  Citation: Alsarayreh, M.; Almomani, F.; Khraisheh, M.; Nasser, M.S.; Soliman, Y. Biological-Based Produced Water Treatment Using Microalgae: Challenges and Efficiency. Sustainability 2022, 14, 499. https:// doi.org/10.3390/su14010499 Academic Editors: Simona Carfagna and Giovanna Salbitani Received: 19 October 2021 Accepted: 19 November 2021 Published: 4 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Review Biological-Based Produced Water Treatment Using Microalgae: Challenges and Efficiency Malak Alsarayreh 1 , Fares Almomani 1, * , Majeda Khraisheh 1 , Mustafa S. Nasser 1,2 and Yousria Soliman 3 1 Department of Chemical Engineering, College of Engineering, Qatar University, Doha P.O. Box 2713, Qatar; ma1808913@qu.edu.qa (M.A.); m.khraisheh@qu.edu.qa (M.K.); m.nasser@qu.edu.qa (M.S.N.) 2 Gas Processing Center, College of Engineering, Qatar University, Doha P.O. Box 2713, Qatar 3 Department of Biological & Environmental Sciences, College of Arts and Sciences, Qatar University, Doha P.O. Box 2713, Qatar; yousra@qu.edu.qa * Correspondence: falmomani@qu.edu.qa; Tel.: +974-4403-4140 or +974-6641-3198; Fax: +974-4403-4131 Abstract: Produced water (PW) is the most significant waste stream generated in the oil and gas industries. The generated PW has the potential to be a useful water source rather than waste. While a variety of technologies can be used for the treatment of PW for reuse, biological-based technologies are an effective and sustainable remediation method. Specifically, microalgae, which are a cost- effective and sustainable process that use nutrients to eliminate organic pollutants from PW during the bioremediation process. In these treatment processes, microalgae grow in PW free of charge, eliminate pollutants, and generate clean water that can be recycled and reused. This helps to reduce CO 2 levels in the atmosphere while simultaneously producing biofuels, other useful chemicals, and added-value products. As such, this review focuses on PW generation in the oil and gas industry, PW characteristics, and examines the available technologies that can be used for PW remediation, with specific attention to algal-based technologies. In addition, the various aspects of algae growth and cultivation in PW, the effect of growth conditions, water quality parameters, and the corresponding treatment performance are presented. Lastly, this review emphasizes the bioremediation of PW using algae and highlights how to harvest algae that can be processed to generate biofuels for added-value products as a sustainable approach. Keywords: produced water; oil and gas production; BTEX removal; biological treatment; chemical treatment; microalgae; harvesting 1. Introduction Produced water (PW) is comprised of an enormous amount of industrial wastewater (WW) generated from oil and gas extraction [1]. This water naturally occurs within the oil reservoir and is generated during the extraction stage [2]. Approximately 250 million barrels of PW are created every day by oil and gas industries, and more than 40% of that is released into the environment [3], which represents a serious environmental threat. The composition of PW is determined by the geological age, depth, geochemical composition of the area carrying hydrocarbons, the chemical composition of crude oil and natural gas in the zone, and the chemicals introduced during the exploratory process [4]. There is no constant volume of PW in oil and gas exploration as it is dependent on the geographic location of the field and the geological formation [5]. The constituents in the PW are toxic organic compounds such as benzene, toluene, ethylbenzene, and xylenes (known as BTEX), inorganic compounds such as heavy metals, total dissolved solids (TDS), chemical additives used during the oil production process, polyaromatic hydrocarbons (PAHs), and other pollutants [4–7]. The presence of these components in PW increases its toxicity, creates significant environmental concerns, and reduces the possibility of treating and reusing the water. Sustainability 2022, 14, 499. https://doi.org/10.3390/su14010499 https://www.mdpi.com/journal/sustainability