Hydrothermal gasification of Acutodesmus obliquus for renewable energy production and nutrient recycling of microalgal mass cultures

Hydrothermal gasification is a process which uses any biomass or carbon-containing source as substrate to generate biogas of regenerative energy production. We used microalgae as biomass source and evaluated the potential of using the residual water of the conversion process as recycled nutrient source for cultivation of microalgae. Nutrient recycling was tested by monitoring growth of Acutodesmus obliquus and Chlorella vulgaris on residual water from hydrothermal gasification of A. obliquus. Four different gasification set ups were tested. After the procedure, all obtained liquid nutrient phases contained, beside nutrients, growth-inhibiting substances affecting photosynthetic activity and biomass yield of the two algal species. At least 28 potential toxic substances were found within one of the batches. Phytotoxicity on cellular structure was verified by electron microscopy. The cell form remained intact but cell compartments vanished. C. vulgaris was not able to recover to a vital growing organism during cultivation, whereas A. obliquus was able to restore cell compartments, photosynthetic activity and growth after 3 days of cultivation. A 355-fold dilution, UV treatment for 4 h and activated carbon filtration of the residual water from gasification finally enabled the discharge to support microalgal growth. UV treatment eliminated 23 substances but generated 4 new substances that were not detected before treatment. Activated carbon filtration eliminated 26 substances. Growth of microalgae obtained in the treated residual water was comparable with that in control medium. This study demonstrated the possibility to recover nutrients after the hydrothermal gasification process when the discharge got remediated to restart the value adding chain of microalgae and lower additional nutrient supply for microalgal cultivation.