Over in the Deep End there are a few individuals dealing with the possible side effects of propylene glycol breaking down and consuming chlorine at a rapid pace during opening.
@Chem Geek suggested a high saline (salt) water concentration as an alternative to propylene glycol, but as noted it would only achieve freeze protection to -21C/-6F. This is assuming it's mixed correctly.
Then something popped into my head - what about Calcium Chloride?
It's readily available at pool stores, some of you with concrete&gunite pools might already have some laying around?
You can buy it as icemelter in any cold area, or Dowflake in pool stores.
It's been used in loading heavy equipment tires forever to increase density and freeze protection in tires. While corrosive to steel I can't see it hurting PVC
A 30% concentration by mass weight should achieve -46.1C/-51F freeze protection.
And when you open up all you have is some added CH to your water