Identification of Selenium Reducing Species Within and Suggested Optimization of a Passive Biological Reactor
Type of DegreeMaster's Thesis
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As concern for the environment moves to the forefront of the issues considered most pressing by society, mankind is becoming increasingly aware of the harmful things it discharges into the ecosystem in the name of industry and progress. One of these compounds that is making itself increasingly known is selenium. This element is a metalloid that occurs naturally within the environment and is a necessary nutrient to many living organisms. The problem comes from human activity creating anthropogenic loadings of selenium species into the environment or accelerating the naturally process that otherwise introduce the necessary amount of selenium into the ecosystem. When selenium becomes too enriched in the environment the impacts upon animal populations can be drastic. It has been well documented, especially in bird and fish species, that exposure to high levels of selenium can impair reproductive processes and thereby have a detrimental impact of organism populations. One method implemented to remove selenium from wastewater streams in an attempt to prevent such things from happening is the use of a passive biological reactor. The type that this study pertains to are of a lagoon structure. These systems receive an organic carbon source for the microorganisms growing within the system, and generally get no additional input like aeration to enhance the process. For this investigation we were tasked to examine such a process and report back with the requested results. The examined systems consisted of three lagoon type reactors operated in series. Each reactor received wood chips as the organic carbon source for the microorganisms present in the reactor. The task presented was to identify the species of microorganisms responsible for the selenium reduction within the system as well as to suggest any means by which to optimize the process. The methods tested by which to potentially optimize the system fall into two categories. The first category consisting of altering the temperature within the reactor. The second category consisted of potential nutrient additives that could potentially increase the potential for and rate of selenium reduction. The additives tested were: Nitrogen, Phosphorous, Micronutrients, Molybdenum, Zinc, and Cobalt. The identification of selenium reducers was performed at every sampling location within the reactors providing a profile for species present across the treatment process. The methods of optimization were tested by observing the rate of selenium reduction when different amounts of the proposed additives present within an isolated batch experiment. The same was done at varying temperatures to determine its effect on the process. Upon completion of the experiments, the rates of reduction were plotted according to the additive tested and the concentration of said additive corresponding to the observed rate of reduction. After completion of the identification process, ten species of selenium reducers were positively accounted for. These species found are: Bacillus Subtillus, Microbacterium aborescense, Enterobacter, Psuedomonas stutzari, Desulfomusa, Desulfomicrobacterium, Desulfovibrio desulfricans, Desulfobacterium, Geovibrio, and Shewanella putrifaciens. These species were found to have changing prevalence from sampling location to sampling location across the process. After taking a closer look at data gathered while performing the species identification lab work it was concluded that the third reactor in process was operating under a surface treatment condition. This conclusion led to the first optimization suggestion which was to decrease the size of the wood chips added to that reactor thereby increasing the surface area at which selenium reduction occurs. Once collected, the data gathered from the procedures to determine the influence of the presented variables for optimization were plotted and examined for the presence of an identifiable trend. The altering of the temperature at which reactor samples were held at, did display an observable impact on the reduction rate of selenium. This suggested that temperature could indeed be a means of process optimization, however the likely economic implications of temperature control for this process very likely make this an impractically means of optimization. Of the additives examined Nitrogen, micronutrients, and Molybdenum were successful in producing an observable impact on the reduction rate. Therefore it was suggested that the addition of these additives could be viable methods for process optimization.