The objective of the present study was to construct a compact retrofit design of Microbial Electrolysis Cell (MEC) within an anaerobic digester. In this design, the cathode chamber is inserted in the anodic chamber for compactness, improved hydrogen production and wastewater treatment efficiency. The performance of the new design is compared with that of a conventional (dual chamber) MEC. A cumulative hydrogen of 40.05 +/- 0.5 mL and 30.12 +/- 0.5 mL were produced at the current density of 811.7 +/- 20 and 908.3 +/- 25 mA/m(2) respectively for conventional and modified MEC system. The cathodic hydrogen recovery (CHR) defined as the recovery of electrons as hydrogen which was observed a maximum of 46.5 +/- 0.8 and 38.8 +/- 0.5% in conventional and modified MEC. The Coulombic efficiency (CE) defined as the recovery of total electrons in acetate as current was observed as 17.25 +/- 0.15 and 16.82 +/- 0.1% for conventional and modified MEC. In addition, the wastewater COD removal efficiency was observed to be 77.5 +/- 1.0% and 75.6 +/- 1.5 in 70 h for conventional and modified MEC designs. As shown in the work below, the modified compact design worked effectively to produce hydrogen under different COD concentrations; anolyte and catholyte concentrations; and applied potentials. Thus the modified compact MEC which is also a retrofit to an existing anaerobic digester can extend the use of anaerobic digesters and improve their economics in waste water treatment. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.