Browsing by Author "Higgins, Matthew J."

Now showing 1 - 5 of 5
  • Thumbnail Image
    Effect of Reactor Feeding Pattern on Performance of an Activated Sludge SBR
    Cubas Suazo, Francisco Jose (Virginia Tech, 2006-09-19)
    The possible effects of changes in the feeding pattern on activated sludge properties related to bioflocculation have been analyzed in lab scale sequencing batch reactors (SBR) in order to determine if these changes in effluent water quality and settling and dewatering properties are significant, so they can be considered in future studies or if they can be recommended as crucial when operating and designing wastewater treatment plants. The activated sludge process is widely used to treat wastewater from both industrial and municipal sources. Biomass from industrial facilities containing high monovalent to divalent ion content usually settles poorly, which leads to low quality effluents that fail to meet environmental requirements. Therefore, the combined effect of feeding pattern plus the addition of sodium to activated sludge reactors was studied in this experiment. A series of SBRs were operated at different sodium concentrations that ranged from 1.5 - 15 meq/L and different feeding times that ranged from 1 minute to 4 hours. Biomass samples were taken from each reactor to study the settling and dewatering properties and effluent samples were used to analyze the amount of organic matter and exocellular polymeric substances present due to deflocculation. As expected, the changes in feeding strategies affected all of the properties measured. When the feeding time was maintained low (pulse feed) the effluent quality and settling properties were the best. As the feeding time was increased the effluent quality, settling, and dewatering properties increased suggesting that the way in which the reactors were fed affected the overall bioflocculation process. The causes of the high deflocculation observed are not well understood, but data suggest that a microbial community change could have affected exocellular biopolymers which are believed to play an important role on bioflocculation. This research demonstrates the importance of the interaction between cation content and feeding pattern when operating a wastewater treatment plants and when reporting lab-scaled results related to settling and bioflocculation.
  • Thumbnail Image
    The Impact of advance purchase deadlines on airline consumers’ search and purchase behaviors
    Hotle, Susan; Castillo, Marco; Garrow, Laurie A.; Higgins, Matthew J. (Elsevier, 2015-12)
    Airlines frequently use advance purchase ticket deadlines to segment consumers. Few empirical studies have investigated how individuals respond to advance purchase deadlines and price uncertainties induced by these deadlines. We model the number of searches (and purchases) for specific search and departure dates using an instrumental variable approach that corrects for price endogeneity. Results show that search and purchase behaviors vary by search day of week, days from departure, lowest offered fares, variation in lowest offered fares across competitors, and market distance. After controlling for the presence of web bots, we find that the number of consumer searches increases just prior to an advance purchase deadline. This increase can be explained by consumers switching their desired departure dates by one or two days to avoid higher fares that occur immediately after an advance purchase deadline has passed. This reallocation of demand has significant practical implications for the airline industry because the majority of revenue management and scheduling decision support systems currently do not incorporate these behaviors.
  • Thumbnail Image
    Mechanisms of Methanogenic Inhibition in Advanced Anaerobic Digestion
    Wilson, Christopher Allen (Virginia Tech, 2009-12-07)
    A series of lab-scaled digestion studies including conventional mesophilic anaerobic digestion(MAD), thermophilic anaerobic digestion (TAD) at a range of treatment temperatures, and mesophilic high solids digestion of thermally pretreated wastewater sludge (THD) were carried out. Enhanced digestion performance in terms of solids destruction and methane generation by THD relative to MAD was achieved, and was largely attributable to the solubilization and subsequent biodegradation of energy-rich substrates within blended primary and secondary sludge. TAD was observed to underperform MAD, especially at elevated temperatures as methanogenic inhibition resulted in the accumulation of headspace hydrogen, thus resulting in poor removal of volatile fatty acids. The thermodynamics of fatty acid metabolism was favorable at each digestion temperature, thus it was concluded that microbial inhibition was the controlling factor in poor thermophilic performance. Inhibition by free unionized ammonia (NH₃) was characterized for THD and MAD biomass. Acetic acid degradation was equally affected over a range of NH₃ concentrations; however, methane generation by THD was less sensitive to ammonia inhibition, thus suggesting that methanogenesis by THD was less dependent on the NH₃-sensitive process of aceticlastic methanogenesis. Total ammonia nitrogen (TAN) and bicarbonate alkalinity were stoichiometrically produced from proteinaceous material during thermal hydrolytic pretreatment and subsequent high solids anaerobic digestion. Combined effects of TAN and high pH resulted in NH₃-inhibition during THD. Kinetic evaluations suggested that a growth rate reduction of approximately 65% was associated with in-situ NH₃ concentrations of the THD reactor. NH₃-inhibition was apparently responsible for a shift in dominant methanogenic community of the aceticlastic Methanosarcina barkeri in MAD to the hydrogenotrophic Methanoculleus bourgensis in THD. A similar shift in methanogenic community was observed between low temperature thermophilic digestion at 47°C, where the dominant order was Methanosarcinales, to high temperature thermophilic digestion at 59°C where the dominant order was Methanobacteriales. These findings support a process-driven pathway shift from aceticlastic to non-aceticlastic methanogenesis between 180 and 290 mg/L NH₃-N. Such a threshold is supported by previous literature related to ammonia tolerance of pure cultures of methanogens and has significant implications for the kinetic design of advanced anaerobic digestion processes.
