I have completed construction of sump at my site which is more than 11000ltrs. The dimensions inside the sump are length 102 inch, breadth 88 inch and depth 80 inch. I had plans for 10000ltrs and roughly made them to excavate based on volume calculations. As I have seen KVS Setty's post of his sump, it helped me to overcome the shuttering issue on the inner side.
step 1: excavated the soil 10 inch more than required on all sides. Then made them to put 40mm jelly concrete and compress it with dhimsu(don know it's proper name in english)

step 2: I made a 6" cement block wall all around like below.

step 3: 10 mm rods all around the sump.

step 4: concreted all around with shuttering for 4" retaining wall and 53 grade cement and waterproofing liquid. (sorry i have no pic with shutters)

Step 5: plastering all around with ppc cement and waterproofing liquid

I had been in person for all steps and arrived late while plastering. To my luck I found they had made plastering with 53 grade cement on all sides. I made them remove all those as it was still wet and remade the plastering with ppc cement. My advice to all owners is to instruct them in detail what to do and what not to.
Kindly let me know how I can paint the inner walls of sump with some waterproof-food grade paint you are aware of.
Thanks,
Bharath