Multiple compartment anaerobic bioreactors for the generation of energy: exploring energy-positive wastewater treatment

Abstract

Wastewater contains resources such as energy, clean water, and nutrients. Under the current wastewater treatment paradigm (i.e., activated sludge), resources are consumed rather than recovered. Anaerobic bioreactors are a potentially viable alternative to traditional aerobic wastewater treatment for several reasons, to include their ability to generate methane-rich biogas while simultaneously reducing volumes of waste sludge and decreasing waste disposal burden. Multiple-compartment anaerobic bioreactors, such as the anaerobic baffled reactor (ABR), are particularly attractive due to the reactor’s low complexity and its ability to generate methane with little to no energy input. Despite these advantages, pilot-scale demonstrations of ABRs, or variations of the ABR, operated under colder wastewater temperatures (11 – 24 degrees C) are extremely limited. Prior to widespread implementation, ABRs treating domestic wastewater require additional pilot-scale demonstration, study of the lifecycle impacts relative to conventional wastewater treatment technologies, and a more complete understanding of the microbial community dynamics for modeling and performance prediction. To address these research needs, this dissertation examines the performance of two low-complexity pilot-scale multiple-compartment anaerobic bioreactors in three different areas: (1) characterizing bioreactor performance over varying temperatures for organic removal, suspended solids removal, and methane generation; (2) modeling lifecycle impacts and energy generating potential relative to conventional wastewater treatment approaches; and (3) characterizing methanogenic community structure over time and space within both bioreactor systems. The pilot-scale anaerobic bioreactors characterized in this study were: (1) an existing four-compartment ABR located at the Plum Creek Water Reclamation Authority in Castle Rock, CO, and (2) a three-compartment ABR coupled with an anaerobic fixed film reactor (AFFR), which was constructed at the Mines Park Wastewater Test Bed. Modeling methods include full treatment train modeling in BioWin 5.2, environmental impact modeling in SimaPro (version 8.0.3.14), lifecycle cost modeling in CAPDETWorks (version 2.5, Hydromantis, Inc.), and uncertainty modeling (Monte Carlo simulation) in Oracle Crystal Ball. Microbial community structure was examined using 16S rRNA gene sequencing. Key findings of this study are numerous, and include: (1) both ABRs studied remove higher levels of organics and suspended solids relative to conventional primary treatment while generating near stoichiometric volumes of methane; (2) ABRs have high chemical energy conversion efficiencies relative to other wastewater treatment reactors, recovering 52% of the chemical energy available in the influent wastewater organics; (3) modeling suggests that energy generated by ABRs coupled with combined heat and power (CHP) with heat recovery is sufficient to power many typical conventional activated sludge systems; (4) lifecycle environmental impacts for treatment trains including ABRs are lower in most impact categories (e.g., fossil fuel emissions, acidification, etc.), but dissolved methane capture is required to reduce greenhouse gas emissions and enhance potential energy generation; (5) lifecycle costs and net energy balances for modeled configurations with ABRs are lower relative to conventional treatment configurations; and (6) both ABRs developed similar methanogen-rich microbial communities dominated by Methanosaeta, an acetate-utilizing methanogen. Additional study is needed in several areas, to include the treatment of residual contaminants in the effluent of ABRs and long-term characterization of the microbial community structure for accurate modeling. However, this study concludes that multiple-compartment anaerobic bioreactors, such as the ABR, are a viable energy-generating alternative to conventional primary treatment. Full-scale demonstrations should be implemented near-term.