We first learn about the sewage field in "Speaking of Courage," when Norman Bowker can't stop driving around a lake while he thinks about what happened to Kiowa. Kiowa, the most moral character in the book and a Native American besides, is a symbol of American decency, drowned in the sewage of the war. The lake in "Speaking of Courage" is a stand-in for the field (as we know from "Notes"), and Norman's driving in circles around the lake shows his inability to talk about any of it. He's not able to cut directly to the heart of the matter – that he feels that decency was destroyed in the war – and so he talks around and around it. Sally and the town, a sort of stand-in for all of small-town America, hear the obscenity he's forced to use instead of listening to the reasons why he's using it. One of the symptoms of PTSD (that's Post-Traumatic Stress Syndrome) is the inability to speak of or remember an event without reliving it. Norman shows this here. Unable to speak about the war, he has to stop driving around and walks into the lake.
Later, in "In the Field," (the field here being both the sewage field that drowned Kiowa and the combat zone) O'Brien discusses the blame for Kiowa's death in the sewage field – or, in keeping with our metaphor, the drowning of American goodness in poop. The soldiers all feel guilty in one way or another – for following orders instead of trusting the Vietnamese, for a moment of stupidity in the field, or for their own brutal and disrespectful natures. Jimmy Cross and Norman Bowker both reflect that the blame is universal. The ignoble death of American decency in war is everybody's fault, in one way or another (which still doesn't mean we should dismiss the idea of personal responsibility).
The next time we see the sewage field is in "Field Trip," when O'Brien goes back to Vietnam to honor Kiowa's memory and to introduce his daughter, Kathleen, to the place that owns such a large part of his soul. Since O'Brien doesn't actually have a daughter named Kathleen and at the time of writing had never been back to Vietnam, we know there's some pretty heavy symbolism going on here.
Kathleen is modern America. She's young, she doesn't really remember the war, and she doesn't understand why her father is so darn obsessed with it, why he's still writing stories. By writing war stories, Tim is metaphorically returning to the sewage field to pay tribute to his dead comrades. He takes us along for the ride to show us what happened. We might not get it – Kathleen certainly doesn't – but that's not what the trip is about. The trip is about honoring the dead and remembering the moral sacrifices that we made in war.