Figure 2.

System for energy delivery from wastes via biohydrogen A fusion of chemical and biochemical engineering for conversion of waste into electricity via integrated biohydrogen technology. Electrodialysis (ED) separates the organic acid (OA) products from the mixed‐acid fermentation of (e.g.) E. coli (formate is converted to H₂ + CO ₂ via formate hydrogen lyase). OAs pass from the dark fermentation medium to the photofermentation, typically being concentrated by ~eightfold via electrodialysis for dilution into the photofermentation vessel. Alcohol is not removed by ED; this would require a catalytic oxidation stage to give the corresponding organic acid; this has been achieved via using Au(0) nanoparticle catalyst made on E. coli cells (Deplanche et al., 2007). Two bio‐H₂ streams are formed from the combined dark‐ and photofermentations, with a third H₂ stream from electrolysis of water. The maximum H₂ yield from the mixed‐acid fermentation is 2 mol sugar⁻¹; hence, the dark fermentation can be viewed as a generator of OAs rather than as the primary H supply. A schematic of upstream waste conversion into sugar feed is shown (see text), and downstream use of hydrogen in a fuel cell for electricity production. Note that bio‐H₂ is free of catalyst poisons, which extends fuel cell life. Not all wastes (e.g. sugary fruits, bakery products) require extensive upstream treatment. The main box is the biotechnology; the grey flow sheet is the chemical engineering required to realize the positive energy balance. Both are equally important.