Question: When I grate vegetables that are shevi’is, do I need to ensure that all the food remains on the grater go into the shemitta bin?
Answer:
It is permitted to wash up utensils that were used for shemitta produce in the regular way, even if the food remains will inevitably be disposed of while washing up.
A precedent for this is found in Derech Emunah (Volume 4, Hilchos Shemitta Perek 5 Tziyun Halocho 24) quoting the Chazon Ish, who allowed a grater to be washed up [in the regular fashion] after being used to grate shemitta vegetables, even though the food remains on the grater would inevitably be disposed of while washing up.
The sefer Mishpetei Eretz (Volume 2 Shemitta Perek 23 Se’if 10) posits that a minimal amount of food remains, that even the economically minded would dispose of and not keep, can be disposed up regularly, and do not need to be treated with kedushas shevi’is.