## 4 Results and Discussion
This arises from more efficient mesobubble formation upstream of the internal electrode arrangement for meso-to-nanobubble conversion. It can be seen from the superior AquaB NB populations that the use of an air compressor, if available, is often helpful if higher performance is desired vis-a-vis the energy input. However, in the case for flow through porous membranes in the non-AquaB examples, there is a non-linear relationship with pressure drop, which, unfortunately, hampers the energy efficiency of their NB production--and can, in time, lead to greater membrane biofouling (although the present study does not concern itself with the thorny issue of biofouling and ongoing operability issues) [41]. The estimated mass of oxygen in NBs is presented in Table 4, although this may be expected to be somewhat of an underestimate, as well as the overall mass created in the mid-range flowrate, and this latter parameter is then normalised per unit power, and the \"low-power\" mode of the AquaB in-line tubular NB generator is also presented (with the 'end user' either \"paying\"--or not--for the use of electric energy for pumping water which is roughly equivalent to the \"mains-water-line\" and \"Venturi-on-unit\" flow-pressure drops). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type/Model** & **Zetasizer Pro** & **Nanosight Pro** & **AC-DC (W)** & **Estd. Pressure-Drop** \\  & & & & **Pump Power (W)** \\ \hline
3V: AquaB In-line & & & & \\ Tubular Pipe & 13 *** \(\pm\) 6 & 14 *** \(\pm\) 3 & 6 \(\pm\) 1.5 & 55 \(\pm\) 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: NB populations (\(\times\)10\({}^{6}\) NBs per mL) and AC-DC electric power during AquaB “OffGrid”/”low-power” air-fed water-flow operation at municipal water tap-driven operation of \(\sim\)10 lpm water flowrate (\(\sim\)0.5 bar g discharge pressure) with Venturi air draw—type 3V. Electric-power water-pumping equivalent of water-flow pressure drop is estimated—which may or may not be borne by the end user of the NB generator. The number of asterisks shows the number of successful detections of nanobubbles out of three attempts. Table 4 shows clearly that the well-engineered flow turbulence of the AquaB radial-vortex flow layout, flowing about electrodes, is the most energy-efficient, in terms of producing one gram per minute of O\({}_{2}\) in NB form in its circa 200 lpm pumped water flow, or a deal more when using the air-compressor mode (as opposed to simply regularly dissolved oxygen--cf. Table 5). The mass-density of AquaB NBs is thought to be on the order \(\sim\)1.5-2 times that of mechanically produced NBs, bearing in mind the Laplace pressure of NBs: it would appear that electrostriction leads to a greater build-up of guest/gas molecules at the water-bubble interface [10]. Of course, overemphasis on the NB population per se (bearing in mind potential underestimates from Nanosight distance-detection diameter cut-off and/or potential spurious detection from the Zetasizer), or indeed the overall mass in NB form, and/or mass-density of the NBs themselves, can potentially distract the operator from considering other aspects that are also very important: chiefly, the presence of reactive oxygen species and the overall oxidative potential of the water (e.g., by way of Oxidative Reduction Potential) [19]. In many cases, it is arguable that NB-imparted oxidative effects [6; 18; 19], via hydroxyl radicals, for instance, are crucially important in securing the best outcomes form NBs. It has been shown that the de facto ROS population from the AquaB electric-field approach generates a significant level of ROS, using ORP and photoluminescence measurements [19], although this was less evident in the present study for mechanically generated NBs.