I do not know of any ...

I do not know of any "international" certificates for a wastewater treatment plant operator. In the U.S., each state has their own requirements for getting a wastewater treatment plant operators license. I am licensed in Pennsylvania (S-A,E: 1,2,3,4) and New Jersey (Industrial N-4). There is an experience requirement where you have to actually work at a wastewater treatment plant before you take the licensing exam. As you did not indicate where you are located, you may want to start looking at local resources first. There are many wastewater treatment courses you can take where you get a Certificate of Completion (after passing a final test) but not an operators license. Feel free to contact me if you want more information.