how much would you NEED to charge? how many litres would you produce and how many would you need to sell per day?
what costs cover maintenance of the system, water bills etc also would you need someone to work in the yard to dispense the water to your customers.
if you need to cover £100 per day for instance.
I have given this some thought myself, but I am still having trouble working it out to be viable. I think it would be different if you had a janitorial supplies shop already set up, so staffing, premises, advertising etc isn't an issue, but to set it up on its own just from your garage and word of mouth could be quite a different thing. there is a place near me that sells it as part of a window cleaning shop set up, but I personally have only used it twice in 4 years.
good luck if you do work it out.