The role of light wavelengths in regulating algal-bacterial granules formation, protein and lipid accumulation, and microbial functions

A review on recent advancement in algal-bacterial aerobic granular sludge: From inoculation approaches to operation strategies

In recent years, the environmental unsustainability of traditional wastewater treatment processes, alongside the growing emphasis on carbon neutrality and self-sufficiency in wastewater treatment plants (WWTPs), has drawn attention to algal-bacterial aerobic granular sludge (AGS) systems. This review synthesizes recent advances in algal-bacterial AGS, focusing on the latest research on its granulation mechanisms under both hydrostatic and hydrodynamic conditions and providing a comprehensive overview of cultivation strategies based on various sludges. We discuss the latest reports on the impacts of process operation strategies for algal-bacterial AGS and emphasize the operation performance and design of different bioreactor configurations. Challenges for commercialization and sustainable applications are highlighted; future application directions are also posted. In summary, algal-bacterial AGS exhibits significant potential in simultaneous efficient wastewater treatment and bioresource recovery, targeting circular bioeconomy and sustainable wastewater industry.

Algal-bacterial granules for circular bioeconomy: Formation mechanisms and biotechnological applications

Cyanobacteria and microalgae are sustainable and renewable biocatalysts for solar energy harvesting, recovering nutrients from wastewater, reducing greenhouse gas emissions from wastewater treatment plants (WWTPs) and enable creation of a sustainable circular bioeconomy. Pure and axenic cultures of photosynthetic microorganisms have been widely studied for synthesizing bio-based products through improving the metabolic pathways via genetic engineering. However, pure cultures suffer from contamination and separation challenges when considered for environmental applications. Mixed microbial communities comprising of photosynthetic organisms and bacteria in the form of either flocs or granules have recently received a lot of attention due to their potential contribution to wastewater treatment, environmental sustainability and circular bioeconomy. The advantages of algal-bacterial granules (ABG) in WWTPs include effective elimination of contaminants and nutrients, reduction in aeration requirement, and production of biomass feedstock for downstream processing. Although ABG are an attractive option for energy positive wastewater treatment, it is not yet matured as technological option for deployment in full-scale WWTPs. Moreover, several aspects of ABG including synergistic metabolism, granulation mechanisms, granular stability, bioreactor operating conditions, cell-cell interactions, extracellular polymeric substances and bio-based products deserve more intense research. This article provides a detailed overview of algal-bacterial communities, their occurrence in natural environments, ABG cultivation in engineered settings, potential biotechnological applications and the recent progress made towards sustainable biological wastewater treatment and circular bioeconomy.

The accumulation and inhibition mechanism of extracellular polymeric substances of Chlorella vulgaris during cycling cultivation under different light qualities

The secretion, accumulation, and composition of extracellular polymeric substances (EPS) are pivotal factors influencing microalgal growth as well as wastewater recycling. Until now, the accumulation and inhibition mechanism of EPS of Chlorella vulgaris during cycling cultivation is not fully understood. The purpose of this study was to explore how different light qualities regulate the secretion, chemical composition, and structure of microalgal EPS, and subsequently influence the recycling of culture wastewater. After four cycles of cultivation, C. vulgaris under green light produced the highest EPS production and lowest biomass production, which were 82% higher and 17% lower, respectively, compared to white light, which yielded the least EPS production and the highest biomass production. EPS under different light qualities all exhibited a fibrillar structure with a sheet-like surface, but differed in composition. Compared with the other groups, EPS under green light showed a significant increase in polysaccharides, proteins, and humic acid-like compounds, as well as an increased proportion of arabinose and rhamnose, according to monosaccharide composition analysis. Transcriptome analysis indicated that the up-regulation of metabolic pathways linked to glycolysis/gluconeogenesis, TCA cycle, lipid synthesis, and ABC transporters promoted EPS accumulation. Additionally, EPS could target light-harvesting complex (LHC) and electron transport chain, down-regulating the photosynthetic pathway, which ultimately inhibited microalgal growth under green light. This study provides a theoretical foundation for the light regulation and circulation culture of microalgae, as well as for microalgal wastewater treatment.

A holistic valorization of treasured waste activated sludge for directional high-valued products recovery: Routes, key technologies and challenges

Global energy shortages and environmental crises underscore the imperative for a circular economy to tackle resource scarcity and waste management. The circular economy model encourages the recovery and reuse of valuable materials, reducing reliance on finite natural resources and lessening the environmental impact of waste disposal. Among urban organic solid wastes, waste activated sludge (WAS) emerges as a potent reservoir of untapped resources (including various inorganic and organic ones) offering significant potential for recovery. This review delves into a comprehensive analysis of directional valorization of WAS to recover high-valued products, including the inorganic matters (i.e. phosphorus, ammonia nitrogen, and heavy metals), organic resources (i.e. extracellular polymers like alginate and protein, volatile fatty acid, methane, hydrogen, and plant growth hormones) and reutilization of WAS residues for the preparation of adsorbent materials - the biochar. Moreover, the main recovery methodologies associated influencing parameters, product application, and attendant challenges for those diverse recovered resources are unveiled. Future research are encouraged to prioritize the development of integrated multi-resource recovery approaches, the establishment of regulatory frameworks to support resource recovery and product utilization, and the systematic evaluation of disposal strategies to foster a more sustainable and resource-efficient future. This work illuminates avenues for sustainable WAS management with high-valued resource recovery towards circular economy.

Nutrients recovery by coupled bioreactor of heterotrophic ammonia assimilation and microbial fuel cell in saline wastewater

Heterotrophic ammonia assimilation (HAA) process had been widely used in the treatment of high salt wastewater, but the electro enhanced coupling process and electron transfer process were rarely studied. In this study, a HAA process coupled microbial fuel cell (MFC) system was established to treat ammonia-containing wastewater under increasing salinity to achieve nitrogen recovery and electricity generation. Up to 95.4 % NH4+-N and 96.4 % COD removal efficiencies were achieved at 2 % salinity in HAA-MFC. The maximum power density and current density at 2 % salinity were 29.93 mW/m2 and 182.37 mA/m2, respectively. The residual organic matter in the cathode effluent was effectively removed by the anode. The increase of salinity not only enhanced the sludge settling performance and activity, but also promoted the enzyme activity and amino acid production of the ammonia assimilation pathway. Marinobacter and Halomonas were gradually enriched at the anode and cathode with increased salinity to promote ammonia assimilation and electron production. This research offered a promising solution to overcome salinity-related challenges in wastewater treatment and resource recovery.

Effects of light intensity and salinity on formation and performance of microalgal-bacterial granular sludge

Photosynthetic microorganisms in microalgal-bacterial granular sludge offer advantages in wastewater treatment processes. This study examined the effects of light intensity and salinity on microalgal-bacterial granular sludge formation and microbial changes. Activated sludge was inoculated into three bioreactors and operated in batch treatment mode for 100 days under different light intensities (0, 60, and 120 μmol m-2 s-1) and staged increases in salinity concentration (0, 1, 2, and 3%). Results showed that microalgal-bacterial granular sludge was successfully formed within 30 days, and high light exposure increased algal particle stability and inorganic nitrogen removal (63, 66, 71%), while chemical oxygen demand removal (>95%) was similar across groups. High-throughput sequencing results showed that the critical algae were Chlorella and diatoms, while the main bacteria included Paracoccus and Xanthomarina with high extracellular polymeric substance production. This study aims to enhance the comprehension of MBGS processes in saline wastewater treatment under varying light intensities.