Date of Award
Fall 2020
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Civil, Construction, and Environmental Engineering
First Advisor
Zitomer, Daniel H.
Second Advisor
McNamara, Patrick, J.
Third Advisor
Mayer, Brooke K.
Abstract
Pyrolysis treats and potentially recovers energy from wastewater solids (WWS). Aqueous pyrolysis liquid (APL), however, is produced and its management is a bottleneck to pyrolysis full-scale application. To overcome this bottleneck, anaerobic digestion (AD) may be a possible method to manage APL, but APL digestion has not been conclusively demonstrated. In AD, a select group of microorganisms convert organic chemicals to biogas containing methane that can be used as a fuel. In this dissertation work, APL derived from WWS pyrolysis was successfully converted into methane as the sole substrate and as a co-digestate with synthetic primary sludge in long-term, continuous anaerobic digesters for the first time. Methane production from APL that was previously hindered by APL recalcitrance and toxicity, increased by applying a low organic loading rate (OLR), high solids retention time (SRT), ozone pretreatment, and selecting appropriate microorganisms that were more proficient at APL biodegradation. APLs’ anaerobic degradability produced at 500 and 700 °C pyrolysis temperatures was evaluated in anaerobic toxicity assays using four different anaerobic inocula. Higher pyrolysis temperature resulted in APL with higher toxicity. Pre-ozonation of APL for 2 h or less improved the methane production rate from 700°C APL. In contrast, ozonation did not have a substantial impact on the methane production rate from 500 °C APL. In long-term, continuous digestion studies, quasi steady state methanogenesis from 700 °C APL was accomplished by employing an appropriate, low APL OLR (0.03 gCOD/L APL) and a sufficiently long SRT (210 days), whereas shorter SRT and higher OLR values inhibited or stopped methane production. Employing a specific anaerobic inoculum from an industrial waste digester that was acclimated to constituents similar to those in APL resulted in more complete and more rapid methane production, compared to municipal anaerobic digester biomass. Microbial communities in digesters inoculated with the industrial biomass were dominated by hydrogenotrophic Methanobacterium, accompanied by an increased relative abundance of syntrophic bacteria belonging to phylum Synergistes and class Clostridia. Bacterial taxa capable of degrading N-heterocyclic compounds, Enterococcus, Eubacterium, and Bacillales, were also enriched. The results demonstrate that long-term methanogenesis from APL as the sole substrate in AD is possible. Anaerobic co-digestion of APL with primary sludge at municipal water resource recovery facilities is also a viable approach at facilities that already have digesters treating sludge. In this scenario, APL from pyrolyzed WWS can be added to digesters for increased biogas production, but care must be taken to control the OLR, SRT and biomass inventory to ensure APL digestion success.