Cellulose fibres are tiny long chains of repeating sugar molecules. Paper is just a wet slurry of these dried and pressed together. The only thing holding them together is that they are sort of tangled together. As soon as the paper gets wet, the fibres start to loosen up from each other and they become untangled. With toilet paper this only takes a few minutes. To answer your question, try an experiment. Get a piece of toilet paper and put it in a bowl of water and just watch for ten minutes. poke it now and then. You will see that very quickly it starts to come apart. There are many different kinds of bacteria, fungi, and other small organisms in the environment in lakes and rivers and certainly in the sewer system that can break up the cellulose chains and use the sugars as an energy source.