I have a similar system where I capture effluent from the washing machine for use in the garden during dry spells. I got a couple of food-grade 55 gallon barrels from the dump (free), and hooked up the washer to pump into them. I use a shallow well pump to pump uphill to the garden. Be advised the water will stink pretty badly if you don't use it in two or three days.
Do you do anything to the water or just use it as is in your garden? Everything I've read said as long as you didn't use bleach in the wash everything should be fine to use on edible plants. I could always use it to water grass and other flowers if so.