■ INTRODUCTION Microbial electrochemical systems and technologies (METs) that can convert chemical energy stored in organic or inorganic matter into energy and/or to valuable chemicals have been studied extensively in the past decade. Generally, a MET reactor is composed of anodes and cathodes separated by membranes. Electrons are released from the microbial oxidation of substrate in the anode and are subsequently accepted by the cathode to complete a reduction reaction. After extensive fundamental studies to understand the electron transfer mechanisms, microbiology, reactor architecture and materials involved, METs are becoming a versatile technology platform for various applications such as wastewater treatment, bioremediation, environmental monitoring, and sustainable production of bioenergy and valuable chemicals. As examination of the technical viability continues, the economic feasibility and assessment aspects are being studied extensively, especially for METs that have come closer to being used in practical applications. The capital costs are one of the decisive economic factors for scale-up and commercialization of METs. Therefore, the reduction of material and construction costs has frequently been cited as the main objective in recent studies. However, the question remains as to whether the capital costs have been accurately reported for the METs that have been developed. Less than 10 out of several thousand publications have provided this information; thus, the answer is obvious. ■ WHY ARE CAPITAL COSTS IMPORTANT? The lack of data on capital costs makes a comparison between different systems, materials or technologies difficult and may create a large gap between the technologies and markets. In many cases, the advantages of the new systems or materials over the conventional ones for the reduction of the capital costs are obvious. For example, several electrode catalysts have been fabricated and have been demonstrated to be promising alternatives to platinum. The determination of which one is the most cost-effective and what the cost savings are is impossible as important information is always missing from reports. Thus, merely listing the technical advantages of a new technology is far from sufficient. Technologies should be costeffective to meet market demands. “How much does your system/technology cost?” is a question that often arises from industrial partners and has motivated us to write this viewpoint.
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