High-rate Resource Recovery from Wastewater with Encapsulated Biomass
2020-01
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High-rate Resource Recovery from Wastewater with Encapsulated Biomass
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2020-01
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This dissertation describes the recovery of a high-value resource during anaerobic wastewater treatment using encapsulated biomass with specialized functions. The encapsulant was formed into a bead and consisted of a customizable alginate gel matrix. Biomass was encapsulated within the bead, enabling the retention of high concentrations of specific communities of biomass in reactors even when operating reactors with a low hydraulic retention time (HRT). The effects of encapsulant customization, including that of cross-linking agents (Ca2+, Sr2+, and Ba2+) and a composite coating on the beads (alternating layers of polyethylenimine (PEI) and silica hydrogel), on the biomass retention ability and mass transport performance was quantified. The diffusion of the organic carbon in brewery wastewater through the alginate encapsulation matrix was not affected by the cross-linking agent and was comparable to that of glucose in water. As a result, the biomass encapsulated in uncoated beads was not substrate limited and high rates of hydrogen production from brewery wastewater were observed, even at an HRT of 45 min. Suspended biomass controls were not able to maintain hydrogen production at this low HRT because the biomass was washed out. Although the coating reduced the biomass escape rate from the encapsulant, it created a mass transport barrier to substrates, reducing both the diffusivity and the partition coefficient of organic carbon. This resulted in lower hydrogen production rates from brewery wastewater compared to the uncoated encapsulation system. A diffusion-reaction model describing the encapsulation system was also developed to predict, and therefore optimize, the hydrogen production rate under various encapsulant customization schemes and operating conditions. Experimental data collected from flow-through reactors with encapsulated biomass fed brewery wastewater were used to calibrate and validate the model. The model was capable of successfully predicting the general hydrogen production trends as a function of HRT, bead size, and wastewater strength. A sensitivity test conducted with the model revealed that the hydrogen production process with encapsulated biomass from brewery wastewater was growth limited and was sensitive to the substrate partition coefficient into the encapsulation matrix, initial encapsulated biomass concentration, and the total volume of beads in the reactor. The effect of encapsulation on hydrogen and methane production when encapsulated biomass was incubated in the presence of several known inhibitors was also investigated to determine whether and how the encapsulating matrix mitigated or exacerbated inhibition by different types of chemicals. The charge of the inhibitors appeared to play a dominant role in how they partitioned into the encapsulation matrix. Dichromate (negatively charged) appeared to be repelled by the alginate matrix while ammonium (positively charged) concentrated into the matrix. Chloroform (uncharged) was unaffected by the matrix and was neither repelled by nor concentrated into it. This was thought to be a result of electrostatic interactions with alginate. As a result, the matrix mitigated dichromate inhibition, increased ammonium inhibition, and had no impact on chloroform inhibition of the encapsulated biomass. Copper, on the other hand, chelated with alginate and the PEI coating, even though it partitioned into the matrix, appeared to be non-bioavailable, completely eliminating inhibition of the hydrogen-producing biomass. These results were also confirmed with an encapsulated methane-producing anaerobic community, demonstrating that the ability of the encapsulant to mitigate or exacerbate inhibition was applicable beyond hydrogen-producing biomass. Finally, a pilot-scale system was built and deployed at a brewery. The system consisted of a single hydrogen-producing reactor containing encapsulated biomass in series with parallel methane-producing reactors, one containing encapsulated biomass and a second containing suspended biomass in a membrane bioreactor configuration. Performance during intermittent operation and perturbations was monitored along with shifts in the microbial community in the two parallel methane-producing reactors. A more rapid recovery after perturbation was observed in the membrane bioreactor. In addition, its microbial community was similar to that sampled from the bulk solution of the reactor containing encapsulated biomass. The encapsulated biomass had a distinct microbial community structure, even though both reactors were inoculated with the same culture. This demonstrated that the community in the membrane bioreactor was able to adapt and change with time, apparently enabling faster recovery from perturbations. This appears to be a potential problem with encapsulated biomass, particularly if highly variable wastewater is being fed to the reactors or the system is expected to experience upsets and operational perturbations. This multi-faceted investigation of a customizable alginate encapsulation system for high-rate recovery of resources during anaerobic wastewater treatment provided insights regarding how to design the system for good performance. It also provided further information regarding potential problems that could be encountered when using encapsulated biomass for treatment. Overall this system offers the potential for low-maintenance decentralized anaerobic wastewater treatment. More work is needed, however, to facilitate robust and reliable treatment and to provide guidance for system optimization and further life cycle assessment.
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University of Minnesota Ph.D. dissertation. January 2020. Major: Civil Engineering. Advisor: Paige Novak. 1 computer file (PDF); 205 pages.
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ZHU, KUANG. (2020). High-rate Resource Recovery from Wastewater with Encapsulated Biomass. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/226664.
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