Can this screen-printing solvent be disposed of in the domestic sewage system or does it pose a risk to the environment?

I can answer this question from our direct experience: the disposal company that we use for our waste considers paper-hand towels that have been dirtied with ink to be hazardous waste with EWC code 15 02 02* (absorbents, filter materials, wiping cloths and protective clothing contaminated with hazardous substances).

As this is a complex matter, one that can be subject to various interpretations, I recommend that you seek the advice of a local specialised advisory organisation, or similar association, or of the specialised company that’s responsible for your waste disposal.