If I understand correctly, the way these work is that microorganisms create a biofilm, and then the biofilm filters the water. The pore size is incredibly small

If I understand correctly, the way these work is that microorganisms create a biofilm, and then the biofilm filters the water. The pore size is incredibly small, so particulates and bacteria and so on are filtered out. In which case, I wonder if you could make a more space-efficient version by producing biofilms, or something similar, on an industrial scale. Whether in bioreactors optimised for film production, or just by making great sheets of agarose gel or something.