Integration of Activated Sludge Kinetics with Microalgae Systems for Carbon Footprint Mitigation in Wastewater Treatment Plants: Case Analysis of the Canary Islands

This study presents a comprehensive approach that combines the kinetic characterization of the activated sludge process with the application of microalgae culture systems to reduce direct and indirect greenhouse gas emissions in wastewater treatment plants (WWTPs). Based on experimental data obtained in laboratory reactors and real operating parameters extracted from a reference plant, the emission routes associated with secondary treatment were analyzed and the mitigation potential by microalgae in the tertiary stage was modeled. Comparative tables, CO2 capture scenarios and integrated operation diagrams were developed. The results show that the integration of both technologies can significantly reduce BOD residual load, improve energy yield and capture up to 135 kgCO2/day depending on the remaining substrate. The reduction in aeration demand reached 25%, and BOD removal increased from a range of 72 to 85% (conventional system) to 87–94% (hybrid system). The work supports the engineering plausibility and relevance of a hybrid sludge–microalgae system as an exploratory strategy for WWTP sustainability. Given the experimental design available in the historical laboratory record, the tertiary-stage outcomes are interpreted here as indicative algal–bacterial scenario responses. Because replicate runs under identical conditions and a parallel non-inoculated control reactor were not available, the manuscript does not present these results as definitive tertiary-performance validation. Complementary plant-scale records from the Canary Islands were considered solely as an external plausibility benchmark for the polishing-stage assumptions and were not interpreted as replicate evidence or as a substitute for a non-inoculated control reactor. The specific objective of this work is to derive activated sludge biokinetic parameters from laboratory assays and to use the experimentally constrained residual load framework to assess, at the scenario level, the mitigation potential of a tertiary algal–bacterial stage under Canary Islands conditions.