Aims: In this study, we evaluated the performance of direct microbial fuel cells using citric acid wastewater as a substrate under different concentrations of MLSS 1000 mg.L-1 and 3000 mg.L-1. 
Materials & Methods: Aspergillus niger yeast was used as a microorganism over 4 days and nights of this experiment. A Nafion membrane was used for proton transfer, and graphite plates were used for electron transfer. COD removal efficiency, maximum open circuit voltage, power, and current density were evaluated. 
Findings: The general trend of energy production and removal efficiency showed that energy production increased with increasing MLSS. The maximum of these variables was recorded for MLSS of 3000 mg.L-1, achieving a removal efficiency of 93%, an open circuit voltage of 500 mV, and power and current density of 24345 µW.m-2 and 444 mA.m-2, respectively. 
Conclusion: Our results showed that the designed MFC suits wastewater treatment and energy recovery.