Full-scale wastewater treatment plant monitoring: Insights from batch assays on nitrogen removal

Wastewater treatment plants (WWTPs) require efficient, cost-effective solutions to optimize performance. Traditional monitoring methods provide only indirect indicators of microbial activity, missing dynamic shifts in key microbial populations. This study demonstrates the advantages of combining batch assays with next-generation sequencing (NGS) for a more comprehensive evaluation of biological processes. Over 10 months, activated sludge was sampled monthly from a full-scale biological reactor to evaluate the activity of key microbial groups: ammonia- and nitrite-oxidizing bacteria (AOB, NOB), nitrate- and nitrite-reducing bacteria (NaRB, NiRB). During the first three months, a significant decline in nitrite oxidation rates was observed, followed by stabilization at 0.5 mg N/g VSS∙h, which was 12.4 times lower than the ammonium oxidation rates. The NOB suppression, accompanied by a reduction of filamentous bacteria, particularly Chloroflexi , suggests a correlation between filamentous bacteria decline and NOB washout. This phenomenon is further supported by a significant correlation between NOB activity and the sludge volume index. Additionally, the abundance of NOB bacteria- Nitrospira , was closely linked to the sludge retention time. The results confirm that integrating batch tests with microbiological analyses is essential to accurately assess nitrogen removal performance and understand microbial interactions that may not be captured through environmental monitoring alone. • Low AOB and NOB abundance did not impair WWTP nitrification efficiency. • Sludge retention time influenced Nitrospira abundance. • Reduction of filamentous Chloroflexi is linked to NOB washout and suppression. • Microbial community shifts influenced nitrogen transformations in the WWT.P