchers" found that sulfide
5
0
•.•.
V
0
Settling
When 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
column after 27.8 min (overflow rate = 0.6 mrnls),
whereas only 18% of the bacteria would be removed in this time. According to the curve for
bacterial settling, the maximum removal of bacteria, even with an infinite settling time, would be
about 40%. When settled together, the removal of
SOin the presence of GSB and the removal of GSB
in the presence of SO were intermediate between
the individual removal curves of SOand bacteria. In
addition, the removal curves for SO were similar
and coincided at an overflow rate of 0.66 mrnls
indicating that the two solids settled together.
n
Run 3
Run 4
Run 5
Triol 9
Trial 10
80
1
o
4
Elemenlal
Sulfur
alone
Bacteria
alone
Elemental
Sulfur
in the
presence
of bacteria
.•• Bacteria
in the presence
of elemental
sulfur
•
V
70
c
o
ii
E
•.
3
o
>
o
:l
C
.,
••
Q:
"0
:l
If)
50
E
o
u
40
2
-•
30
o
"0
20
'"
I
o
R2
=
0.961
LL~~~~~~~~~~~~~~~
2
o
Sulfide
Looding
(e.eluding
Run 5)
I
10
o
3
4
5
6
7
Role (mg/h·L)
Fig. 3-The effect of sulfide loading on sulfide consumption
~~~~~~~~~-L~~~~~~
0.0000
0.0005
0.0010
Overflow
0.0015
Role
0.0020
0.0025
(m/s)
Fig. 4--R.emoval of elemental sulfur and bacteria by plain
settling
HENSHA W et af.: BIOLOGICAL
CONVERSION
OF SULFIDE TO ELEMENTAL
SULFUR
207
Forty-four per cent of both the SOand GSB were
removed from a I m depth of column after 26 min.
The reported elemental sulfur removal values in
the combined experiment were lower than the actual SOremoval, since SOwas measured at only one
depth in the column. These results show that during simple settling, the association between bacteria and SO remains and therefore this technique
cannot be used to separate these two types of suspended solids.
is essentially equal to the value at neutral pH in the
presence of bacteria. For bacteria, this value is
roughly equal to that for bacteria settling alone at
neutral pH. From these experiments, it can be concluded that the association between the bacteria
and SOis weakened by raising the pH. At a pH of
8.8, elemental sulfur was released from the sulfurbacteria complexes to settle independently. At pH
8.6, bacteria were released to settle at their normally slow rate.
Separation of bacteria and elemental sulfur can
Settling at elevated pH
be achieved by this method. The best separation
The settling rates of bacteria and elemental sul- occurred at pH 8.6 and an overflow rate of 0.22
fur were increased by elevating the pH. At pH 8.8, mmls when 70% of the elemental sulfur was setthe removal rate of bacteria at an overflow rate of tled from a mixed suspension compared to 27% of
0.6 mmls was 50% (Fig. 5) as compared to 43% at the GSB. The viability of the bacteria after settling
neutral pH (Fig. 4). More importantly though, the at a pH of 8.6 was not tested.
rate of removal of SO was different from that of
GSB, being 75% at an overflow rate of 0.6 mm/s. Filtration
The difference in removal rates between elemental
The results of preliminary tests, where a clean
sulfur and bacteria is even more pronounced at pH separation of elemental sulfur from the bioreactor
8.6 (Fig. 5) but the overall settling rates are lower contents was achieved, were not reproducible in
than at pH 8.8. In fact, the removals of SO and the confirmatory tests. In preliminary tests, all of
bacteria at pH 8.6 at an overflow rate of 0.6 mm/s the SObut only 30% of the GSB were trapped on a
were 37 and 14% respectively. For So, this removal
5 urn pore size membrane filter (Table 2). Similarly, 100% of the elemental sulfur but only 50%
100r-~--~~----~~==~======~
of
the bacteria were removed by a glass fibre filter
pH = 8.8
V £Iemental Sulfur
that retained particles larger than 1.2 urn. Later
•• Bacteria
90
studies using membranes with pore sizes ranging
pH = 8.6
o £Iemental Sulfur
from 1.2 to 8.0 urn resulted in the removal of all
• Bacteria
80
suspended material at all pore sizes. The greater
removal in the latter study was attributed to longer
70
GSB chains as a consequence of the longer bacteria
retention time in the reactor. Thus, the batch-to..-, 60
~
batch
variation in bacterial characteristics
pre'-'
0
cludes filtration as a reliable method of separating
50
>
0
bacteria and elemental sulfur.
E
• 40
0::
Exploratory studies looked into the possibility
of diluting, blending or raising the pH of the reactor contents prior to separation by filtration. None
30
of these techniques resulted in differential separa20
tion of elemental sulfur from the reaction mixture
(Table 2).
10
Centrifugation
0
0.0000
0.0005
0.0010
Overflow Rote (m/s)
Fig. 5-Removal
at elevated pH
of elemental sulfur and bacteria by settling
In preliminary tests, the use of a centrifuge to
produce a strong separating force, was shown to
accomplish a good separation of 90: 10 (% SOremoved: % GSB removed) from the contents of a
batch bioreactor. Further tests (Fig. 6) achieved
208
INDIAN J. ENG. MATER. SCI., AUGUST 1998
Table 2---Removal of elemental sulfur and bacteria by filtration
~m
Treatment of
mixture
0.45
0.8
1.2·
regular
regular
regular
1.2
regular
1.5.
