Date of Thesis
Anaerobic treatment of domestic wastewater has many advantages compared to aerobic treatment. For example, organic matter is converted into methane, which can be used as cooking fuel, or converted into heat and power. In addition, anaerobic treatment produces less solids and has lower operating and maintenance costs compared to aerobic systems. However, 40-60% of the methane produced in anaerobic treatment of domestic wastewater is not captured, as it is trapped in the dissolved phase, and is subsequently released to the atmosphere after the effluent is discharged to waterways. Dissolved methane is a greenhouse gas with 28 times the impact of CO2. In this work, we test a rotating biological contactor (RBC), for its ability to oxidize methane, dissolved in a synthetic anaerobic effluent. We used laboratory data combined with engineering process models, life cycle assessment (LCA) and technoeconomic analysis (TEA), to compare the environmental, and economic life cycle impacts of three treatment scenarios treating 2 MGD of domestic wastewater in Pennsylvania, over 30 years: (1) an anaerobic baffled reactor (ABR) system (2) an RBC combined with anaerobic digestion (RBC+AD) system, and (3) a system consisting of an ABR coupled with an RBC+AD (ABR + RBC). Additionally, we conducted a social LCA to compare social risks for the supply chain of the ABR and RBC+AD assemblies.
Full scale performance of the ABR and RBC were modeled by scaling up laboratory data, including uncertainty. The three sustainability assessments, environmental, economic and social, were based on the same life cycle inventory. The social hotspot database (SHDB) was used to conduct social LCA.
The bench-scale RBC oxidized approximately 80% of the methane dissolved in the effluent from an ABR and 96% of the chemical oxygen demand (COD). The reactor’s ability to oxidize dissolved methane before it volatilized confirmed our hypothesis that microorganisms were able to maintain a low enough concentration of dissolved methane, such that volatilization was minimized. Additionally, we hypothesize that in the RBC, COD was oxidized first, methane was oxidized second, and ammonia oxidized third.
Four environmental damage categories were assessed using environmental life cycle assessment (LCA). For three of the damage categories— ecosystem quality, resource depletion, and human health—the ABR incurred impacts that were 82-99% and 5-13% more beneficial than the RBC+AD and ABR+RBC assemblies, respectively. For climate change, however, the ABR incurred impacts that were 95% and 87% more harmful than the RBC+AD and ABR+RBC assemblies, respectively. Additionally, harmful impacts for climate change associated with the ABR+RBC varied based on the amount of dissolved methane oxidized in the RBC. Ninety-one percent of the dissolved methane must be oxidized in the RBC for the ABR+RBC assembly to yield a beneficial impact on climate change. When uncertainty is accounted for, impacts of the three treatment systems on climate change were harmful, except for the ABR+RBC, where 20% of the Monte Carlo simulations in the climate change category have beneficial impacts.
TEA results indicated that the ABR had the least capital costs, operating costs, and net present value (NPV). In terms of NPV, the ABR assembly costs 27% and 34% less than the RBC+AD and ABR+RBC assemblies, respectively. The low costs for ABR can be explained by low operation costs, due to low solids production, as well as the production of biogas. The higher costs of the RBC+AD and ARB+RBC systems are due to solids management – belt filter press, anaerobic digestion and sludge disposal.
Similar to economic costs, the ABR assembly was associated with lower social risk across the five social categories assessed in the Social Hotspot Database (SHDB) compared to the RBC+AD assembly. Social risk along the supply chain results from three main drivers within each relevant country specific economic sector (CSS): (1) economic contribution, (2) labor intensity, and (3) social conditions. For the ABR assembly, the ABR was the unit process with the highest social risk. For the RBC+AD assembly, AD and disposal of solids were the two unit processes with the highest social risk. The AD, ABR, and solids disposal unit processes corresponded to four country specific sectors (CSSs): Public Services, Construction, and Machinery in the USA; and Commerce in Angola. These CSSs are identified as social hotspots (i.e., sectors with relatively high social risk) for aerobic and anaerobic wastewater treatment systems. The main social issues identified in the USA include lack of collective bargaining protections, inadequate social benefits (such as maternity leave), exposure to toxics and hazards, and a high rate of occupational injuries and fatalities. The main social issues identified in Angola, one of the US’s sources of oil (an important sector for solids disposal) include widespread corruption and a weak legal system. Most of the challenges that Angola faces are due to the 27-year civil war that ended in 2002 and rampant corruption. Much of Angola’s infrastructure was destroyed during the civil war, which was reflected in the social indicators evaluated here.
No single technology was optimal based on cost, environment and social impact. The ABR assembly has the lowest cost and social impact, but the most harmful environmental impacts. On the other hand, the ABR+RBC assembly has the most beneficial environmental impacts, but the highest net present value.
global warming, social life cycle assessment, dissolved methane
Masters Thesis (Bucknell Access Only)
Master of Science in Environmental Engineering
Mohammed, Mona, "Sustainability Assessment of Coupling a Rotating Biological Contactor with an Anaerobic Baffled Reactor for Municipal Wastewater Treatment" (2019). Master’s Theses. 226.