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SULFUR FORMATION AND RECOVERY IN A THIOSULFATE-
OXIDIZING BIOREACTOR
A. González-Sánchez a; R. Meulepas ab; S. Revah c
a
Departamento de Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana,
Iztapalapa, México D.F., Mexico b Departamento de Procesos y Tecnología, Universidad Autónoma
Metropolitana-Cuajimalpa, c/o Department of Process Engineering, UAM- Iztapalapa, 09340 México
D.F., Mexico c Sub-department of Environmental Technology, Wageningen University, 6703 HD
Wageningen, The Netherlands
To cite this Article González-Sánchez, A., Meulepas, R. and Revah, S.(2008) 'SULFUR FORMATION AND RECOVERY IN
A THIOSULFATE-OXIDIZING BIOREACTOR', Environmental Technology, 29: 8, 847 — 853
To link to this Article: DOI: 10.1080/09593330801987657
URL: http://dx.doi.org/10.1080/09593330801987657
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2. Environmental Technology, Vol. 29. pp 847-853
© Taylor & Francis, 2008
SULFUR FORMATION AND RECOVERY IN A THIOSULFATE-
OXIDIZING BIOREACTOR
A. GONZÁLEZ-SÁNCHEZ1, R. MEULEPAS1,3 AND S. REVAH2*
1
Departamento de Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana, Iztapalapa, México D.F., Mexico
2
Sub-department of Environmental Technology, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands
3
Departamento de Procesos y Tecnología, Universidad Autónoma Metropolitana-Cuajimalpa, c/o Department of Process
Engineering, UAM- Iztapalapa, Av. San Rafael Atlixco No. 186, Col. Vicentina, 09340 México D.F., Mexico
Taylor and Francis
(Received 18 June 2007; Accepted 3 December 2007 )
10.1080/09593330801987657
ABSTRACT
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This work describes the design and performance of a thiosulfate-oxidizing bioreactor that allowed high elemental sulfur
production and recovery efficiency. The reactor system, referred to as a Supernatant-Recycling Settler Bioreactor (SRSB),
consisted of a cylindrical upflow reactor and a separate aeration vessel. The reactor was equipped with an internal settler
and packing material (structured corrugated PVC sheets) to facilitate both cell retention and the settling of the formed
elemental sulfur. The supernatant from the reactor was continuously recirculated through the aerator. An inlet thiosulfate
concentration of 100 mmol l−1 was used. The reactor system was fed with 89 mmol l −1 d−1 thiosulfate reaching 98 to 100%
thiosulfate conversion with an elemental sulfur yield of 77%. Ninety-three percent of the produced sulfur was harvested
from the bottom of the reactor as sulfur sludge. The dry sulfur sludge contained 87% elemental sulfur. The inclusion of an
internal settler and packing material in the reactor system resulted in an effective retention of sulfur and biomass inside
the bioreactor, preventing the oxidation of thiosulfate and elemental sulfur to sulfate in the aerator and, therefore,
improving the efficiency of elemental sulfur formation and recovery.
Keywords: Thiobacilli, thiosulfate, thiosulfate-oxidizing reactor, sulfur production, wastewater
INTRODUCTION Elemental sulfur production is preferred because smaller
amounts of chemicals are needed to neutralize the formed
The anthropogenic emission of sulfur compounds SO42− , lower energy consumption is required as the partial
produces environmental and health problems. Petroleum oxidation to S° needs four-fold less oxygen, and the S° can be
refining, flue gas desulfurization and photographic film recovered by sedimentation thus sequestering the sulfur from
processing, among others, generate wastewater containing further transformation in the water. Oxygen concentration
undesirable amounts of thiosulfate. In petroleum refining and the feed molar oxygen/sulfur (O2/S2−) ratio have been
the presence of thipsulfate is due to the aqueous partial air shown to be the key parameters to control the extent of oxida-
oxidation of sulfides originally present from waste gas tion [6, 8, 9]. Different reactor configuration systems have
scrubbing. In the above-mentioned application, thiosulfate been implemented to remove reduced sulfur compounds
concentration varies between 0.001 and 50 g l−1 [1]. Thiosul- from liquid waste streams by biological oxidation including
fate, (S2O32−), has a negative impact on the resident biotic stirred tank reactors [10, 11], biotrickling filters [12], air lift [8]
community in the receiving streams of wastewater by both and reverse fluidized loop reactors [13].
increasing the biochemical oxygen demand (BOD) and In most reactor configurations used, the oxidation of
reducing the pH through the sulfuric acid produced from its sulfur compounds to S° is controlled by direct air injection to
oxidation [2]. the aqueous phase containing the sulfur-oxidizing bacteria.
