It all depends on the digestion method. I attach one of my own papers for further information. 

Dear Dr.A.R.karbassi,

Thanks for your help.

The theoretical methane potential for Scenedesmus obliquus, based on its gross composition is 0.59-0.69 L CH4 g VS−1 (Sialve et al. 2009, see attached file).

The actual methane production is much lower (0.18 L CH4 g VS−1) and pretreatment may be useful to increase this production (Gonzalez-Fernandez et al., 2012)