Harvesting and pre-treatment of microalgae cultivated in wastewater for biodiesel production: A review

•Microalgae biodiesel has high potential to replace fossil diesel.•Wastewater could serve as alternative nutrients source to cultivate microalgae.•Harvesting of microalgae is one of the obstacles to produce microalgae biodiesel.•Pre-treatment is required to enhance lipid extraction efficiency from microalgae.•Ozone bubble is a new method to harvest and pre-treat microalgae simultaneously.