You need to take it to the local hazardous waste disposal sight, of course its going to cost you money to dispose of, and you have to let them know what it is also. Tell them its fertilizer from hydroponically grown Tomatoes or something, not Pot of course.
I put my used ferts on my veggie garden in summer and down the drain in winter. I am going to try the grass this comeing spring and see if that makes much difference.