Chemical treatments have been used for the removal of Direct aeration prevents the adequate control of the oxygen/
S2O32− from wastewater streams [1] but microbiological sulfur ratio, thus reducing the S° yield . Separating aeration
processes are often considered to be low-cost alternatives [3]. from biological oxidation improves the S° yields by strict
Typical microorganisms capable of using S2O32− as an energy control of the oxygen supply [14, 15].
source are the chemolithoautrophic sulfur-oxidizing bacteria Complete oxidation of 9 mmol l−1 of S2O32− was
and in particular the thiobacilli [4–7]. The two main biological reported [2] in a continuous-flow reactor containing both
oxidation reactions are the partial conversion to elemental heterotrophic and autotrophic bacteria. The S° formation and
sulfur (S°) and the complete oxidation to sulfate (SO42−). recovery from synthetic sour water in a bioreactor-settler
847

3. system treating S2O32− had both a flow and a static zone, The feed dilution rate (D), recirculation dilution rate
which allowed sulfur sedimentation, and an external aeration (DR), thiosulfate loading rate (QS O 2− ), % S2O32− removal
2 3
vessel [16]. Steady state S2O32−-oxidizing cultures were estab- efficiency (%RE) and sulfur compound yields are defined by
lished at different O2/S2− molar ratios. At an O2/S2− ratio 1.0, Equations (i) to (vi):
SO42− was the main product while under O2/S2− molar ratios
around 1.0, S° formation was observed. The maximum sulfur F
D= (i)
formation (60%) was obtained at a molar ratio of 0.98. Most of Vr + Va
the SO42− formed was in the aeration vessel and simultaneous R
oxygen-sulfide limitation was observed in the settler, where DR = (ii)
Va
the maximum specific growth rate, µmax (8.6 d−1) decreased
90% under S°-producing conditions [17]. Cell retention inside QS O − = D[S2O2 − ]in (iii)
2 2
the settler is required to achieve high S° yield, and to avoid
cell washout.
%RE = 100 * 
[
 S O 2− − S O 2−
2 3
in
2 3 ] [ ]out


The aim of this study was to improve S° formation and (iv)
its recovery in a Supernatant-Recycling Settler Bioreactor



S2O32−
[in
] 


(SRSB) system with enhanced biomass retention.
Yield SO 4 − =
2
[SO ] 2−
4
{ }
(v)
− [S O ]
MATERIALS AND METHODS
2 [S2 O 3 − ]in
2
2
2−
3
out
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Sulfur-oxidizing Consortium and Medium Composition
Yield S 0 =
[S ]
0
(vi)
The inoculum was extracted from the S2O32−-oxidizing [S2O3 − ]in
2
− [S2O3 − ]out
2
bioreactor as described by Velasco et al. [16]. The sulfur-
oxidizing chemoautotroph consortium was composed mainly Note: SO42− yield ranges from 0.5, when thiosulfate is trans-
of thiobacilli [15]. The mineral medium used [4] contained formed in equimolar proportion to sulfur and sulfate, to 1.0
(g l−1): NaHPO4.7H2O (1.2), KH2PO4 (1.8), MgCl2.6H2O (0.4), when thiosulfate is fully oxidized to 2 mol sulfate; S0 yield is
NH4Cl (0.5), CaCl22H2O (0.03), MnCl2.4H2O (0.02), FeCl3. between 0 and 1.0. Temperature was maintained at 30°C and
6H2O (0.02), Na2HCO3 (2.0) and Na2S2O3.5H2O (24.8). pH at 5.0–5.5 by automatic addition of sodium hydroxide
(2 mmol l−1). Inoculation was made with 3 l of active biomass
Supernatant-Recycling Settler Bioreactor (SRSB) suspension.
