In a reverse electrodialysis setup, a chamber is separated by a stack of membranes that allow charged particles, or ions, to move in only one direction. Filling the chamber with salt water on one side and fresher water on the other causes ions to try and move to the fresher side. That movement creates a voltage. Adding more membranes increases the voltage, but at a certain point it becomes unwieldy.
By putting the bacteria in the side of the reverse electrodialysis chamber with the fresh water, and using only 11 membranes, Logan was able to generate enough voltage to generate hydrogen. Ordinarily he would need to generate about 0.414 volts. With this system, he can get .8 volts, nearly double. (The microbial part of the cell generates 0.3 volts and the RED system creates about 0.5.)
Using seawater, some less salty wastewater with sewage or other organic matter in it and the bacteria, Logan's apparatus can produce about 1.6 cubic meters of hydrogen for every cubic meter of liquid through the system of chambers and membranes. Another bonus is that less energy goes into pumping the water - if anything, flow rates and pressure have to be kept relatively low so as not to damage the membranes.