The moment analysis of pulse responses was applied to a biological activated carbon (BAC) bed to evaluate the microbial activity. Glucose and activated carbon fiber (ACF) were employed in experiments as a model tracer and an activated carbon respectively. No significant difference in biodegradation rates of glucose by microorganisms attached on different solid supports such as activated carbon, unactivated carbon and glass fibers was observed. However, higher microbial activity was obtained from microorganisms supported on the ACF in terms of the apparent degradation of total organics including metabolic byproducts by microorganisms, since part of the metabolic byproducts was irreversibly deposited onto the ACF.