Aerobic granular sludge and microplastics in wastewater treatment: interactions, retention, process impacts and downstream fate

Wastewater treatment plants remove microplastics (MPs; 1 μm–5 mm) mainly by transferring them to solids. Aerobic granular sludge (AGS), with its dense, rapidly settling aggregates, and extracellular polymeric substances (EPS)-rich matrix, may favour particle association and solids separation. This review evaluates whether AGS outperforms conventional activated sludge (CAS) in MPs capture and retention, and assesses whether improved retention translates into reduced environmental release when the subsequent fate and management of sludge are considered. A structured search of Scopus, Web of Science and ScienceDirect (2,310 records screened) and citation searching identified 29 direct AGS studies and 172 field studies of MPs removal in full- or pilot-scale plants. Only nine AGS studies quantified the particles, and only one measured removal in a full-scale AGS train: 92%, against 93% for an oxidation ditch on the same sewage. Across 108 field studies of activated-sludge processes, median removal was 91% (interquartile range 79–97%), so AGS performs within that range; an advantage is plausible but has been examined in only one matched field comparison with solid-phase measurements. MPs may associate with AGS through attachment, EPS binding, entanglement, entrapment and co-settling, but they can also alter EPS, destabilise granules and impair denitrification. Most evidence comes from pristine particles in laboratory sequencing batch reactors, and long-term retention (beyond accumulation in laboratory reactors) or biodegradation of conventional polymers has not been demonstrated. AGS is best regarded as a plausible capture and partitioning platform whose net environmental value depends on stable retention and downstream sludge management. Priorities include matched AGS–CAS field comparisons, realistic particles and plant-wide mass balances.