Algal–bacterial granular sludge (ABGS) exhibits pronounced intragranular dissolved oxygen (DO) heterogeneity. However, the internal DO microenvironments under different oxygenation strategies remain insufficiently understood. In this study, intragranular DO distributions in ABGS were characterized under darkness, illumination, and artificial aeration. Results show that intragranular DO distributions varied with granule size and were differently influenced by artificial aeration and photosynthetic oxygenation. After 60 min of artificial aeration at an air uplift velocity of 2.8 cm s−1, DO at a depth of approximately 0.8 mm in granules with a diameter of around 3 mm remained nearly 0 mg L−1. In contrast, oxygen generated in situ via photosynthesis rapidly elevated intragranular DO levels, exceeding 4 mg L−1 at the same depth after 30-min illumination. This study shows that intragranular DO in ABGS can be dynamically restructured in response to distinct oxygen supply and consumption processes, which also provides an in-depth insight into better ABGS design and operation.

Reducing CO₂ emission and energy consumption is crucial for the sustainable management of wastewater treatment plants (WWTPs). In this study, an algal-bacterial aerobic granular sludge (AGS) system was developed for efficient carbon (C) assimilation and nitrogen (N)/phosphorus (P) removal without the need for mechanical aeration. The photosynthetic O₂ production by phototrophic organisms maintained the dissolved oxygen (DO) level at 3-4 mg/L in the bulk liquid, and an LED light control system reduced 10–30% of light energy consumption. Results showed that the biomass assimilated 52% of input dissolved total carbon (DTC), and the produced O₂ simultaneously facilitated aerobic nitrification and P uptake with the coexisting phototrophs serving as a C fixer and O₂ supplier. This resulted in a stably high total N removal of 81 ± 7% and an N assimilation rate of 7.55 mg/(g-MLVSS∙d) with enhanced microbial assimilation and simultaneous nitrification/denitrification. Good P removal of 92–98% was maintained during the test period at a molar ∆P/∆C ratio of 0.36 ± 0.03 and high P release and uptake rates of 10.84 ± 0.41 and 7.18 ± 0.24 mg/(g- MLVSS∙h), respectively. Photosynthetic O₂ was more advantageous for N and P removal than mechanical aeration. This proposed system can contribute to a better design and sustainable operation of WWTPs using algal-bacterial AGS.

A reasonable recovery of excess sludge may shift the waste into wealth. Recently an increasing attention has been paid to the recycling of extracellular biopolymers from conventional and advanced biological wastewater treatment systems such as flocculent activated sludge (AS), bacterial aerobic granular sludge (AGS), and algal-bacterial AGS processes. This review provides the first overview of current research developments and future directions in the recovery and utilization of high value-added biopolymers from the three types of sludge. It details the discussion on the recent evolvement of cognition or updated knowledge on functional extracellular biopolymers, as well as a comprehensive summary of the operating conditions and wastewater parameters influencing the yield, quality, and functionality of alginate-like exopolymer (ALE). In addition, recent attempts for potential practical applications of extracellular biopolymers are discussed, suggesting research priorities for overcoming identification challenges and future prospects.