The algal-bacterial aerobic granular sludge (AB-AGS) system, which combines microalgae with aerobic granular sludge, is a sustainable and promising wastewater treatment method. The algae embedded in the aerobic granulation process allow for the algal and bacterial cells in the sludge to interact and form granular flocs, which increases the treatment efficiency and helps separate the treated wastewater from the biomass. Additionally, AB-AGS biomass can be harvested for the extraction of biolipids and alginate-like exopolymers, contributing to resource recovery. This study reviews the applications and resource recovery potential of the AB-AGS system, highlighting its proven effectiveness in treating a range of wastewaters, from low-strength municipal to high-strength leachate. Keyword co-occurrences analysis further revealed pollutant removal, extracellular polymeric substances, the microbial community, and wastewater treatment as research hotspots. Research trends indicated by the keywords with the most recent publication year focused on system application and granulation factors. Additionally, an increasing number of studies regarding AB-AGS for saline and antibiotic-containing wastewater have also been reported. This review also identifies several research gaps and suggests directions for future investigations in this field.

Trichloramine (NCl₃) is an inorganic chloramine that causes a pungent chlorine-like odor, and it is difficult to remove its precursors (nitrogen organic compounds and/or ammonia) completely from water. Powdered activated carbon, ozonation, and UV treatment have been applied for decomposing NCl₃, but free chlorine was also decomposed. So, it is necessary to develop a technique that can selectively control NCl₃ without losing free chlorine. UV light-emitting diodes (265, 280, and 300 nm) and plasma emission UV sheet (347 ± 52 nm, hereafter 350 nm) were compared to find the optimal wavelengths that decompose NCl₃ but not free chlorine. As a result, 90.6, 96.7, 92.5, and 77.8% of NCl₃ were removed at 265, 280, 300 (3,600 mJ/cm²), and 350 nm (14,400 mJ/cm²), respectively. On the other hand, free chlorine at neutral pH (hypochlorous acid is dominant) and slightly alkaline pH (hypochlorite ion is dominant) was not decomposed at 350 nm, but at other wavelengths (i.e., 265, 280, and 300 nm) the removals were more than 64%. Therefore, UV radiation at 350 nm can be candidates to remove NCl₃ while maintaining free chlorine. However, this method requires high input energy, and further study is needed for evaluating the practical applicability of this method by considering optimal reactor design.

In some sewage treatment plants in Japan, treated water with increased ammonia concentration is discharged to supply for the lack of nutrients in the seaweed farms. However, chlorinating treated water, including ammonia, reduces the disinfection effect and generates secondary products. In this study, the behavior of residual chlorine was investigated in a tidal river receiving effluents by seasonal operation. The findings indicated that the seasonal operation increased NH₄⁺-N, which acted as a chlorine consumer, consequently increasing the proportion of combined chlorine as a secondary product. In the field survey, the residual chlorine in the river channel under the seasonal operation demonstrated a tendency to disperse in tidal areas by the tides. Additionally, it is suggested that there is a possibility of a decrease in total residual chlorine concentration originating from the effluent and the increase in concentration due to lateral inflow from the numerical simulation result of a planar two-dimensional model that accounts for the temperature dependence of water in winter. Then, the residual chlorine decay coefficient r₀ was roughly estimated to be 3 × 10⁻⁵ 1/s. Finally, it was shown that the residual chlorine from discharged water under seasonal operation does not necessarily significantly affect the receiving water.

Using a lab-scale activated sludge system equipped with an external selector for suppressing nitrite oxidising organism (NOO), the microbial population dynamics were investigated during about 400 days of the continuous operation. In the varied selector operations and excess sludge withdrawal, the nitrification, partial nitritation, and nitrification loss followed by the recovery of partial nitritation consecutively took place. The DNA analysis with the high-throughput sequencing revealed that only Nitrosomonas spp. retained during the nitritation–nitrification and only Nitrospira spp. could grow in nitrification, respectively. Proteobacteria was the most predominant heterotroph in the main aeration tank, whilst Bacteroidota was abundant in the external selector in association with the reduced microbial diversities. In the external selector, due to the high nitrite and ammonium, the ammonium oxidising organism’s (AOO) inherent enzyme genes (Amo and Hao) per total microbial gene copies were as low as about 50% of that in the main aeration tank. The dynamically calculated AOO and NOO concentrations using the IWA-ASM1-based model could be linearly correlated to both relative abundances in the activated sludge. The correlations suggested that the DNA analysis could be potentially utilised as an alternative tool to estimate the nitrifier biomass of activated sludge instead of performing kinetic modelling.

In this study, we analyzed changes in the community structure of sulfur-oxidizing bacteria (SOB) in a wastewater treatment process handling urban sewage over a 1-year period. Facultative anaerobic SOB, which can use nitrate as an electron acceptor, were predominant over obligate aerobic SOB in the activated sludge tank and clarifier/settler throughout the investigation period, with mixotrophic SOB being the dominant group. Facultative anaerobic SOB were predominant in most wastewater treatment facilities included in our comparative analysis, confirming observations in the investigated facility. Although the constituent species of SOB in the activated sludge tank and clarifier/settler remained consistent over the investigation period, the proportion of SOB relative to all analyzed sequences fluctuated between 0.91% and 6.20%, with periodic increases and decreases. Sulfuritalea hydrogenivorans, which was the predominant SOB in the activated sludge tank at the investigated facility, exhibited maximum and minimum 11.1- and 6.8-fold differences in abundance between the open and closed tanks, respectively. We conclude that sulfur oxidation by SOB during wastewater treatment can contribute to denitrification under anaerobic conditions. Furthermore, the population of predominant SOB species in the investigated wastewater treatment facility exhibited cyclical fluctuation.

Nitrous oxide (N₂O), known to be the highly greenhouse effect gas, is emitted from the biological wastewater treatment process. However, microbial control methods to reduce N₂O emission have not been established. Therefore, in this study, we focused on nitric oxide (NO) which is a precursor of N₂O produced by ammonia-oxidizing bacteria (AOB), to clarify the mechanisms of N₂O production by AOB and to experimentally verify the effectiveness of NO as a novel engineering indicator for reducing N₂O production. We verified the emission characteristics and interrelationships of NO and N₂O during the ammonia oxidation process using nitrifying bacteria cultivated in the laboratory-scale batch reactor. The results indicated that N₂O production by AOB can be predicted by observing NO emissions. It was also confirmed that AOB regulates the supply and demand of NO by themselves depending on the amount of ammonia oxidation, and produce N₂O as the way to decompose excess NO. Furthermore, we tested the applicability of our new findings to the full-scale wastewater treatment plant and experimentally demonstrated the possibility of reducing N₂O production by controlling NO concentration or chemical NO removal, even in a highly diverse microbial community that includes denitrifying bacteria.