There seems to be an assumption that you must dispose of the liquid after etching when using salt water. The salt water used during electrolytic etching can be reused over and over again; though, over time, you may need to add more pickling salt and distilled water. The method for dealing with the liquid is to let the particulate copper fall out of suspension and to then decant the saltwater into another dedicated container for reuse.
You can wipe the copper slurry out with paper towels or allow it to evaporate further and then carefully wipe out the powder. I prefer to wipe it out when it is still wet and cannot go airborne. Either way, you are left with copper that can be disposed of in a closed container in your regular household trash or be taken to a hazardous waste facility. The key is not washing that copper residue down the drain. As long as nothing goes down the drain, you won't have to worry about aquatic toxicity. This method is convenient because it doesn't leave you with bottles of toxic liquid that need to be stored until they can be taken to the hazardous waste facility in your area.
Please remember that the liquid that remains after etching is not the only copper in an aqueous solution that you need to deal with in an environmentally conscious manner. How are you dealing with your spent pickle? I save mine and use it with steel wool so that I can plate silver solder seams on copper items. I also plate brass with it to produce a more pronounced antiquing in recesses. You'll know when that plating liquid is ready for disposal because the blue green color will be gone and the liquid will have taken on a rusty appearance. The blue green is gone because the copper is now a part of the item that you plated (and plated onto the steel wool) rather than in the spent pickle. The rusty color is actually rust from steel wool particles in the acidic pickle. After neutralizing the pickling acid with baking soda, it can be safely poured down the drain. What kind of pickle do you use? I prefer to use an inexpensive mixture of white distilled vinegar and pickling salt (1 C vinegar : 1 Tbsp. p. salt). The more prolific metalsmiths might consider that the mist from pickle pots has been shown to cause pitting of the tooth enamel of those who frequently work (factory environment) with sulfuric acid pickles. I'm not willing to take that risk--however slight it may be.
Here is an article that explains how copper that is "bound to organic matter" is rendered inactive or not bioavailable.
Ferric chloride is not as non-toxic as some might believe. I strongly suggest that anyone who works with chemicals or is even a casual, occasional metalsmith take the time to read the material safety data sheets on the products that they use. Here is the MSDS for ferric chloride. A lot of information is readily available to us if we choose to be proactive in protecting our health and the environment.
Here is the MSDS for copper sulfate which is the compound (sludge at bottom of container) that remains after electrolytic etching.
Stay informed and stay safe!