Hello tapmbot,
You can find an LCI of the process you want to use. I would recommend searching in literature. If you have an LCI, input it in OpenLCA, in a new process, and you can use that process as a supplier/waste processing process.
Alternatively, you can build it yourself from proxy processes. Similarly to the first option, you would need to make a new process, and then add input and output to that.
If no data is available, you will have to find alternatives with lower data quality, such as using a proxy process for the waste treatment.
Best of luck