Acute dysfunction of granular-based anaerobic reactor triggered by short-term exposure to colloid-containing...

Inert colloidal substances (ICS) are ubiquitous in industrial wastewater; however, their effects on the stability of anaerobic granular sludge beds remain insufficiently understood. Using polyacrylamide as a model ICS, this study demonstrates that cationic colloids exert the strongest destabilising influence on granules, despite inducing only mild suppression of methanogenic activity (4.9-29.4 %). Strikingly, short-term colloidal exposure triggered acute reactor failure accompanied by irreversible sludge bed flotation. Mechanistic analysis revealed that the failure did not result from direct inhibition of microbial activity, but rather from a cascade of process destabilisation along the reactor height-initiated by spatial disruption of the sludge bed. Specifically, colloidal coating induced failure through a physical pathway: by hindering biogas release, it prolonged bubble detachment (up to several hours) and increased bubble size (to 1.23 mm), thereby generating persistent buoyancy sufficient to lift the sludge bed. The resulting flotation delayed pollutant-sludge contact and disrupted the critical anaerobic reactions that normally occur as wastewater flows through the lower sludge bed zone. This irreversible flotation ultimately caused severe deterioration of reactor performance. Overall, the findings highlighted the necessity of controlling ICS to ensure the stable operation of anaerobic granular sludge reactors.