The conical-shaped bottom of the reactor included
A lab-scale SRSB was used (Figure 1). It consisted of a 0.86 l of static zone where the settled sludge, containing
cylindrical glass reactor with a diameter of 100 mm and mainly sulfur and cells, accumulated and was intermittently
volume (Vr) of 3.4 l, and a glass aeration vessel with a recovered manually through a valve.
diameter of 66 mm and volume (Va) of 0.67 l. In all experi-
ments, the reactor was amended with structured PVC pack- Analyses
ing Plasdek 12060 (Munters Corporation, Fort Myers, FL,
USA) to improve the biomass and solids retention. The Daily samples were taken from the influent and effluent.
packing consisted of stacked, corrugated sheets of triangu- The settled sludge was removed intermittently for analyses.
lar wave-like flutes, 0.028 m wide, and had a specific area of The concentrations of SO42− and S2O32− were analyzed
223 m2 m−3 and a void fraction of 0.95. The total area by capillary electrophoresis (Waters CIA Analyzer, Milford,
supplied by the packing was 0.51 m2. MA, USA). For biomass and S° analyses, independent
After 19 days, the reactor was equipped with a conical
Figure 1. Supernatant-Recycling Settler Bioreactor (SRSB).
samples of the effluent, the settled sludge and the reactor
top settler, which had a maximum diameter of 200 mm and suspension were centrifuged and dried at 105°C for 24 h.
the settled liquid was discharged via an overflow into a The dry weight, carbon content and S° contents were deter-
channel. mined. When the experiment was terminated, the packing
Different feeding flow rates (F) were applied to the SRSB, material was removed and washed, and the amount of
with a inlet S2O32−concentration [S2O32−]in of 100 mmol l−1 biomass on the packing and in suspension were similarly
together with mineral medium being recirculated between the determined.
aerator and the reactor. The molar oxygen/S2O32− consump- Elemental analyses were carried out with a 2400 Series
tion ratio was controlled by adjusting the recirculation flow II CHNS/O Analyzer (Perkin Elmer, Boston, USA) using
rate (R) to obtain a recirculation dilution rate, DR, of 0.9 to cystein (C3H7NO2S) as reference. Biomass was estimated
3.5 min−1, and by the control of the dissolved oxygen concen- from the carbon in the solids, assuming cells contain 50%
tration in the aerator by varying the air bubbling rate. The carbon.
effluent was continuously drained from an overflow tube in The dissolved oxygen concentration was measured
the aerator. in the aerator, [O2]a, and in the top settler, [O2]r, with a
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4. Downloaded By: [Wageningen UR] At: 11:52 18 February 2010
Figure 1. Supernatant-Recycling Settler Bioreactor (SRSB).
polarographic probe (Applisens, model Z01005902020; respiration rate (during stage ii) multiplied by the corre-
Applikon, Schiedam, the Netherlands). The oxygen uptake sponding experimental suspended biomass concentration.
rate (OUR) in the reactor (OURr) was evaluated by mass
balance: RESULTS
(
OURr = DR [O2 ]a − [O2 ]r ) (vii) Set-up and Performance of the SRSB
The OUR in the aerator (OURa) was estimated from the Figure 2 shows the global results from the operation of
respiration rate (qO2X) under the conditions in the aerator the SRSB. For the first 18 days, the reactor operated without
([O2]a > 3 mg l−1), as reported previously [17]. the top settler (stage i), from day 19 to 52 the top settler was
For some data points OURa and OURr were calculated adapted to the top of the reactor (stage ii). Throughout the
from the theoretical oxygen consumption in the SRSB, experiment, the S2O32− loading rate was increased stepwise
considering that 0.5 and 2.0 oxygen moles were needed to and no significant [S°]out was detected. At day 52, the packing
oxidize S2O32− to S0 and SO42−, respectively [6]. Additionally, material was removed and the attached and suspended
OURa was calculated taking into account an average specific biomass was determined.
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5. Downloaded By: [Wageningen UR] At: 11:52 18 February 2010
Figure 2. Operating conditions and performance of the SRSB during stages i and ii.
