Well the safest way is to dilute it in alot of water, then if you were being really good you could take it to the council dump and they can dispose of it, or, what I do, take it somwhere well clear of any water sources, dig a hole and tip it in and fill in the hole. It might not sound very enviro. friendly, but if you dilute the solution enough, and bury it, the ground will filter it out better than any man made filtration system.
