It depends on what solution you are using. Most ultrasonic cleaning solutions are biodegradable surfactants which are safe to flush down the drain as long as they are mixed with water to the correct cleaning ratios. As for the dissolved carbon and lead from the casings, almost all water treatment plants leach harmful lead and other toxins from the water before it enters outgoing pipelines. The real potential worry for lead contamination in the water supply is in the pipe material itself which can contain trace amounts of lead in the metallurgy.