Raising the pH to 7 directly on a solution is not enough to remove all of the base metals as many base metal hydroxides are soluble at pH 7. Personally I raise the pH to 9.5 filter the sludge out, then neutralize to pH 7. What remains is salt water (sodium sulfate and sodium chloride) that can be disposed of via dehydration and disposal of the salt at the dump or the liquid can be taken to a waste water treatment plant that will accept the liquid. It would be wise to have the clear waste water tested first before deciding on a disposal route. The sludge is taken to the local landfill and given to the HHW (household hazardous waste) manger who stores it in a lined steel drum and has it hauled off when full for a fee. I pay $330 to have a single 55 gallon drum disposed of in this fashion. This cost does not include the cost of lye, waste disposal containers, labor, and PPE (personal protective equipment) on my end.
Here's a few posts and text that discuss the solubility of various base metal hydroxides:
Various Precipitation pH's of Base Metals
Hydroxide Solubility Curves
More On removing Metals from Solutions
Using lime is cheaper than sodium hydroxide (lye), but it's much less soluble in water and necessarily increases the volume of your waste solution for this reason. If you use a steel that contains chromium to cement the copper from the solution, you will be adding chrome ions to the solution as iron does not cement chromium. If you do not completely saturate the solution with iron, there is no guarantee that you have removed all of the other base metals from the solution. If you use junk steel that is painted or has coatings, or if you processed circuit board material and/or components with your solutions, they may contain organic compounds which complex with some metals, which can prevent normally cemented metals from precipitating on iron. If you have large excess of free acid in the solution, it can dissolve multiple metals on the activity series chart simultaneously in the same solution without precipitation.
If you choose to collect the copper from the iron only cementation, be aware that it may very well contain other dangerous metals such as arsenic (common dopant in integrated circuits and components), barium (found in surface mounted capacitors and components), cadmium, antimony, lead, nickel, mercury, and cobalt. Several of these metals produce dangerous gases when heated (eg.: when melting the copper).
If you have a known type of clean steel and know your solution only contains copper and nickel, then you should be alright simply using iron cemetation to saturation and raising the pH to 7.
As I mentioned before, if you have ammonia in your waste solution, you will likely produce a choking ammonia and/or ammonium chloride vapors when neutralizing the solution.
Know your scrap, reagents, and wastes.
Steve