I also have a separate septic tank rider that specifically calls out nothing except toilet paper and enzymes can go into the tank.

I would then treat with enzyme (google enzyme for pet urine) usually available at local janitorial supplies.

I recommend looking into enzyme cleaners as well, as they are also effective in removing odors.

Enzymes. 

The sale was not triggered by the urine, but after washing the carpet it is somewhat apparent it had been urinated on, while the carpet was damp the smell got very strong.Professional carpet cleaners use an enzyme-based product for urine. 

It contains the enzymes that, in nature, break down the compounds in the urine, feces, etc.

For one tenant that had a cat (inherited tenant and the only cat I've allowed), I had to soak slab with enzyme cleaner, didn't work well, then soaked in vinegar, replaced subfloor boards upstairs, replace trim all around flooring -- it's a huge undertaking requiring multiple applications.     

The chemicals interfere with with enzyme process.

Remove anything that you can throw away, destroy any urea crystals with a sprayed on pet enzyme cleaning product and encapsulate whatever is left with a strong primer.

I put killz all over the walls, will probably end up replacing the trim boards, and drowning the corner in some kind of enzyme eliminating solution.