The alternative would be to collect and ship it back which would require tons of energy that will have to be in the form of fossil fuels given current technolog

The alternative would be to collect and ship it back which would require tons of energy that will have to be in the form of fossil fuels given current technology. Said fossil fuels have a huge environmental footprint (greenhouse gas emissions and depletion of fossil fuel reserves) that is affecting us now. In contrast, the ice shelf is huge and still has a lot of capacity to contain sewage. It should remain frozen for the foreseeable future - if it melts (to the level where this sewage will become liquid) it means the global warming situation is so bad that there's likely no longer any humans around to actually witness this sudden deluge of sewage. As long as we manage to keep that ice shelf frozen, we're fine. If we somehow fail at it it's likely there wouldn't be anyone around to complain about the sewage anyway.