 It has to be kept in mind, that these systems have a limited sustainability as the foils have to be replaced from time to time.For full balances, the investment for required support systems has to be calculated as well.Porous Substrate Bioreactor (PSBR), being developed at University of Cologne, also known as the twin-layer system, uses a new principle to separate the algae from a nutrient solution by means of a porous reactor surface on which the microalgae are trapped in biofilms.