Raw Sewage is a burger made by Plankton, to finally make a Krabby Patty without always failing. It only appears in the episode "Patty Caper." In that episode, Plankton tells SpongeBob that he is on a list in the alphabet, and that he is on the letter R, and to steal the Krabby Patty Formula.
At the beginning, SpongeBob thought that Plankton stole the secret ingredient for the Krabby Patty, until he finds out that the burger isn't a real Krabby Patty and just a failed duplicate attempt.
The Raw Sewage burger looks similar to a Krabby Patty except it's darker in color. It also has green sauce on it which resembles raw sewage.