  • Thumbnail Image
    The roles and interactions of cations, proteins, and polysaccharides in the settling and dewatering of activated sludge
    Higgins, Matthew J. (Virginia Tech, 1995-08-05)
    The roles of cations and exocellular biopolymer on the settling and dewatering of activated sludge was investigated. Both laboratory and full-scale activated sludge systems were examined. The results of the study showed the settling and dewatering properties of the activated sludge were dependent on the calcium, magnesium, potassium and sodium concentrations added to the feed. A minimum of 0.72 meq/L each of calcium and magnesium in the feed was necessary for acceptable settling and dewatering. Two types of microbial cultures were observed, one required both calcium and magnesium in the feed while the other required either calcium or magnesium, but not both, for optimization of settling and dewatering. Sodium addition to the feed improved the settling of activated sludge when the ratio of sodium to calcium plus magnesium was equal to approximately one on a meq/L basis. When this ratio was greater than two, the settling and dewatering properties deteriorated, but the deterioration could be reversed by increasing the calcium and magnesium concentration in the feed. In general, the data supported the cation bridging model for bioflocculation, and the cations act to bind protein to the biofloc structure. Results from the full-scale activated sludge plants correlated well with results from laboratory activated sludge systems and demonstrated the cation content in these systems had a direct impact on the settling and dewatering properties. Field trials in which divalent cations were added to activated sludge systems resulted in dramatic improvements in the settling properties of these systems. Characterization of the exocellular protein extracted from laboratory, industrial and municipal activated sludges revealed the presence of a single protein, which appears to be a lectin. The molecular weight of the protein measured by SDS PAGE was approximately 16,000 Daltons with similar amino acid composition as microbial lectins. Also, amino acid sequencing analysis indicated the N-terminal sequence of the protein was consistent with those of microbial lectins. In addition, the activated sludge cultures exhibited lectin activity as demonstrated by binding site inhibition experiments. A model of bioflocculation that includes the role of protein was proposed.
  • Thumbnail Image
    A study of multi-stage sludge digestion systems
    Kim, Jong Min (Virginia Tech, 2010-07-14)
    Various combinations of multi-stage thermophilic and/or mesophilic anaerobic sludge digestion systems were studied to evaluate their solids reduction, odor generation after centrifugal dewatering and indicator organism reduction in comparison to single-stage thermophilic and/or mesophilic anaerobic digestion systems. Pre-aeration of sludge in a thermophilic temperature was also tested followed by single or multi-stage anaerobic digestion systems. It was found that multi stage systems were capable of greater solids removal and placing thermophilic system in multi stage system enhanced indicator organism destruction below EPA Class A biosolids requirement. However, all the digestion systems in the study showed less than 3 log reduction of indicator organism DNA/g solids, which was much smaller than indicator organism reduction measured by standard culturing method. It was also found that the thermophilic anaerobic digestion system could increase organic sulfur-based odors from dewatered biosolids while placing a mesophilic digester reduced odors. It was exclusively observed from sludges containing high sulfate such as ones in this study. A combined anaerobic and aerobic sludge digestion system was also studied to evaluate their solids and nitrogen reduction efficiencies. The aerobic digester was continuously aerated to maintain dissolved oxygen level below 1 ppm and intermittently aerated. It was found that 90 % or more nitrogen removal was possible at the aerobic SRT greater than 3 days and the optimum aeration ratio could be determined.