[Strawbale] rainwater catchment for toilets?

[rick]

The only way the City water could be contaminated is if there is any  
cross-connection with its pipes. It sounds like your systems will be  
completely independent of each other so there is no problem. Only if  
both systems go to the same place at the same time, like a pipe from  
each going into the same tank, or direct physical connection of the  
two systems can there be any contamination.

If you plan to run the cistern system with supplemental flow from the  
City if it gets low, put a reduced pressure backflow preventer on the  
City line into the cistern. This type of backflow preventer has a  
physical air gap that vents to the atmosphere instead of allowing  
water to go into the City mains. This is the best type to use (it is  
good enough for hospital and morgue use) and should satisfy the  
inspector.

Rick