Biochemical oxygen demand (BOD) remains the key indicator of wastewater quality, yet the standard five-day test is far too slow to support real-time process control. We have developed a self-powered microbial fuel cell (MFC) biosensor that delivers continuous, near real-time BOD measurement, tackling the two barriers that have long held the technology back: signal instability and high material cost. The biosensor pairs a macroporous 3D-printed resin anode with an exsolved rhodium air-cathode that uses less than one-eighth of the noble-metal loading of conventional platinum designs, cutting electrode costs by 25%.
Crucially, we validated the device over 45 days of continuous operation using real urban wastewater from a UK treatment plant, testing both settled and unsettled streams to reflect plant conditions. The biosensor tracked changing organic load linearly across a wide range (up to roughly 1900 mg/L COD), with the electroactive biofilm responding to load changes within seconds. The low-noble-metal cathode proved markedly more durable, losing only 3% of its current response over 45 days against 17% for the conventional design.
These results demonstrate a credible, affordable route to online BOD monitoring for data-driven, digital process control.