Date of Award

6-18-2021

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Civil and Environmental Engineering

Supervisor

Nihar Biswas

Supervisor

Rajesh Seth

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

Anaerobic digestion is a widely used, convenient and long-standing method of sludge treatment before disposal. The process includes hydrolysis, acidogenesis/acetogenesis and methanogenesis. A single or multiple reactor (tank) can be used. However, fluidized bed reactors (AnFBRs) are increasingly becoming popular to promote anaerobic digestion when digesting sludge before final disposal. AnFBRs are characterized by a uniform temperature gradient in the reactor. However, this study has been carried out to establish which media are most optimal for the AnFBRs. Likewise, research has been done to establish the efficiency of the process when either mesophilic or the thermophilic organisms are dominant, keeping in mind the optimum temperature range where these organisms are most efficient. The third part of the study was devoted to investigate how how vomitoxin (deoxynivalenol [DON]), associated with plant pathogens, affected biological methane production kinetics during mesophilic anaerobic digestion.The role of different media on mesophilic anaerobic digestion of simulated thin stillage using fluidized bed reactors was investigated in the first phase. Zeolite, a mineral based medium and plastic, a synthetic media was employed in two lab-scale AnFBRs. The reactor with zeolite showed an increase in the amount of attached biomass (32.4 mg VSS/g of media) when compared with plastic media (19.7 mg VSS/g of media). The maximum thickness of biomass in zeolite was 370 µm compared with 200 µm in the plastic media. From an operational perspective, zeolite was more suitable for the AnFBRs with less floating issue. The zeolite media was better in resisting floatation compared to the plastic media, which floated to the top even with 50% less superficial velocity that was sustained by the zeolite media. The second part compares the operational parameters and efficiency between mesophilic and thermophilic anaerobic digestion of simulated thin stillage conducted in AnFBRs. Two 7-liter working volume AnFBRs were operated mesophilically (37±1˚C) and thermophilically (55 ±1˚C) over different hydraulic retention times (HRT). The plastic media with a diameter of ( dm) in the range of 600-2000 µm was employed as carrier media. Each experimental run continued over a six-month period. At 16, 6 and 4-days HRT, while maintaining the constant chemical oxygen demand (COD) concentration in the feed, Methane composition in biogas from the thermophilic reactor was between 10-15 % more than the amount of methane produced by the mesophilic reactor. Results also suggest that the thermophilic reactor could be operated at lower HRT without the loss of performance. Also, during this operation, it was observed that there was no foaming issue in the reactor. The third part of the study investigated the effects of vomitoxin on biogas and methane production. The anaerobic digestion was conducted in batch reactors. Results indicate that the presence of vomitoxin, even at a high level of 20 ppm, did not harm the anaerobic digestion process of the liquefied corn. The cumulative methane production from reactors with vomitoxin and without vomitoxin remained almost the same. Liquefied corn with 0 g/L vomitoxin (control) produced around 458 mL of methane, while the samples with 1, 5, 10, and 20 ppm of vomitoxin produced around 443, 456, 453, and 459 mL, of methane, respectively. Methane fractions in the biogas for all the bottles were between 51 and 56%. Biogas production and the methane content both remain unaffected even under the presences of 20 ppm of vomitoxin.

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