Stoichiometry and kinetic modeling of an anaerobic baffled reactor, The

Abstract

The current domestic wastewater treatment paradigm is centered around aerobic biological treatment processes. The aerobic treatment of wastewater requires a substantial energy input, while effectively wasting a significant portion of the potential chemical energy found in the raw wastewater. This energy intensive process is perpetuated downstream of the aerobic treatment scheme due to its effectiveness in maximizing solids production- an added cost and energy component for digestion, dewatering, and exportation. Mainstream anaerobic treatment exhibits promising potential of becoming the sustainable wastewater treatment process of the future because of its energy generation potential from organics that are otherwise converted to solids under the current paradigm, lower input energy requirements, and reduced solids generation. A concern with mainstream anaerobic treatment of municipal wastewater is the potential impact of a lower water temperature on organic removal rates and lack of knowledge about the overall process stability in locations with temperate climates. Anaerobic baffled reactors (ABRs) are essentially a series of compartments in which wastewater is directed through down-flow and up-flow conditions, passing through a solids layer that is retained in each compartment. This design allows for a decoupling of the solids and hydraulic residence time, meaning that solids are retained under varying hydraulic loading conditions. The ABR configuration fosters a naturally occurring spatial distribution of microorganisms that perform the sequential steps of hydrolysis, acidogenesis, acetogenesis, and methanogenesis, in the conversion of complex organic material to methane. An understanding of the stoichiometry and kinetics of an ABR receiving raw wastewater influent at ambient temperatures, is lacking. It is critical to develop this understanding in order to characterize the organic removal potential for further upscale and design of the ABR technology. A unique data set was available from a four-compartment ABR located in the headworks building of Plum Creek Water Reclamation Authority in Castle Rock, CO, treated raw domestic wastewater for two years. The 1000-liter pilot reactor was operated at a 12-hour hydraulic residence time, treating the raw wastewater at ambient water and air temperatures as low as 12 ºC. The data set was reduced to produce monthly averages of organics for particulate, dissolved and gas phase fractions. Four stoichiometric equations were developed based on model compounds. A simple reactor model with pseudo first order kinetics was fit to the monthly averages for measured and operationally defined organic fraction that were represented by model compounds. The model was used to relate the removal of different fractions of organic matter to process design and operation variables (e.g. volatile suspended solids, temperature). In this study, eight parameters were used to describe the anaerobic degradation of soluble and particulate biodegradable substrate in the ABR, and three substrates were used to develop the stoichiometry. Modeling was used to evaluate how the processes of hydrolysis, fermentation, and methanogenesis are interrelated in each compartment based on the operationally defined organic fractions. Such analysis can provide a framework to better inform anaerobic reactor design and understanding.