This study was conducted to evaluate the effect of commercially available engineered iron oxide nanoparticles coated with a surfactant (ENP Fe-surf) on effluent water quality from a lab-scale sequencing batch reactor as a model secondary biological wastewater treatment. Results showed that ∼8.7% of ENP Fe-surf applied were present in the effluent stream. The stable presence of ENP Fe-surf was confirmed by analyzing the mean particle diameter and iron concentration in the effluent. Consequently, aqueous ENP Fe-surf deteriorated the effluent water quality at a statistically significant level ( p < 0.05) with respect to soluble chemical oxygen demand, turbidity, and apparent color. This implied that ENP Fe-surf would be introduced into environmental receptors through the treated effluent and could potentially impact them.