Effect of Anaerobic Reactor Compartmentalization on Excess Sludge Reduction in ASSR-Based Systems

Excess sludge management represents a major challenge in wastewater treatment, requiring effective minimization strategies. Anaerobic side-stream reactors (ASSR) have emerged as a promising approach to reduce sludge production. The effect of anaerobic reactor hydrodynamics on sludge minimization was evaluated under identical anaerobic exposure time (AET). A laboratory-scale activated sludge system was coupled with a compartmentalized ASSR composed of six reactors in series, approximating plug-flow behavior, and compared with a fully mixed configuration. The compartmentalized system achieved significantly lower sludge yield (Yobs), approximately two times lower than the single-compartment reactor at the same AET. This was attributed to the formation of redox gradients, which enhanced microbial stress, endogenous decay, and biomass breakdown. EPS destructuration was observed, with decreasing EPS and increasing SMP, confirming the role of hydrolysis in sludge reduction. Despite a slight decrease in sludge settleability, COD removal remained high. Reactor hydrodynamics was found to play a key role, and compartmentalization proved to be an effective and costefficient strategy for sludge minimization.