This document describes a study that used a photosynthetic bioreactor with the green bacteria Prosthecochloris aestuarii to remove hydrogen sulfide from a pond. A STELLA model was created based on past studies to simulate the bioreactor. The model showed about 80% reduction of hydrogen sulfide concentration, from 0.02903 mmol/L initially to 0.006 mmol/L in the effluent, by using a light intensity of 25 W/m2 and initial biomass of 0.16 g. The study concluded that this approach could yield up to 80% hydrogen sulfide reduction from the pond water with elemental sulfur as a recoverable byproduct.

1. Model of Removal of Hydrogen Sulfide from a
Pond using a Photosynthetic Bioreactor
Katie Bickle, Conor Bury, Cassidy Laird, Dr. Caye Drapcho
BE 4100, Dept. of Environmental Engineering & Earth Sciences,
College of Engineering & Sciences, Clemson, SC, 29632
Abstract
The ability of the green bacteria Prosthecochloris aestuarii to oxidize
hydrogen sulfide was analyzed using a CSTR model in STELLA. The rate at
which hydrogen sulfide was converted was highly dependent on the algae
biomass and light intensity in the reactor. Using a light intensity of 25 W/m2
and an initial biomass of 0.16 g, the hydrogen sulfide content was reduced by
approximately 80%, from an initial concentration of 0.02903 mmol/L to an
effluent flow of 0.006 mmol/L.
Introduction
Hydrogen sulfide is a major problem associated with anaerobic
stabilization of sulfur-containing organic compounds, and has an unpleasant,
rotten-egg-like odor. As the most reduced form of sulfur, H2S has a high
oxygen demand, and in water it reacts rapidly with dissolved oxygen and may
cause a depletion of O2. Common methods for removal of H2S today are
physicochemical processes which involve either air stripping or oxidation
(Kobayashi, et al.).
The most common way to treat water contaminated with H2S is either
aeration or chemical compounds such as manganese dioxide or chlorine
dioxide. These methods are energy intensive and costly, and do not leave an
economically viable product that is able to be captured, which was the goal of
the project (Kobayashi, et al.).
In order to reduce the H2S in the system as well as capture a usable
product, the bacteria Prosthecochloris aestuarii was chosen to be used in a
CSTR reactor. P. aestuarii is a green sulfur bacterium that is
photolithotrophic, a strict anaerobe, and usually found in mud with a high
hydrogen sulfide content. P. aestuarii uses sulfide as an electron acceptor
during photosynthesis, with elemental sulfur being deposited as “extracellular
globules” (Takashima, et al.). These deposits can then be harvested as a viable
economic product.
Materials and Methods
Materials:
•Research Articles
•Hydrogen Sulfide test
•STELLA
•Microsoft Excel
Methods:
The experiment began with testing the hydrogen sulfide levels in samples
taken from the Duck Pond in the Botanical Gardens, using the hydrogen
sulfide test. The test was unsuccessful, and therefore a past level of 1 mg
hydrogen sulfide/L was assumed. After research, a journal article titled
“Anaerobic Oxidation of Dissolved Hydrogen Sulfide in Continuous Culture
of the Phototrophic Bacterium Prosthecochloris aestuarii.” was used as
guidance in the design, giving modeling constants that were used to generate a
STELLA model. Once this was done, the variables were altered to determine
the optimal parameters.
Results
Using the STELLA model with the calculated numbers, a final
concentration of 0.0060 mmol H2S/L (0.208 mg/L) was found (see Figure 2).
This results in an approximately 79.5% reduction of the concentration of
hydrogen sulfide in the Duck Pond. It was also noted that this final
concentration did not change with alteration of various parameters (see
Figures 2 and 3).
In regards to the data, there appears to be a point of saturation, where the
concentration of the hydrogen sulfide cannot be reduced further. This is seen
in Figure 2 as well. This may be due to the bacteria not being able to use any
more hydrogen sulfide.
Conclusions
Based on the data collected, this solution could yield up to an 80%
reduction of the hydrogen sulfide concentration in the Duck Pond in the
Botanical Gardens. Given a one to one molar ratio, the sulfur produced can be
predicted based on the H2S consumed. Overall, the proposed design is an
anaerobic plug flow bioreactor with a hydraulic retention time of 21.85 hours
and an inoculation size of 0.16 g of P. aestuarii. Other design details include
using a material to limit the amount of incidental light to the chosen level of
25 W/m2
, as well as adding a buffer as needed to regulate the pH levels.
References
Kobayashi, H.A., Stenstrom, M., Mah, R.A. “Use of Photosynthetic Bacteria for Hydrogen Sulfide Removal
from Anaerobic Waste Treatment Effluent.” Water Research 17.5 (1983): 579-87. Science Direct. Clemson
University, 10 Apr. 2013. Web. 29 Nov. 2014.
National Climatic Data Center. National Oceanic and Atmospheric Administration. 2014. Web. Accessed 24
Nov. 2014.
Takashima T., Nishiki T., and Konishi Y. “Anaerobic Oxidation of Dissolved Hydrogen Sulfide in
Continuous Culture of the Phototrophic Bacterium Prosthecochloris aestuarii.” Journal of Bioscience and
Bioengineering 89.3 (1999): 247-51. Science Direct. Clemson University, 13 June 2000. Web. 26 Nov. 2014.
Acknowledgements
We would like to thank Dr. Caye Drapcho for her guidance and supervision of this project. We would also like to thank
Clemson University and the Biosystems Engineering Department for the facilities provided.
Model Development
Based on the problem statement, it was concluded that a type of photosynthetic sulfur
bacteria would be a possible solution. These algae use sunlight to cleave hydrogen sulfide
instead of water, yielding hydrogen ions and elemental sulfur (Kobayashi, et al.).
H2S + sunlight  2H+
+ S0
A plug flow reactor (PFR) was chosen as the bioreactor type, since it would allow a
gradient of biomass and substrate, and would help the sulfur to settle to the bottom for easier
collection.
After setting up a mass balance equation (see above) for the concentrations of biomass (xB)
and hydrogen sulfide (S), Monod’s model was used to determine the other parameters of the
bioreactor. Using past bioreactor data (Takashima, et al.) for a type of green sulfur bacteria
called Prosthecochloris aestuarii, the following data was used to create a model:
Incidental light on average over a day for South Carolina (National Climatic Data Center)
was used to determine the amount of incidental light that would yield an acceptable hydraulic
retention time (in hours). These final values (see below) were used to create a model in STELLA
(see Figure 1) to determine the effects of inoculating the PFR with 0.16 g of the algae chosen.
Figure 2 (left). Graph
from STELLA model
showing the H2S
concentration in the
effluent after 400
hours. The
concentration can be
seen approaching the
final concentration of
0.006 mmol/L.
Figure 3 (right).
Graph from STELLA
model showing the
H2S concentration in
the effluent after
1500 hours. The
concentration levels
out at 0.006 mmol/L,
or 0.208 mg/L.
I0 = 25 W/m2
μ = 0.055775 hr-1
τ = 21.85 hours
Figure 1. STELLA model
designed to show the
proposed bioreactor. The
biomass model is on top, and
the hydrogen sulfide on the
bottom.