Wastewater sludge management is a significant challenge for small-scale, urban wastewater treatment plants (WWTPs). Common management strategies stabilize sludge for land disposal by microbial action or heat. Such approaches require large footprint processing facilities or high energy costs. A new approach considers sludge to be a fuel which can be used on-site to produce electricity. Electrical power generation fueled by sludge may serve to reduce the volume of hazardous waste requiring land disposal and create economic value for WWTP operators. To date, no detailed system designs or techno-economic analyses have been found for small scale sludge fueled power plants. Fortunately, a literature base exists describing the fundamentals of applying thermochemical conversion (TCC) technologies to sewage sludge. Thermochemical conversion of sludge is established for large WWTPs, however large system design techniques may not be applicable to small systems.To determine the feasibility of small scale power generation fueled by sludge, this work evaluates several thermochemical conversion technologies from the perspective of small urban WWTPs. Literature review suggests wet oxidation, direct combustion, pyrolysis, and gasification as candidate front-end TCC technologies for on-site generation. Air and steam blown gasification are found to be the only TCC technologies appropriate for sludge. Electrical power generation processes based on both air and steam blown gasification are designed around effective waste heat recovery for sludge drying. The systems are optimized and simulated for net electrical output in ASPEN Plus R . Air blown gasification is found to be superior. Sensitivity analyses are conducted to determine the effect of fuel chemical composition on net electrical output. A technical analysis follows which determines that such a system can be built using currently available technologies. Finally, an economic analysis concludes that a gasification based power system can be economically viable for WWTPs with raw sewage flows of 0.115 m 3 /s, or about 2.2 million gallons per day.
The objective of this study was to evaluate the lifecycle impacts of anaerobic primary treatment of domestic wastewater using anaerobic baffled reactors (ABRs) coupled with aerobic secondary treatment relative to conventional wastewater and sludge/biosolids treatment systems through the application of wastewater treatment modeling and three lifecycle-based analyses: environmental lifecycle assessment, net energy balance, and lifecycle costing. Data from two pilot-scale ABRs operated under ambient wastewater temperatures were used to model the anaerobic primary treatment process. To address uncertain parameters in the scale-up of pilot-scale anaerobic reactor data, uncertainty analysis and Monte Carlo simulation were employed. This study demonstrates that anaerobic primary treatment of domestic wastewater using ABRs can be incorporated with existing aerobic treatment strategies to reduce aeration demand, reduce sludge production, and increase energy generation. The net result of coupling anaerobic primary treatment with aerobic secondary treatment is a more favorable net energy balance, reduced environmental impacts in most examined categories, and lower lifecycle costs relative to conventional treatment configurations; however, the removal and/or capture of dissolved methane is required to reduce global warming impacts and increase on-site energy generation. With further study, anaerobic primary treatment can be a path forward for energy-positive wastewater treatment.
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