Microbial fuel cell sensors have shown great promise for continuous monitoring of toxic substances in wastewater, but a persistent problem is the signal interferences due to fluctuations in organic strength. An approach to eliminate the interferences is to saturate the sensor with an added organic substrate. In this study, signal stabilization using acetate and glucose addition (150, 300 and 500 mg COD/L) to domestic wastewater was examined. Addition of acetate (500 mg COD/L) gave the best performance, increasing the robustness of the sensor by reducing signal interference (decrease in baseline current) from 65% to 15% for a sudden 75% decrease in organic strength. The sensor sensitivity remained unchanged at current drop of 0.16%/(mg/L Cu(II)) for a toxicity event (300 mg/L Cu(II)). Addition of acetate (300 mg COD/L) and glucose (150, 300 and 500 mg COD/L) also resulted in increased robustness but led to a reduced sensitivity to Cu(II).