I've been working on Isolation of Pure cultures such as Scenedesmus sps, Phormidium Sps. etc. But I face a lot of hinder with Chlorella dominating other cultures.
Use BG11 or FOGG's medium. During the preparation of plates in laminar, streak near the flame. wrap it immediately with clean film and grow it in a growth room.
you can wash your sample with several time centrifuging, then dilute the culture and make streak plates to get single colonies. finally isolate the single colony and re-culture in a fresh sterilized medium. if it is not successful you can use antibiotic treatment.