This is a green microalgae. It does look like Scenedesmus or Desmodesmus in unicellular form. 

Can you give us the information regarding the size (length and width) of the organism. Is this a marine or freshwater species? 

Thanks

Dear Eldrin,

Thank you so much for your answer! They are microalgae grown in digestate, so it is quite likely that it's Scenedesmus. Can you identify also the sorrounding bubbles?

I didn't find any Scenedesmus colonies in the picture, only unicellular organisms are visible.  Further it is better to isolate one or two unicellular structures and take the photograph.

the second photo looks Dictyosphaerium-like.  i also dont see Scenedesmus 

Thanks a lot

It will be necessary better images and high magnifications. (chlorococcales)

I am not sure waht do you mean with "the sorrounding bubbles" but the "round empty" structures looks like central diatom frustules