Comparing the effects of nanobubbles and conventional aeration on wastewater properties: can nanobubbles reduce disinfectant demand?

Utilities across North America are striving to find ways to optimize their treatment processes to meet permit limits in a more cost-effective manner. For treatment facilities that receive significant contributions from industrial users, this can be especially challenging. The International Standard Organization (ISO) defines nanobubbles (NBs) as gas-filled cavities with a volume equivalent diameter of less than 1 μm (Figure 1) [1]. Macro and microbubbles are larger than NBs and have been widely and effectively applied in water and wastewater treatment processes where gas-liquid two-phase flows are implemented (e.g., flotation, ozonation, aeration, oxidation, filtration) [2]. As the concentrations and composition of refractory organic and inorganic compounds in wastewater become more complex, conventional treatment processes become less effective and the development of new technologies becomes critical [3,4]. The benefit of NBs comes from their behaviour in liquids (Figure 2). Microbubbles in the aqueous solution disappear quickly in water, either rising up and collapsing at the surface or shrinking and dissolving in water, which limits their overall efficiency [5]. NBs are highly stable and authors claim that they can exist in water from a few hours up to several months [6]. Besides their high stability, NBs possess other distinctive properties that make them attractive to wastewater treatment applications, including high surface area-to-volume ratio, high negative zeta potential, low buoyancy, ability to generate radicals. This would allow NB to contribute to physical, chemical and biological water treatment processes in many ways [7,8]. Given the potential for lower energy use, reduction in chemical usage, and hypothesized free radical generation, NB are considered a green nanotechnology [9]. Many authors have documented the benefits of using NB on wastewater treatment in municipal and industrial settings and in natural water bodies. The most relevant studies highlight that NBs have positive impacts on BOD, COD, TSS, TDS, conductivity, TP and TN concentrations, as well as increasing dissolved oxygen (DO) levels and oxygen transfer rates (OTR) [2-7]. Studies examining removal of pathogens, indicator bacteria and other organisms highlight the efficacy of NBs in reducing bacterial and fungal targets, but they were mainly focused on the efficacy of ozone nanobubbles [4-5]. As the most prevalent BNR process, any improvements to the conventional activated sludge (CAS) process are highly desirable in the water industry. CAS requires significant aeration and most current technologies to expand CAS capacity are very costly. In comparison, NB have emerged as an interesting technology with the potential to revolutionize wastewater aeration due to its unique physics. Despite their presence on the market for over a decade, NB adoption remains slow and limited number of pilots have been commissioned worldwide with variable success rates and at smaller plants [5]. NBs have interesting properties that have been applied in many fields, but controversies, limitations and disagreements in the published literature interfere with practical implementation [1, 5, 7-9]. To our knowledge, the effect of nanobubbles on disinfectant demand and its surrogates has not been evaluated before. In September 2024, we conducted two weeks of field work at a wastewater treatment plant (WWTP) in Tennessee, USA, to better understand the potential improvements that can result from NB use. A 10L bench scale setup was used to compare the effect of NB generation (Trident TNS-1; mean bubble diameter 135.5 +/- 2.9 nm in DI water) against conventional fine bubble aeration (10 to 50 nm in diameter in DI water) in primary influent (Figure 1). The study was designed to address three main around differences in performance of conventional fine bubble aeration and NB, differences in performance at peak flow and the impacts of industrial discharge. Experiments were run for 30 minutes and samples collected every ten minutes (n=17). Four 90-minute runs were also completed for comparison. Samples were analyzed for total suspended solids (TSS), turbidity, apparent color, true color, total COD (tCOD), readily biodegradable COD [rbCOD, calculated through filtered (fCOD) and flocculated filtered COD (ffCOD)], total coliforms (TC) and E. coli (EC) using Standard Methods for the Examination of Water and Wastewater. Results showed that on average conventional aeration reduced apparent color, TSS, tCOD and rbCOD by 21%, 35%, 11%, and 77% respectively. It increased true color by 10% and had no effect on turbidity. In comparison, NB reduced true color, turbidity, TSS, tCOD, and rbCOD by 14%, 10%, 18%, 16%, and 97%, respectively. It increased apparent color by 5%. Results showed that the only variable that was statistically significantly different by two-way ANOVA between NB and conventional aeration was true color (F ratio = 9.035, p = 0.004) (Figure 4). Concentrations of total coliforms and E. coli performed by culture based methods were unreliable due to cells becoming viable but not culturable. Results showed little impact of between peak flow on NB performance, but industrial discharge had impacts on true and apparent color. We conclude with lessons learned and recommendations for utility managers interested in the technology.