In the beginning, human wastes will not really be a problem. However, as Oceania grows, the lack of land-fill, and the hope that we can be both economically viable and also have a low impact on the environment means that a solution will have to be found.
Given that the entire system is being built from scratch, waste recovery can be high up the list. The central problem is that the whole area is flat, and at a few feet above sea level, meaning that any system will have to be pumped rather than gravity driven. Whilst drainage for “grey” water, such as from sinks and excess rain water, can be allowed to flow straight through drain holes into the ocean below, “black” water, containing sewage and the like will have to be pumped somewhere for treatment before discharge. Large floating underwater septic tanks would probably be the best way to go, as this would allow gravity to be used, with the tank being pumped every few years. The main problem with this is the empty tank will be very buoyant, whilst the full tank will be very heavy. For more frequently used areas, a better solution would be a more traditional sewage system. I am not a waste management engineer, so any suggestions would be appreciated.