Septic tank access

[dnoah]

I have a formed inground tank that has two hugh lids with no access other than to dig up area to remove lids and pump out. I would like to know the best way to install an access port? how many? and what size?

[Stuart Meade]

The size of the access holes in the top of your tank are going to determine the size (diameter) while the buried depth will determine the height of riser that you will need.  The riser should encompass the entire hole.  This will ensure that water and soil don't have a way to get into your tank.  You will want to install two risers if you have a two compartment tank.  If not, one riser on the outlet side will suffice.
 
Measure the hole in the tank that you just uncovered.  Contact your local concrete precaster and tell them the size of your hole and buried depth of the tank.  They will give you some options on PVC and concrete risers.  Concrete will be heavier and harder install but will provide a better seal at the tank surface.   Be sure to clean the top of the tank well and put down tar tape between the top of the tank and the riser.  The heavy concrete riser will push down on the tar tape and give you a good seal to protect against groundwater infiltration. 
 
A PVC riser works the same way except for it is much lighter weight.  Be sure to get a heavy lid to fit on top of the PVC.  This keeps the kids out and helps to seal the tar tape.
 
Your riser(s) should be tall enough to extend above ground.  Don't give surface water a chance to seep in at your riser lid.
 
Good luck!