Go here for the MSDS of HCA : https://www2.itap.purdue.edu/msds/docs/9725.pdf
The working solution is up to 50 g/l Na2SO3 which has a very high COD. It ranks up there with some developers and near or below hypo itself! I don't deny it is useful, but rather I argue that if you dump a 1 L tray, you have to contend with normal hypo/silver in wash #1, HCA that you dumped, and HCA / silver in wash #2. It is the hca dumped and wash #2 that I am pointing to. These must be at least handled properly.