This peer-reviewed study provides the first comprehensive analysis of ammonium oxidizing bacterial (AOB) populations in South African activated sludge wastewater treatment plants. Six municipal plants across the City of Cape Town (CPT) and City of Ekurhuleni (COE) were evaluated using high-throughput sequencing and advanced statistical modelling.

The findings demonstrate that activated sludge (AS) nutrient chemistry — not climate or influent composition — is the primary driver of AOB selection. Specifically, activated sludge ammonia (NH₃/NH₄⁺), total phosphate (TP), and total chemical oxygen demand (CODt) were identified as the most significant environmental determinants of AOB community structure.

The genus Nitrosospira was found to dominate in plants with higher activated sludge nutrient concentrations, particularly in Cape Town facilities. Importantly, no significant seasonal shifts in AOB community structure were observed, indicating that winter temperatures in South Africa do not materially impair nitrification performance.

These results provide actionable insight for wastewater treatment plant operators, reinforcing that activated sludge chemistry monitoring is critical for nitrification stability and optimisation, rather than relying solely on influent metrics or seasonal assumptions.