2.5·
3.0·
3.0
regular
regular
regular
regular
0
68
20
5.0
regular
II
51
56
45
Pore size
5.0
blended
8.0
regular
8.0
regular
8.0
pH=8.82
8.0
diluted
Reduction on filtration (%)
Preliminary Tests
Confirmatory Tests
Bacteria
Elemental sulfur
Bacteria
Elemental sulfur
105
99
0
33
28
35
109
117
68
90
91
92
77
39
63
100
100
100
101
99
102
100
100
100
101
105
93
98
98
92
95
96
95
95
94
91
97
100
100
100
100
100
100
100
100
100
100
100
100
100
60
115
79
125
99
99
100
• effective retention
similar results, in which the elemental sulfur removal approached 100% as the centrifuging time
exceeded 20 min. Conversely, even after prolonged centrifuging, the bacterial removal from
suspension was only 35-40%. There is more scatter
in the bacteria removal data than the SO removal
data. Presumably, unaccounted-for characteristics
of the bacteria batches which affected the removal
of bacteria were expressed. Good separation of
90:29 was achieved when the sample was spun at
143 gravities for 9 min. Centrifuging at lower
speeds (56 gravities) resulted in less removal
(65:26) of both elemental sulfur and bacteria from
suspension and the difference in removal percentages was less.
Extreme agitation of the sample in a household
blender to break the SO-bacteria association prior to
centrifuging gave similar results to those reported
for unblended elemental sulfur removal, but the
variability in bacteria removal after blending was
much greater than in the unblended case. These
results are not shown because the GSB removal
variability precluded assigning a statistically-based
curve to the bacterial data, as was done for the unblended samples.
Even with centrifuging, the low elemental sulfur
concentrations used in these experiments resulted
in low SOconcentrations in the pellet. To illustrate
this, consider that the average elemental sulfur
concentration in these experiments was 19 mg/L. If .
HENSHAW
et al.: BIOLOGICAL
CONVERSION
9'
80
:6.-
,, .t:..
P-
70
60
t?
~
D
>
0
50
"
,
,,
,
, •••
,, ...
.0
•
•
...
.•.
30
BOOrpm
Bchl. 800 rpm
6.- Sulfur. 500 rpm
••• Bchl. 500 rpm
800 rpm
._-- 500 rpm
•••
.
~.
.
E
0 Sulfur.
•••
,~
,,
0
•
'"
--
,
,,
,,
,,
•
•
•••
..
.-~-.---.- -.-.- ..
t
•••
•
20
.•.
10
•
•••
•••
0
0
10
Time.
Fig. ~Removal
gation
30
20
SULFUR
209
poor test performance of some of these methods of
separation may have been due to the inconsistent
nature and low concentration of the bacteria. The
tests described were all conducted on bacteria
grown in batch reactors. It is hoped in the future
that the most promising of these techniques can be
repeated on the effluent from a continuous reactor.
100
90
OF SULFIDE TO ELEMENTAL
.0
t (min)
Acknowledgment
This research was supported by funding from
Imperial Oil Canada and the Natural Sciences and
Engineering Research Council of Canada. The experiments on the separation of elemental sulfur
were conducted under the supervision of the
authors by Yvette Ly, David Diemer and Brian
Dantas for their senior year engineering projects in
the Department of Civil and Environmental Engineering at the University of Windsor.
References
1
Cadena F & Peters R W, )WPCF, 60 (1988) 1259.
2
Kobayashi H A, Stenstrom M & Mah R A, Water Res, 17
(1983) 579.
Losier L, Environmental status report of the canadian
petroleum refinery industry (Supply and Services Canada,
Ottawa), 1990
Sittig M, Petroleum Refining industry: energy saving and
environmental control (Noyes Data Corp., Park Ridge,
N.J.),1978
West J R & Duecker W W, in Reigel's handbook of industrial chemistry (7th ed.), edited by Kent J A (Van
Nostrand Reinhold Co., Toronto), 1974, 62
of elemental sulfur and bacteria by centrifu-
90% of the SO was removed from aIL
sample,
17 mg would end up in the pellet. The average bchl
concentration was 39 mg/L. If 29% of this was
removed from aIL sample, 11 mg bchl would be
in the pellet after centrifuging. Assuming 3% of
the VSS in the biomass is bchl!', the VSS in the
pellet would be 11 mg/0.03 = 370 mg. Therefore,
the percentage of elemental sulfur in the pellet
would be about 100(17/(17+370» = 4% on a dry
mass basis,
3
4
5
6
7
8
Conclusions
A continuous stirred tank bioreactor successfully removed dissolved sulfide at loadings of up
to 5 mg S2-/L h. At higher loadings the bioreactor
failed and sulfide accumulated within the reactor.
Four methods of separating elemental sulfur
from a suspension of SOand GSB were tested on a
bench scale: settling, settling at elevated pH, filtration and centrifuging. Of these, centrifuging was
the most consistent and gave the highest difference
in removal of SOand GSB from the bioreactor suspension. Even so, the SO content in the sulfurenriched fraction after separation was low, due to
the low SO concentration in the bioreactor. The
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