Ideally, hydroponic solution should be recycled through the system repeatedly, but when it's time to replace it, take care to dispose of it in an environmentally responsible manner. One great option is to dilute it 1:1 with clean tap water and use it to feed your houseplants and garden plots. Your trees will probably enjoy a little boost as well.
If you've got too much solution to spread out that way, offer some to your neighbors! The worst way to get rid of it is to dump in a nearby storm drain. Stormwater often flows directly into nearby surface water, like lakes and streams, and the high concentration of nutrients can severely damage the local ecosystem by triggering large algae blooms. Even if the algae itself isn't toxic, the disregulation of the nutrient cycle can lead to the death (eutrophication) of the entire aquatic environment.
