## 3 Materials and Methods  
In order to compare various types of commercially available NB-generation approaches for NB generation in water, five commercial liquid-flow-based NB generators were used in the present study, as well as two commercial submersible NB generators-- submerged either partly or fully in water. These are detailed in Table 1, with label numbers (1 to 7); in addition, the AquaB flow units are appended by \"V\" or \"C\" to denote Venturi- or compressor-fed for the upstream gas-input section. These units were not run for more than a few hours, given that the current study is not focussed on longer-term use, biofouling, or maintenance issues: rather, the focus is on absolute and comparative NB-generation efficiencies. It was decided to carry out aeration with the five flow generators (1 to 5), whilst air was supplied to the submersible units (6 and 7); neither ozone nor CO2 was considered for the submersible units in the present study. Although the \"hammermill-rotation\" Acriti microStar (6) is submerged in water, the associated 400 W motor needs to be kept above the water line, as this is not water proof. Of the five flow units, the Anzai one (5) requires more of a pre-pressurised gas source--air or oxygen, at typically up to 4.5-5 bar g regulator/delivery pressure--although, the best result for boosting DO requires the use of essentially pure O2 for efficient flow through the micro-nano-porous-medium membrane. To be fair in comparisons to other water-flow NB generators in the present study (1 to 4), we only discuss the use of air. For the Moleaer Kingfisher (1) and Acriti Turbiti (2), on the one hand, internal water pumps and compressed air (with partial internal ozone mixing) and, on the other, Venturi air-draw approaches are leveraged, respectively. This adds substantially to overall product complexity (including maintenance considerations) and operational energy requirements by way of larger pressure drops across the units. The energy consumption will be considered later. The AquaB flow units--both the in-line tubular pipe-style model (3) and the larger-flowrate vertically mounted radial-vortex unit (4)--may operate either with lower-pressure-drop coarser-bubble Venturi sections (V) or else with compressors or gas-cylinder feeds (C). The \"default\" of using a Venturi section for ambient-air capture works comparatively well vis-a-vis compressor or cylinder gas feed for, e.g., nano-aeration, when considering the level of Venturi-driven NB production, given the pressure-drop equivalence of Venturi use against air-compressor energy budgeting (vide infra). In terms of the water source, filtered tap-water was used: a three-stage water reverse-osmosis filter from AquaClear Ltd. was used for Dublin-area tap-water, and filtering was carried out until the derived count rate in a Malvern Zetasizer Pro (from Malvern Panalytical) was just below the 100 k.c.p.s. threshold, so as to minimise the level of (light-scattering) impurities. This is important for reproducibility and to allow reasonable levels of statistical error to be found. It was found that there was no NB detection found in background water samples without being passed through water-flow-based NB generators or used in conjunction with submersible units. For Dynamic Light Scattering (DLS), a Malvern Nanosight Pro was also used in the case of the flow units for comparison purposes, to use the NTA particle/bubble-tracking approach as arguably a more consistent bubble-counting/detection metric than Zetasizer Pro multi-angle DLS (MA-DLS), although nanobubble populations can be truncated for detection purposes in some cases below 35 nm, and the lower concentration detection