In many countries, the algae of “green tides” are harvested in the fight against pollution. Ulva often represents the main component of the tide, and intensive research has been conducted on the possibility to use the algae as a methanisation substrate. However, methanisation is hampered by various practical obstacles, which requires a compromise between productivity and biological yield. The process described here calls upon a pre-digestion phase of Ulva which, besides the economy of time and volume of the digestion, makes it possible to obtain a biogas of good quality. The methanisation substrate is the hydrolysis juice collected by draining, followed by pressing. The cake resulting from the pressing process can be used as organic enriching or fertilizing agent in agriculture. Various presses were tested. The screw press was found the most suitable to recover a great quantity of sufficiently loaded pressing juice after only a short hydrolysis time. For a 3 month hydrolysis period, the different fractions amounted to 158 L of hydrolysis juice, 192 L of pressing juice, and 0.075 m3 of cake per m3 of initial algae. The bi-phasic anaerobic digestion with forced recuperation of juices offers interesting pollution abatement perspectives, with total and soluble chemical oxygen demand cleaning rates of respectively 79 and 95% during the methanogenic phase, for a volume productivity of 1.5 m3 CH4 m−3digester day−1. The quality of the Ulva juice also makes it suitable for use as substrate for industrial processes or co-substrate of methanisation in pre-existing reactors, so that subsequent investment could be avoided.
