The acrylic polymer will safely disperse while it remains wet. Dried portions will remain in clumps but may break up depending on how big they are. The thing to be concerned about are the pigments, particularly those with heavy metals like cobalt and cadmium (and even worse, lead and mercury, which are not available in acrylics but are in oil). These can contribute to groundwater or soil contamination with a septic system, but are usually collected in the sludge during sewage treatment. The amount of heavy metal waste from a single art painter over a lifetime is probably not significant, especially compared with industrial applications. Of greater concern are waste solvents, but that's more of a problem for oil painters.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?