Through novel application of small-angle neutron scattering, we examined the fate of silica nanoparticles (SiO(2)NPs) during simulated primary wastewater treatment, by measuring, in real time, the colloidal behavior of SiO(2)NPs in wastewater (sewage). We examined the effects of surface functionality on SiO(2)NP fate in wastewater, by comparing both unfunctionalized (uncoated or "bare") SiO(2)NPs and SiO(2)NPs functionalized with a thin coating of a nonionic surfactant (Tween 20), which is widely used in personal care and household product formulations containing engineered oxide nanoparticles. Our results show new evidence that the surface functionality of SiO(2)NPs plays a crucial role in their flocculation and sedimentation behavior in wastewater, and thus the likely efficacy of their removal from the effluent stream during primary wastewater treatment. Uncoated SiO(2)NPs did not flocculate in wastewater over typical residence times for primary treatment. Conversely, surface-functionalized (Tween-coated) SiO(2)NPs underwent rapid flocculation in wastewater. Our results show that the surface-functionalized SiO(2)NPs are likely to be removed by sedimentation to sewage sludge (typically recycled to land), whereas uncoated SiO(2)NPs will continue through the effluent stream. While nanoparticle design is driven by use purpose, this study shows new potential for exploiting surface functionalization of nanoparticles to modify their environmental pathways.