BACKGROUND: Ferric chloride (FeCl3) is widely used as a flocculating agent during wastewater treatment but can detrimentally lower pH and increase iron concentration. Microbial fuel cells (MFCs) are a promising technology for treating waste while concomitantly producing electricity and so were tested under the extreme conditions imposed by the addition of FeCl3. MFCs were fed eight concentrations of FeCl3 over two 8-week periods and the effects on power, pH, conductivity, metal content and COD were examined. RESULTS: MFCs generated highest power (3.58 W m(-3)) at 1.6 mmol L-1 FeCl3 (pH 3.46), however cells reversed when fed 2 mmol L-1 (pH 3.29). During the second run, power almost doubled and MFCs were more resilient at higher loadings up to 2.8 mmol L-1 (pH 3.02). Conductivity and pH increased following treatment while soluble phosphorus, sulphur and iron levels decreased significantly in all feedstock up to 1.6 mmol L-1 FeCl3. COD reduction was observed but efficiency may have been affected by the presence of alternative electron donors such as hydrogen sulphide. CONCLUSION: These findings demonstrate the robustness and versatility of MFCs in hostile conditions. They also confirm that MFCs can complement current wastewater treatment processes, even downstream from FeCl3 dosing where conditions might be deemed unsuitable for operation. (C) 2014 Society of Chemical Industry