A novel nutrient control method to deprive green algae of sulphur and initiate spontaneous hydrogen production

The green alga Chlamydomonas reinhardtii has the ability to produce clean and renewable molecular hydrogen through the biophotolysis of water. Hydrogen production takes place under anaerobic conditions, which may be imposed metabolically by depriving the algae of sulphur. Sulphur-deprivation typically requires the spatial and temporal separation of the algal growth and hydrogen production stages. This would typically require separate photobioreactors for each stage as well as a costly and energy intensive medium exchange technique such as centrifugation, making the process difficult to scale up. m of this paper is to show how these two stages are able to take place in a single reactor and hence eliminate the need for a separation step and for an additional reactor. To achieve this we have investigated the sulphate and acetate consumption and uptake rates during algal growth under different illumination conditions. The experiment has been repeated in various photobioreactor geometries in order to determine a reactor-independent relationship between the algal growth and nutrient consumption kinetics. Using this relationship, the initial sulphur and acetate concentrations of the algal medium have been optimised so that these nutrients run out at the exact moment when the maximum algal cell density is reached. This nutrient control method allows a fully-grown algal culture to enter spontaneous hydrogen production mode, eliminating the need for a medium separation technique and for an additional photobioreactor. en production rates and yields were measured by membrane-inlet mass spectrometry (MIMS) in a novel photobioreactor designed specifically to facilitate the green algal hydrogen production process. The nutrient control method of sulphur-deprivation has proven to be superior to the traditional methods of centrifugation and dilution. Hydrogen production by nutrient control reached a maximum rate of 1.30 ml/l/h and a yield of 112.7 ml/l, compared to maximum rates of 0.18 ml/l/h and 1.11 ml/l/h, and yields of 28.0 ml/l and 102.7 ml/l for the dilution and centrifugation methods, respectively. Nutrient control has been identified as the method of choice for green algal hydrogen production.