Is It Okay to Throw Holy Water Down the Sink?

No. Holy Water is a sacramental, a material object that is blessed and used to increase devotion and should not be thrown out into sewage systems. Holy Water can remain in receptacles for long periods of time if need be or if there is a need to clear out fonts or holy water receptacles, one should take the water and use it to water the earth outside a church or it can be poured down the sacrarium in the sacristy which is a sink where the pipes go directly into the earth.