Traditionally, etched plates which have toxic residue on them have been washing in the sink, discharging waste into the water system through drains and sewers. Electrolytic solutions do not affect the aquifier (the water system) if certain precautions are taken.
ZINC SULFATE and FERROUS SULFATE SOLUTIONS
Plates that have been etching using a zinc sulfate or ferrous sulfate solutions may be washed directly into the sink with generous quantities of water, because they are not harmful and does not affect the sewerage systems or water table.
COPPER SULFATE SOLUTION
However, it is recommended that plates that have been etched using copper sulfate solution be wash in a rinsing tank. Prepare a rinsing tank which will comfortably accomodate the etching plate, and fill it with water and either clean iron filings free from machine grease, or a piece of fine iron wool for polishing which is sold in hardware shops. On contact with iron filings, copper ions quickly turn into harmless and easily disposable copper particles. The solution in the rinsing tank will turn yellow and, once filtered, can be emptied down the drain (but see comment below). When the plate is removed from the rinsing tank it can be washed with water in the sink.
Rinsing tank containing an etched plate and steel wool Use Iron Filings or Steel Wool in your tank.
Flushing the wash water down the drain may not be a problem but in these days of changing environmental legislation, local disposal regulations should be checked first. Generally educational institutions will have protocols for the disposal of wastewater and chemicals; sometimes this may preclude any flushing down the drain.