Q.grubs eating roots of a lime tree
What is a safe treatment to save the tree and have the fruit safe to eat?

Certified GKH Gardening Expert
The main grub that feeds on the roots of citrus trees is Diaprepes, also called the citrus root weevil. Its larva is a damaging pest.
https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-root-weevil.htm
http://ipm.ucanr.edu/PMG/r107305001.html
http://entnemdept.ifas.ufl.edu/creatures/citrus/diaprepes_root_weevil.htm
http://edis.ifas.ufl.edu/document_cg006
These articles also include advice on treatment. Biological control is the safest option. This involves releasing natural enemies of the pest. Check with an arborist or local university extension office to see what control methods are available in your area.