Ferric chloride and copper chloride ( the etching bi-product) are not environmentally safe and require proper disposal. If you have the money for it, chemical waste pick-up companies exist.
However, I prefer to take a different route. It may require a bit more work, but the cost is a fraction of hazmat fees.
You'll need
Solid sodium hydroxide
A proper blowtorch or oven retired from cooking
Lastly, it is very important to use a steel or copper alloy dish that can hold liquid waste, not aluminum trays. The leftover acid in the waste will react with the aluminum and potentially dissolve a hole in the aluminum tray.
The process will take the copper and iron chloride salts dissolved in the spent etchants and turn them into rust, water, and table salt. All things that are perfectly safe to dispose of in normal means.
To begin, add a 50/50 mix of spent etchant and water into a glass container. The container should be at maximum halfway full of liquid waste
Next, the solid sodium hydroxide needs to be added. This needs to be done extremely carefully. Sodium hydroxide is an extremely caustic material and if it gets wet will create a liquid capable of dissolving skin. I say this not to scare anyone, but to stress the serious respect this chemical demands.
The etching elixir and ferric chloride waste are both extremely acidic chemicals, whereas sodium hydroxide is a very basic chemical. When an acid and a base are brought together they neutralize each other creating salt and water. In this case, the hydrochloric acid in solution will react with the sodium hydroxide making water and sodium chloride, table salt.
The sodium hydroxide will also react with the metals dissolved in liquid form. The copper and iron chloride will react creating copper and iron hydroxide. This is important because unlike their previous forms, copper and iron hydroxide are insoluble in water and will sink to the bottom of the glass container.
Now that, that bit of theory is over it's back to the instructions!
Using an old steel spoon, scoop an amount equivalent to the size of your thumbnail and dump it in the spent etchant. Stir the mixture after the NaOH has been added. I like to use an old copper wire but a glass rod would be preferable if you have one,
After the mixture settles, and if the color is still a murky green color, more NaOH is needed. I like to wait about 12 hours between NaOH additions just to be safe.
Keep slowly adding and stirring the mixture and eventually, you will reach a critical point where there will be a clear liquid on top of a reddish or greenish sludge.
Once the water is totally clear it's safe to pour down the drain, I like to use a coffee filter to make sure none of the sludge accidentally makes its way down the drain. Pour the sludge into a steel container with walls like an old baking or muffin tray.
Once the sludge is separated we have iron and copper hydroxide that need to be broken down into iron and copper oxide, which we would call rust and have no issue tossing in the recycling. This process is done by heating the hydroxides into oxides.
What I like to do is to pour the copper and iron hydroxide sludge into a stainless steel tray with at least a half inch remaining between the top of the sludge and the rim of the tray.
Next, use a blow torch to heat the sludge until it starts bubbling. This signifies the reaction is working, extra hydrogen atoms are being removed from the iron and copper hydroxide, turning them into copper and iron oxide. A good way to check to see if this is working, is the bubbles produced when brought into direct contact with a flame will produce a small pop and an orange flame, this is the hydrogen combusting.
Keep torching the sludge until no more bubbles are produced from the heat of the blowtorch, the sludge should turn a rust color and be crunchy and brittle when the process is complete. After all the chemicals have been converted to their respective rusts. they can both be thrown away without issue.