Model of Removal of Hydrogen Sulfide from a Pond using a Photosynthetic Bioreactor

Katie Bickle, Conor Bury, Cassidy Laird, Dr. Caye DrapchoBE 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 MethodsMaterials: •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.

ReferencesKobayashi, 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.

AcknowledgementsWe 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 DevelopmentBased 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.