Don't go below 50% without Water Wetter. The WW has anti-corrosion agents and can be used with pure water without corrosion problems. The WW works better with pure water and is less effective as the percentage of a/f increases.
As to disposal, I hope nobody is dumping it on the ground, or even worse, into a storm sewer. It causes more damage than you can possibly imagine. If you have city sewer for your toilets, dump waste coolant down the toilet. The water treatment plants have no problem filtering it. If you have a septic tank, you need to bottle it up and take it to a recycling place, or haz mat collection center - most counties will take it free but you may need to make a couple of phone calls to find out where. Most people who live in the city are lucky, you can dump it into the toilet... I moved to the country (septic tank) and I miss that convenience!
