What Makes Aerobic Granular Sludge Different from Flocculent Sludge

Diagram showing conversion of sbCOD to rbCOD in an STP. Particulate organic matter in a non-aerated collection tank is hydrolyzed under anaerobic conditions into soluble, readily biodegradable carbon. The converted rbCOD improves biological treatment efficiency, denitrification, phosphorus removal, and biomass activity in the reactor.

What Makes Aerobic Granular Sludge Different from Activated Sludge

While both aerobic granular sludge and activated sludge perform biological wastewater treatment, their physical structure and biological behaviour are fundamentally different. These differences explain why AGS-based SBRs outperform Activated Sludge SBRs in nutrient removal, stability, and efficiency, as introduced in Article 6.

Biomass Structure Defines Capability

Activated sludge forms loose, irregular flocs that remain fully mixed in the reactor. All microorganisms experience similar oxygen and substrate conditions, limiting biological diversity.

Aerobic granular sludge forms dense, compact granules. Oxygen penetration decreases toward the centre, naturally creating aerobic outer layers and anoxic or anaerobic inner zones.

Internal Zoning Enables Simultaneous Reactions

In Activated Sludge SBRs, nitrification and denitrification must be separated in time. In AGS, these processes occur simultaneously within the same granule due to internal redox gradients.

This capability directly supports the BNR pathways explained in Article 4.

readily biodegradable carbon is limited and valuable. Its premature consumption leaves insufficient substrate for denitrification and biological phosphorus removal, resulting in partial BNR or dependence on external carbon dosing.

Settling Performance Drives Stability

Activated sludge settles slowly and inconsistently, requiring longer settling times and conservative operation. AGS granules settle rapidly and predictably, allowing short settling phases that act as a biological selection mechanism.

This settling advantage improves biomass retention and reactor capacity.

Biomass Retention Without High Sludge Age

Activated Sludge SBRs rely on long sludge ages to retain nitrifiers. AGS systems retain biomass through physical structure rather than time, enabling higher treatment intensity in smaller volumes.

Greater Resistance to Load Variability

Granules protect internal microbial populations from hydraulic and organic shocks. Activated sludge flocs respond poorly to such variability, often requiring operator intervention.

This resilience addresses the influent behaviour described in Article 1.

Reduced Sludge Production

AGS systems typically produce less excess sludge and offer improved dewaterability compared to Activated Sludge SBRs, reducing downstream handling challenges.

Biology Replaces Mechanical Control

Activated Sludge SBRs rely heavily on mechanical control. AGS-based systems rely on biological selection through cycle design, reducing operational complexity.