Current status, potential assessment, and future directions of biological treatments of unconventional oil and gas wastewater.

Unconventional oil and gas (UOG) extraction techniques typically involve the production of large volumes of so-called flowback and produced water (FPW), a site-specific wastewater stream characterized by complex organic and inorganic composition. Sustainable and cost-effective management of FPW, as well as mitigation of its environmental risks and impacts, represents substantial challenges for governments, industries, and societies worldwide. Among various treatment technologies, biological processes have gained interest due to their low installation and operational costs. However, the interaction of FPW's complex composition with microorganisms poses challenging scientific and engineering questions. This review examines the water quality characteristics and sources of FPW from twelve UOG extraction sites in China and North America, revealing strong spatio-temporal heterogeneity of organic, inorganic, and microbial components across different reservoirs. The complex and variable water quality, large wastewater volumes, and high treatment demands have driven the exploration of biological treatments for FPW. This work systematically reviews and analyzes the operating conditions, treatment efficiency, and technical applicability of suspended sludge reactors, attached sludge reactors, mixed systems, and resource/energy recovery systems. Developing skid-mounted equipment based on suspended sludge reactors to handle variations in wastewater quantity and innovating the form of attached sludge reactors, especially in enriching salt-tolerant microbes for in-situ FPW treatment, are deemed essential. The dominant microorganisms playing a key role in the biological treatment are also discussed, with focus on two different inoculation sources (activated sludge and FPW). Roseovarius from FPW and Pseudomonas from activated sludge have strong adaptability to different reactors. The review further underscores the need to integrate biological treatments with complementary technologies. Finally, it advocates for the establishment of robust and scalable biological treatments through research in three main directions: (i) exploring microbial resources in original FPW; (ii) using omics technologies to elucidate microbial function and species interaction; (iii) pre-designing environmental and operational conditions to optimize treatment efficiency.