Stage (i). Operation without top settler
Figure 2. Operating conditions and performance of the SRSB during stages i and ii.
0.89 and NaOH addition decreased concurrently. A global
Figure 2 shows that after inoculation the SRSB immedi- sulfur balance for the duration of the experiment showed
ately reached removal efficiency close to 100% with complete that 93% of the converted S2O32− was found as biomass, SO42−
transformation to SO42− at day 6. After day 9, when a technical and S°.
problem occurred in the aeration vessel, the SRSB was oper- Figure 3. Evolution of oxygen uptake rate (OUR), biomass and yield in stage ii. Void markers (❍, ∆) were calculated as reported in Materials and Methods.
ated with a lower loading rate to allow the recovery of the Influence of O2 Consumption on Elemental Sulfur Production
activity. The DR was also decreased to reduce the oxygen feed
to the reactor; however, at day 14 the S2O32− was again The biomass concentration in the effluent, which
converted mainly to SO42−, as shown in Figure 2 was the same as in the well-mixed aerator, is presented in
Figure 3A. The reduction in biomass concentration at days
Stage (ii). Operation with top settler 31 and 39 resulted from the washout of the suspended
From day 19 the reactor was amended with the top biomass after increasing the dilution rate at days 29 and 38.
settler. During this stage the loading rate was increased step- Subsequently, higher S2O32− loading rates resulted in
wise and, consequently, the R to supply sufficient oxygen increased biomass production. As seen also in Figure 3A, a
was increased. Figure 2A shows that at D > 0.9 d−1 the RE similar tendency was followed by OURa, confirming in
was below 100%. Figure 3A shows that between day 19 and this case the proportional relation between qO2X and the
29 S2O32− was almost completely converted to SO42−. After an suspended biomass in the aerator. The reduction of
increase in the loading rate at day 29, more S° was produced suspended biomass at days 29 and 38 resulted in less O2
and recovered to attain a yield of 0.75. After day 47, the S° consumption in the aerator and, consequently, more oxygen
production and recovery increased further up to a yield of availability in the reactor, so favoring higher S° production up
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Figure 3. Evolution of oxygen uptake rate (OUR), biomass and yield in stage ii. Void markers (❍, ∆) were calculated as
reported in Materials and Methods.
to a yield of 0.7 (Figure 3B). Days later, the OURa increased At day 52 the packing material was removed, and next
provoking a decline in the S° yield (down to 0.4). day there was still complete S2O32− oxidation by the
In all measurements, [O2]r was very close to 0. From days suspended biomass, but with increased SO42− production
19 to 30, the main portion of the transferred O2 was consumed because the suspended S° sludge and biomass were not
in the aerator. During the same period, S° production was very retained in the reactor.
low (Figure 3A). At day 22, the [O2]a was 2.3 mg l−1 which
promoted complete oxidation of the reduced sulfur [16] and DISCUSSION
restricted the O2 supply to the reactor to 0.40 mol O2/mol
S2O32−. To increase the O2 feed, the R was increased at day 22 Continuous reacting systems with recirculation
and 25 (Figure 2), however, this did not reduce the OURa perform as stirred tank reactors (STR) when the recirculation
because of the high suspended biomass. flow rate is considerably higher than the feeding flow rate.
At day 52, the biomass in the reactor with the top settler For a similar reactor system, a STR performance with a R/F
was 2380 mg and 29% of that biomass was attached to the ratio greater than 10 was reported [18]. The SRSB was oper-
packing (690 mg) while the rest was suspended as small flocs ated at a minimum R/F ratio of 250 so complete mixing of the
with a concentration of 320 mg l−1. Because of settling, the aqueous phase can be assumed. Nevertheless, the design of
biomass concentration in the aerator was only 87 mg l−1. the reactor allowed biomass-sulfur floc formation and settling
Therefore, most of the biomass was retained in the reactor by due to low upflow linear velocities and the reduced drag
biofilm formation and sedimentation. provided by the structured packing.
