How much are you talking about? 5 gallons? Take it to the local toxic waste day. 50 gallons? Call a lab and ask what they do or consider removing the silver yourself. This can be done with steel wool or with reclaiming machines (electrolysis). Once you get the silver out, dump it down the drain or take it to a town with sewage treatment and dump it down the drain. Remember that all of SF sewage and street runoff goes right to the ocean. Nice.