Determination of dissolved oxygen limitation in aerobic biofilm reactors

Abstract

The concentration of dissolved oxygen (DO) strongly influences the performance of aerobic biofilm reactors because organic oxidation is limited by the availability of oxygen. However, it is not necessary to maintain a high DO level in the reactors in order to overcome this limitation. Excessive aeration wastes energy. Therefore, the determination of the onset of DO limitation against organic substrate removal in aerobic biofilm reactors is important for their effective operation. This study is aimed at developing an expression to determine the onset of DO limitation and hence to control the aeration system. The expression developed is as follows: C(b) < R(b) · (D(ws)/D(wc)) · S(b), where S(b) and C(b) are the bulk concentrations of organic substrate and DO, respectively; D(ws) and D(wc) are the diffusion coefficients of organic substrate and oxygen in the reactors respectively; and R(b) is an overall ratio of oxygen consumption to organic substrate removal in the reactors. The latter is the key parameter in the equation, and is determined by the characteristics of the substrate, biofilm, and reactor. In order to measure the value of R(b), the authors have developed a micro- biofilm reactor. The value of R(b) was determined to be 0.13 (mg O2 mg-1 COD(cr)) for glucose removal with this reactor. The equation has, subsequently, been verified with data from batch and continuous experiments.