Yes Bobad, there is always the weight of water. However an air diffusion style aerator/mixer actually moves a lot more water with a lot less energy. For instance, one style of our diffusers will pump about 12 MGD with 4.5 CFM. The 1/3 HP Thomas Wobble compressors can deliver this amount even at 15 PSI. Now, I'm not 100% sure, but I don't think that there are any 1/3 HP water pumps that can come anywhere close to pumping 12 MGD. This becomes important for us because when our aerators are used in an agricultural/industrial application (especially waste water) we can save our customers a lot of money (up to 50% of capital costs)by qualifying the customer for energy credits from our local energy company (PG&E) simply based on energy savings. The largest rebates seem to come from when a customer switches from a system that pumps or moves water (i.e. paddle wheels & aspirators) to an air diffusion style aerator in which the bubbles do most the work or to a solar mixer. Sorry to throw more into the mix sullpond but I have a bit of a passion for energy conservation. I'll be awaiting the hippie jokes.
