Anaerobic digestion of selenium-rich wastewater for simultaneous methane and volatile fatty acid production and selenium bioremediation
Logan, Mohanakrishnan
Logan, Mohanakrishnan
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Publication Date
2022-09-09
Keywords
Anaerobic digestion, Methane production, Volatile fatty acids, Selenium bioremediation, Dissolved air floatation slurry, Glycerol, Food waste, Dairy wastewater, Granular activated carbon, Conductive materials, Up-flow anaerobic sludge blanket reactor, Sequential batch reactor, Microbial community, Integrated approach, Science and Engineering, Natural Sciences, Microbiology
Type
Thesis
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Abstract
This research aimed at investigating the anaerobic digestion (AD) of selenium (Se)-rich and lipid-rich wastewaters via evaluating the operational parameters in sequential and continuous bioreactors. In the first part of the study, batch AD of dissolved air floatation (DAF) slurry derived from dairy wastewater showed methane yield was improved up to 177% after up to three folds dilution. Similarly, adjustment of the initial pH to 6.0 resulted in a higher methane yield, which was 23% higher than the control without pH adjustment. Following this, the potential of granular activated carbon (GAC) supplementation to enhance the AD of dairy wastewater was studied in sequential batch reactors (SBR). The methane production increased by up to 500% in the GAC-amended SBR when compared to the Control SBR. GAC addition led to faster lipid degradation and promoted the activity of syntrophic and electroactive microorganisms such as Geobacter, Synergistes, Methanolinea and Methanosaeta. The second part of the study evaluated the effect of Se oxyanions on AD of different model substrates and real waste/wastewater. AD of dairy wastewater based DAF slurry supplemented with 0.05 and 0.10 mM Se oxyanions achieved a similar cumulative methane yield of 180 mL/g COD as that of digestion of Se free DAF slurry after 65 days of incubation. The IC50 was 0.08 mM for selenate and 0.07 mM for selenite. Following this, the influence of pH, heat treatment (HT) of inoculum and Se oxyanions on volatile fatty acids (VFA) production from food waste was studied. The highest VFA yield (0.516 g COD/g VS) was achieved at pH 10, which was 45% higher than that at pH 5. The VFA composition was dominated by acetic and propionic acids at pH 10 with non-heat treated inoculum, which diversified at other test conditions. HT and Se resulted in VFA accumulation in alkaline pH, but HT was detrimental for selenate reduction (< 15 % Se removal after 20 days). Finally, a long term, continuous and simultaneous methane production and Se bioremediation was demonstrated in up-flow anaerobic sludge blanket reactors. About 150 mL/g COD daily methane yield was achieved which was comparable with that of the control until 400 µM selenate. Simultaneously, more than 90% Se removal was accomplished. Biosynthesis of Se nanoparticles and metal selenides were observed, supported by X-ray diffraction, scanning and transmission electron microscopy. However, methane production deteriorated when selenate was increased to 500 µM, due to inhibition in the activity of Methanosaeta. Remarkably, Se facilitated sludge granulation. The selenate concentration, but not the COD/selenate ratio, was found to govern the AD of selenate rich wastewaters.
Publisher
NUI Galway