In article , says...

I would like to add a couple of risers to the manholes of my septic tank.
The tank is a concrete unit with 24" openings. I've been told that there are
concrete as well as plastic risers. I was wondering which is best and how
would you seal the risers to the tank? Also if anyone knows where I might
buy risers it would be appreciated.

Thanks
Joe


http://www.tuf-tite.com/risers.html