.4. The DO concentration in different depths of bio fi lm in Phase 1. In Phase 1, only four membrane modules were aerated. The DO concentrations profiles within different biofilm depths are shown. in Fig. 5 (A). With air provided, the biofilms are all in aerobic. condition. With oxygen provided inside the hollow fiber mem-. branes, the DO concentration near the membrane surface is higher,. reaching up to 10.5 mg / l and then oxygen gradually diffuses out-. wards. Due to mass transfer resistance and consumption by. microbes during the diffusion process, the DO concentration in the. external biofilm significantly decreases to 3.1 mg / l. As shown from Fig. 5 (a), DO concentrations of the Unaerated. membrane modules decreases from outer to inner. The biofilm is almost in anaerobic condition and only a small part near the bulk. liquid is in aerobic condition. Due to oxygen diffusion from the water to the biofilm, DO concentration decreases to 0.3 mg / l inside. the biofilm and anaerobic zone forms there.