Complex Biochemical Reactors for Selenium and Sulphate Reduction: Organic Material Biodegradation and Microbial Community Shifts
Biochemical reactors using complex organic materials for treatment of mine-affected waters are attractive low-cost solutions, but their widespread adoption is severely limited by poor reliability and limited longevity. This is partly due to lack of guidance on which organic materials to use, their degradation over time, and how this affects the microbial community composition, which in turn influences reactor performance.In this project, continuous-flow column bioreactors containing differing ratios of a wood, hay and manure mixture were operated for 159 to 430 days including successful and decline phases of performance.
Reactor performance, detailed organic matter composition and microbial community structure were measured for reactors with different wood to hay ratios and after different times of operation. Reactors with more hay than wood reduced sulphate from 500 mg/L to less than 100 mg/L and selenium from 20.3 µg/L to less than 0.2 µg/L with a retention time of 14 days for the whole period of operation. Whereas reactors with a high wood to hay ratio operated successfully for 100-200 days after which performance fluctuated.
Increase in more readily available organic compounds with decrease in recalcitrant fibrous materials was charted over time and correlated with changes in microbial community composition. More hemicellulose and alpha-cellulose were consumed in the bioreactors with more hay content. Lignin content remained the same for the wood rich bioreactors, and increased in the hay-rich columns. Ash content in bioreactors with either organic mixture increased over time. The labile components, determined as neutral detergent and water soluble compounds, fluctuated cyclically.
The microbial communities that evolved in the bioreactors were distinctly different from those present initially. At the early stages, the communities were rich in organic matter degraders classified in the Bacteroides, Parabacteroides and Ruminococcaceae taxonomic groups. There was a shift towards Methanogens and Mollicutes and Spirochaetes classified groups for the longer running bioreactors. Sulphate reducing bacteria were mostly Desulfobulbus and Desulfovibrio related and they were more prevalent in the presence of high sulphate throughout the reactor history.
Sequestration of Carbon Dioxide: A Biological Approach to Mineralization of Carbon Dioxide in Depleted Oil and Gas Reservoirs
Biological sequestration of carbon dioxide is one of the proposed methods to reduce concentration of carbon dioxide in the atmosphere. In this method, CO2 is converted to a mineral such as calcium or magnesium carbonate. An enzyme is used to promote the hydration reaction of CO2 and precipitation of the mineral. Enzyme carbonic anhydrase is known as the suitable enzyme for this purpose.
In the present work, the effect of bovine carbonic anhydrase on the hydration of carbon dioxide and also its precipitation in the form of calcium carbonate was studied through various experiments. The change in the rate of hydration reaction was measured indirectly, by measuring the change in pH of the aqueous solution. The results of the experiments showed that bovine carbonic anhydrase enhanced the hydration reaction.
The effect of enzyme concentration, as well as temperature, on the hydration reaction was studied. The rate of pH drop increased with both the enzyme concentration and temperature. The precipitation of calcium carbonate was enhanced in the presence of the enzyme. The concentration of enzyme did not impact the precipitation; however temperature was a factor and at higher temperatures less calcium carbonate was formed. Also, when the enzyme was present in the solution, calcium carbonate settled down faster. The pH did not influence the enzyme's activity, but it affected the formation of calcium carbonate.
As one aspect of the present research, a mathematical model was developed to get an understanding of the precipitation of CO2 in a reservoir. The outcome of the model was in a close agreement with the experimental results, which introduce the model as a reliable source for future estimations.
Effect of Hydraulic of UASB Reactor on Performance of Sulphate-Reducing Bacteria
Presence of sulphate in domestic and industrial wastewaters could have adverse effects on the environment. One method to remove sulphate from wastewaters is converting sulphate to hydrogen sulphide by anaerobic sulphate-reducing bacteria in Upflow Anaerobic Sludge Blanket (UASB) reactors.In this project effect of hydraulic parameters, namely COD concentration and flowrate, on the performance of sulphate-reducing bacteria in a pilot UASB reactor was studied.
The UASB reactor was receiving a synthetic wastewater with a COD of 500 mg/L or 1000 mg/L and sulphate concentration of 500 mg/L. The results showed that at flowrates higher than 1 m/hr, SRB were washed out of the reactor. Unlike Methane-producing bacteria (MPB), Sulphate-reducing bacteria do not form granules, and since they are lighter than MPB they easily get washed out of the system at high flowrates.
Sulphate removal at low COD/SO4 ratios was the most efficient at flowrates of 1.5-2.5 m/hr. At lower flowrates concentration of hydrogen sulphate can get very high, which inhibits the bacteria. On the other hand at higher flowrates the COD removal efficiency drops due to a lower hyudraulic retention time. An increase in COD/SO4 ratio resulted in an increase in COD rmoval efficiency, however it did not have any efect on sulphate removal efficiency.