Treatment of low strength wastewaters using microbial fuel cells (MFCs) has been effective at hydraulic retention times (HRTs) similar to aerobic processes, but treatment of high strength wastewaters can require longer HRTs. The use of two air-cathode MFCs hydraulically connected in series was examined to continuously treat high strength swine wastewater (7-8 g/L of chemical oxygen demand) at an HRT of 16.7 h. The maximum power density of 750 +/- 70 mW/m(2) was produced after 12 days of operation. However, power decreased by 85% after 185 d of operation due to serious cathode fouling. COD removal was improved by using a lower external resistance, and COD removal rates were substantially higher than those previously reported for a low strength wastewater. However, removal rates were inconsistent with first order kinetics as the calculated rate constant was an order of magnitude lower than rate constant for the low strength wastewater. (C) 2016 Elsevier Ltd. All rights reserved.