[Strawbale] Hot water storage concept - insulation

[Tom Oswald]

Hi,
We are working out ways to store heat in water-mass for home hot water and limited/back-up space heating.  We plan to heat the mass by passive solar, fire-box, and possibly surplus PV.  We are considering using a concrete septic tank to hold the water in, even though it may not be the most green material to use.  We are considering placing the tank outside the foundation, under a deck, on the earth.  We want to avoid using foam insulation around and under the tank as it is not a green material.  We are thinking about using saw dust but are concerned about it getting wet.  We are seeking suggestions on how to do this most efficiently and most ecologically.  
Thanks,
Tom 
North Stonington, CT