I'd be careful making nitrites from nitrate as a means of water treatment- if there are any amines around, even downstream of your effluent tailpipe, you could end up with a nitrosamine problem. Nitrosamines like nitrosodimethylamine (NDMA) are carcinogens and regulated to very low levels (parts per trillion in drinking water). They don't stick to activated carbon, they don't air strip, and they don't biodegrade rapidly, so they're not easy to treat. UV and UV/peroxide treatments do destroy NDMA, but it's far better not to make it in the first place. If you're going to de-nitrify, make sure the bugs take the nitrogen all the way to N2 gas. If this an "electroless" process and there are amines in the electroless plating solution, it would be best to stay away from denitrification. That being said, if you're interested in destroying nitrate prior to disposal, there are relatively few options available to you other than denitrification.
You'll also need to deal with the nickel- it's a toxic metal and is regulated in most jurisdictions. Plus it's a non-renewable resource which it would be great to recover and re-use rather than to lose forever. At 200 L/d and 1% nickel, you're only talking about ~ 2kg of nickel daily, which is only worth about US$10 or less, so there's no pure economic driving force to recover it- but that doesn't mean that you shouldn't do it.
Unfortunately, conc nitric acid with peroxide in it is a powerful solvent for nickel. It's doubtful that you could recover the nickel by IX or SX or EW efficiently from such a strong solvent to merely regenerate your acid, but others can comment on that much more knowledgeably than I can.
Neutralizing that much acid will take a lot of base, also costing $$$ and leaving you the nitrate to deal with, though it will permit you to precipitate most of the nickel as the hydroxide or rendering it more amenable to IX or EW. If you're going the neutralizaton/disposal route, perhaps you could switch to HCl instead of HNO3 (if this will work). At least then you'll only have to pay for the base and the metal sludge disposal- the chloride probably won't bother the regulators that much.
So you have a tough problem to solve! Perhaps others out there can comment on what people are doing with spent pickle liquour- enormous amounts of it are produced each year...
