The wastewater may contain recalcitrant compounds commonly including pharmaceuticals, hormones, pesticides, humus, lignocellulose, antibiotics, microplastics, and aromatics compounds. Bioremediation is a common approach to wastewater treatment due to its advantages of high efficiency, cost-effectiveness, and environmental friendliness, in which microorganisms play an important role in pollutant removal. However, the biodegradability index of wastewater, as indicated by the ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD), plays a significant role in determining the effectiveness of biological treatment. A low biodegradability value (below 0.5) indicates a lack of nutrients and the presence of poorly biodegradable substances that may be harmful to microbes, inhibiting their activity and resulting in high COD levels in the effluent, insufficient denitrification, and reduced bio-phosphate removal. Meanwhile, various oxidation techniques have been employed as a pretreatment methods to improve the wastewater biodegradability. Different types of reactors and bioreactors are used for chemicals and biological reactions in wastewater treatment such as fixed or packed bed reactors, fluidized bed reactors, agitated reactors, and bubble column reactors. Airlift reactors (ALRs) and airlift bioreactors (ALBRs) are a unique type of bubble column reactors with a simple design and flexible capacity that performs dual aeration and mixing functions consisting of the riser, downcomer, gas separator, and bottom zones. Internal and external loop ALRs offer advantages over conventional systems, such as simplicity of construction, reduced risk of contamination, and efficient gas-liquid dispersion with low power consumption, and are usually operated under atmospheric pressure. They have been applied for efficient treatment of high-strength wastewater through chemical coagulation, electrochemical oxidation, flotation, photocatalysis, ozonation, microfiltration, hydrolysis, ultrasound, and the Fenton process. They also provide efficient biological treatment of through high biomass concentration in a small space, shorten the retention time of microorganisms, and even provide liquid circulation and mixing due to hydrogen micro-bubbles generated during water electrolysis.

Despite significant progress in the field, scaling-up industrial slurry reactors remains challenging due to limited data on hydrodynamics and mass transfer. Thus, conducting an economic evaluation of different ALR and ALBR designs using life cycle assessment (LCA) becomes crucial for sustainable development in both developed and developing societies.