Modeling of simultaneous removal of easily degradable substrates and chlorinated phenols in UASB reactors.

Abstract

A dynamic model describing the simultaneous degradation of easily degradable substrates (sucrose and acetic acid (HAc)) and 2,4-dichlorophenol (2,4-DCP) in upflow anaerobic sludge blanket reactors (UASB) was developed. Two critical factors considered in the multiple substrate degradation processes were sorption and substrate interaction during degradation. Experimental investigations on the sorption of chlorinated phenols under dynamic situations as well as the degradation kinetics of cosubstrates and 2,4-DCP considering sorption and substrate interaction were conducted first. It was found that partitioning was the dominant mechanism in sorption of chlorophenols to anaerobic granules and that metabolically mediated diffusion during sorption was negligible. Under a dynamic situation, anaerobic sorption of chlorophenols which follow sorption linearity and sorption-desorption singulanty in isotherms, can be described by a dynamic model incorporating linear sorption equation. Nonequilibrium sorption caused by diffusion limitations in anaerobic reactors was found to be negligible because of the strong hydrodynamic dispersion that prevails in anaerobic reactors and the high porosity of anaerobic granules. However, rmnor nonideal sorption phenomena were observed for 3,4-dichlorophenol (3,4-DCP) and pentachlorophenol (PCP), both of which showed sorption-desorption isotherm hysteresis. Substrate interaction during degradation of cosubstrates and 2,4-DCP resulted in the inhibition of acetogenesis and methanogenesis by 2,4-DCP. The effect of electron donors on 2,4-DCP degradation was found to be minimal. A modified Haldane type inhibition function was proposed to described the degradation of 2,4-DCP. On the basis of model discrimination results, the degradation kinetics of HAc and propionic acid (HPr) were defined by the uncompetitive inhibition and Haldane type inhibition functions, respectively, with 2,4-DCP as inhibitor. Acidogenesis of sucrose to HAc or HPr followed the Monod equation since no inhibiting factor was found for this degradation process. Knowledge obtained from the above investigations was used to develop a dynamic model for UASB reactors. Data that were obtained from experimental investigations on multiple substrate degradation in continuous UASB reactors were used to validate and veritjl the dynamic model. The model predicts the system responses for 2,4-DCP, 4-monochlorophenol (4-MCP), HAc, HPr and chemical oxygen demand (COD) concentration in the effluent. Based on model fitting results, it was found that the degradation rates of 2,4-DCP and cosubstrates, HAc and HPr, changed inversely as a function ofthe specific organic loading rate ofthe UASB reactors. The implication ofthis finding was fully discussed.