Gas–liquid mass transfer in a circulating jet-loop nitrifying MBR

Based on airlift configuration, a novel circulating jet-loop submerged membrane bioreactor (JLMBR) adapted to ammonium partial oxidation has been developed. Membrane technology and combined air and water forced circulation are adopted to obtain a high biomass retention time and to achieve a separate control of mixing and aeration. This study is intended to determine how gas–liquid mass transfer is affected by operating conditions. In a first approximation, liquid was assumed to be perfectly mixed. A classical non-steady state clean water test, known as the “gas out–gas in” method, was used to determine the gas–liquid mass transfer coefficient kLakLa. Air and recirculated liquid superficial velocities were gradually increased from 0.013 to 0.019ms-1 and 0.0056 to 0.011ms-1, respectively. Subsequently, the gas–liquid mass transfer coefficient kLakLa varied from 0.01 to 0.02s-1. It appears to be influenced by the combined action of air and recirculated liquid flowrates in the range 0.72–1.03Nm3h-1 and 0.30–0.58m3h-1, respectively, for air and liquid. Correlations are proposed to describe this double influence. Experiments were performed on tap water and a culture medium used for the autotrophic growth of nitrifying bacteria, respectively. Oxygen transfer appeared to be not significantly affected by the mineral salt (0.48gl-1) encountered in this medium.