Hydrodynamics and mass transfer of concentric-tube internal loop airlift reactors: A review

A critical review of ozone-based electrochemical advanced oxidation processes for water treatment: Fundamentals, stability evaluation, and application

In recent years, electrochemical advanced oxidation processes (EAOPs) combined with ozonation have been widely utilized in water/wastewater treatment due to their excellent synergistic effect, high treatment efficiency, and low energy consumption. A comprehensive summary of these ozone-based EAOPs is still insufficient, though some reviews have covered these topics but either focused on a specific integrated process or provided synopses of EAOPs or ozone-based AOPs. This review presents an overview of the fundamentals of several ozone-based EAOPs, focusing on process optimization, electrode selection, and typical reactor designs. Additionally, the service life of electrodes and improvement strategies for the stability of ozone-based EAOPs that are ignored by previous reviews are discussed. Furthermore, four main application fields are summarized, including disinfection, emerging contaminants treatment, industrial wastewater treatment, and resource recovery. Finally, the summary and perspective on ozone-based EAOPs are proposed. This review provides an overall summary that would help to gain insight into the ozone-based EAOPs to improve their environmental applications.

Performance and mechanism of simultaneous nitrification and denitrification in zeolite spheres internal loop airlift reactor

An internal loop airlift reactor was constructed with zeolite spheres as biofilm carriers (ZS-ALR), and the performance and mechanism of nitrogen removal were investigated. The results indicated that the TN, NH₄⁺-N and TOC removal efficiencies of ZS-ALR reached 96.12%, 100% and 94.54% under appropriate conditions (HRT of 6-8 h, aeration rates of 80-120 mL/min, C/N ratios of 4-6), and the highest TN removal rate constant was 0.01156 min^-1. Further investigating the influence of ammonia-N concentrations on nitrogen removal and biofilm stability revealed that catabolism was important in TN removal, and the prominent genera for nitrogen removal included Sphaerotilus (42.20%), Flavobacterium (17.47%) and Fusibacter (6.14%). Meanwhile, the abundance of amoA, napA, narG and nosZ genes was markedly influenced by ammonia-N concentrations. The nitrogen removal of ZS-ALR was mainly through ammonia-N adsorption by zeolite spheres and simultaneous nitrification and denitrification by biofilm.

Use of Pressurized and Airlift Bioreactors for Citric Acid Production by Yarrowia lipolytica from Crude Glycerol

Citric acid production is generally carried out in an aqueous medium in stirred tank reactors (STR), where the solubility of oxygen is low and the oxygen demand of microbial cultures is high. Thus, for this bioprocess, providing adequate oxygen mass transfer rate (OTR) from the gas phase into the aqueous culture medium is the main challenge of bioreactor selection and operation. In this study, citric acid production by Yarrowia lipolytica W29 from crude glycerol, in batch cultures, was performed in two non-conventional bioreactors normally associated with high mass transfer efficiency: a pressurized STR and an airlift bioreactor. Increased OTR was obtained by raising the total air pressure in the pressurized STR and by increasing the aeration rate in the airlift bioreactor. An improvement of 40% in maximum citric acid titer was obtained by raising the air pressure from 1 bar to 2 bar, whereas, in the airlift bioreactor, a 30% improvement was attained by increasing the aeration rate from 1 vvm to 1.5 vvm. Both bioreactor types can be successfully applied for the citric acid production process using alternative ways of improving OTR than increasing mechanical stirring power input, thus leading to important operating saving costs.

Recent progress on converting CO2 into microalgal biomass using suspended photobioreactors

Inhomogeneous light distribution and poor CO₂ transfer capacity are two critical concerns impeding microalgal photosynthesis in practical suspended photobioreactors (PBRs). To provide valuable guidance on designing high-performance PBRs, recent progress on enhancing light and CO₂ availabilities is systematically summarized in this review. Particularly, for the first time, the strategies on elevating light availability are classified and discussed from the perspectives of increasing incident light intensity, introducing internal illumination, optimizing flow field, regulating biomass concentrations, and enlarging illumination surface areas. Meanwhile, the strategies on enhancing CO₂ light availability are outlined from the aspects of generating smaller bubbles, extending bubbles residence time, and facilitating CO₂ dissolution using extra additives. Given the microalgal biomass production using current PBRs are still suffering from low productivity and economic feasibility, the possible future directions for PBRs implementation and development are presented. Altogether, this review is beneficial to furthering development of PBRs as a practical technology.