Evaluation of Sanafoam Vaporooter II stress on activated sludge performance for carbonaceous removal and nitrification

Root intrusion control in sewer pipelines often relies on chemical herbicides, yet their unintended impacts on downstream biological treatment remain underexplored. This study investigated the effects of Sanafoam Vaporooter II (SVII) on carbon removal and nitrification in activated sludge reactors (ASRs) under long-term operation. Two parallel bench-scale ASRs were installed and operated at Subiaco municipal wastewater treatment plant in Perth, Western Australia: one was exposed to incremental SVII concentrations (1-3000 mg/L), while the other served as an unamended control. Stoichiometric analysis revealed stable organic matter removal across all SVII doses. In contrast, nitrification decreased progressively at concentrations exceeding 128 mg/L. Within the 750-1500 mg/L range, partial inhibition of ammonia oxidation was observed, resulting in an effluent ammonia concentration of 5.36 ± 3.4 mg/L. At 3000 mg/L, irreversible deterioration occurred, with >30-fold ammonia accumulation and Dichlobenil (DI) residues exceeding 500 μg/L. Acute exposure at 128 mg/L caused no loss of core performance, but nearly doubled DI residues (71.5 μg/L) compared with acclimated biomass, indicating reduced resilience without prior adaptation. A non-competitive inhibition model described a dose-dependent effect (R² = 0.76) and yielded a nitrification inhibition constant (k_i) of 4732.5 mg/L. This high k_i value suggests relatively weak inhibitory potency compared with many organic toxicants, though sustained high-dose exposure destabilises nitrifiers. Overall, SVII concentrations above 128 mg/L progressively impaired nitrification, while extreme doses (3000 mg/L) triggered irreversible process collapse, underscoring the need to consider herbicide applications in sewer systems when assessing downstream treatment performance.