851

7. When the top settler was absent or the packing material A similar beneficial effect was found when increased
was removed, there was an inefficient retention of suspended O2 supply to the reactor resulted in the production of S°
biomass in the reactor, and consequently more active biomass sludge with good settling properties, thus reducing the
was recirculated and in contact with the high oxygen concen- concentration of thiosulfate in the recirculation liquid.
tration in the aerator, thus promoting an elevated SO42− yield. Therefore, OURa remained low even when the suspended
This situation was found in the first few days of the experi- biomass concentration recovered, as seen beyond day 47
ment and from days 19 to 30 where OURa reduced the O2 when the S° yield attained a maximum around 0.89. The
concentration in the aerator to 2.3 mg l−1 which was only 42% settled S° recovery efficiency at days 22 and 25 was 86% and
of the maximum solubility (5.45 mg l−1 at 2300 mg and 30°C). 76%, respectively. Recovery improved after day 25 and was
Sulfur aggregates formed by bacteria were damaged by the in the 93–99% range due to the higher S° production which
continuous aeration in an air lift reactor, promoting complete promoted better flocculation.
oxidation to SO42−, even under oxygen-limiting conditions Nevertheless, the requirement for a short retention
(below 0.1 mg l−1) [8]. time in the aerator is not an issue when the S° and the biom-
During stage ii, the low O2/S2O32− molar ratio (by the ass are efficiently retained, which is consistent with the
increase in the S2O32− loading rates) and the inclusion of the top results of Janssen et al. [14]. In any case, the performance
settler, promoted S° formation and sedimentation, respec- and volumetric oxidative capacity can be improved by cell
tively, in the reactor up to a yield of 0.89 after day 47 with the retention in the packing. Then, S° production and recovery
concurrent decrease in NaOH. The top settler reduced the depends not only on cell retention but also on biomass aggre-
upflow linear velocity to 3.1 m h−1 which was sufficient to foster gation. The high sulfide-elimination capacity of retaining
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settling, considering that the average sedimentation velocity biofilm, which formed biomass on buoyant carrier particles,
for the biologically produced sulfur, with particle size between was attributed to the retention of the active biofilm without
100 and 300 µm, was 8.9 m h−1, as previously reported [16]. fouling [13].
The sulfur mass balance showed that 7% of the trans-
formed S2O32− was not accounted for. Some of this sulfur was CONCLUSIONS
observed deposited on the wall and the packing. It is also
possible that a fraction was present in the effluent as sulfite The successful operation of the SRSB was achieved by
(SO32−), as a result of S2O32− disproportionation which occurs controlling the O2 supply and by removing from the system
at around neutral pH [19]. most of the formed S° in order to reduce further oxidation.
Low liquid retention time in the aerator is desirable to High thiosulfate conversion and S° yields were favored by
decrease SO42− production by the suspended biomass from separating O2 supply from the biological oxidation. The oper-
both soluble S2O32− and suspended S°. This condition can be ating conditions in the reactor allowed sufficient mixing for
reached by using a small aerator size. On the other hand, the soluble compounds (thiosulfate and oxygen) while both
sufficient O2 supply to the reactor requires efficient mass the packing and the top settler segregated the insoluble frac-
transfer in the aerator. The ratio volume between aerator/ tion (S° and biomass), thus reducing the sulfate production in
reactor in the SRSB (0.09) was smaller than that applied by the aerator. Higher volumetric rates were achieved by effi-
Janssen et al. [14] (0.21). cient biomass retention. This is a critical aspect because
During stage ii, the temporary decrease in the autotrophic bacteria normally have low growth rates and
suspended biomass concentration in the aerator (days 28 to yields and merits further research.
35), after increasing the D, can explain the improved S° yield
by reducing the SO42− production which is favored by higher ACKNOWLEDGEMENTS
O2 concentrations. Consequently, as seen by the OUR profiles
in Figure 3A, the OURr was greater than the OURa during This study was supported by the National Council of
this period. Cell retention in the packing allowed the dilution Science and Technology of Mexico (Basic Science Project no.
rate to increase up to 1.06 d−1 which is higher than the value 26024-2006), CONACYT. We thank Sergio Hernández,
of 0.86 d−1 which has been found under oxygen-limiting Carmen Fajardo and Isidoro Antunez for their technical assis-
conditions [19]. tance, and Antonio Velasco for his useful opinions.
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