The treatment of different types of wastewater by physicochemical or biological (non-microalgal) methods could often be either inefficient or energy-intensive. Microalgae are ubiquitous microscopic organisms, which thrive in water bodies that contain the necessary nutrients. Wastewaters are typically contaminated with nitrogen, phosphorus, and other trace elements, which microalgae require for their cell growth. In addition, most of the microalgae are photosynthetic in nature, and these organisms do not require an organic source for their proliferation, although some strains could utilize organics both in the presence and absence of light. Therefore, microalgal bioremediation could be integrated with existing treatment methods or adopted as the single biological method for efficiently treating wastewater. This review paper summarized the mechanisms of pollutants removal by microalgae, microalgal bioremediation potential of different types of wastewaters, the potential application of wastewater-grown microalgal biomass, existing challenges, and the future direction of microalgal application in wastewater treatment.

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The present study investigated the growth, harvesting, biocrude conversion, and recycling of the HTL aqueous phase for one self-settling (i.e., Chlorocystis sp.) and another non-settling (i.e., Picochlorum sp.) marine microalgae. Both the strains were grown simultaneously in 2 identical 25,000-L raceway ponds in the Qatari desert. The cell size of Picochlorum sp. was small (2-3 µm), and its biomass was harvested using a centrifuge. Cells of Chlorocystis sp. (6-9 µm) formed flocs that settled spontaneously in a sedimentation chamber. Harvested biomass of these two strains was then converted to biocrude oil, using a 500-mL Parr reactor. The biocrude yield of Picochlorum sp. and Chlorocystis sp. was 39.6 ± 1.15% and 34.8 ± 1.65%, respectively. The energy content of the biocrude oil was 32.78 and 33.38 MJ/kg for Chlorocystis sp. and Picochlorum sp., respectively. Both the strains were capable of efficiently utilizing more than 95% nitrogen of the HTL aqueous phase. Although lower biocrude yield was obtained from Chlorocystis sp., compared to Picochlorum sp., harvesting of Chlorocystis sp. would require much lower energy compared to Picochlorum sp. Therefore, a self-settling microalgae (e.g., Chlorocystis sp.) could potentially be a better candidate, over non-settling microalgae, for producing biofuel feedstock.