This study reports an investigation of the effect of the anode surface area on the performance of a single chamber microbial fuel cell (SCMFC) based biosensor for measuring the organic content of wastewater. A packed bed of graphite granules was used as the anode. The surface area of the anode was changed by altering the granule bed thickness (0.3 cm and 1 cm). The anode surface area was found to play a role in the dynamic response of the system. For a granule bed thickness of 1 cm and with an external resistance of 500 W, the response time (defined as the time required to achieve 95% of the steady-state current) was reduced by approximately 65% in comparison to a SCMFC biosensor with a carbon cloth anode.