Link to SAFE WATER Report that explains the extreme need for future change. The amount of overflow depends on the individual cities attempts to overcome this. Link to the the old way to get rid of black water in the USA non-urban areas.
As water becomes more valuable because of scarcity, the purification of water will become even more important. New technologies can be large efforts that urban areas are trying, to smaller individual home or building units. All units have to go through a series of processes to make the water usable again. Here is a detailed article on this process: Black Water Purification.
I would think that future waste transportation systems will use other methods to carry waste away. Maybe instead of a garbage disposal system in sinks, the waste material would be carried away by augers or pressure to transport the waste to a composting container. The future homes will have to use ideology of aviation and space programs that have self-sustaining contained water/ sewage systems. Maybe chemicals and pharmaceuticals will need to use 50% less water to apply or transport the product to its needed function. Maybe the solids from the black water could be separated and burned for another home energy or heating source. Link to waste energy study.
European homes are already using in home black water /purification units. Public rest rooms can be built and not hooked to the grid. This may not mean much at first glance, however hooking to the grid costs all taxpayers over decades of use. Not to mention the actual cost to hook the unit to the grid. Like cars that verge several different energy supply sources and different methods to achieve the transportation need, homes will have to become more complex as well. Smart homes will be equipped with sensors in the waste plumbing to determine matter density and decide to dispose or direct it in a manner most effective to achieve efficient process for water purification and/or usage . Link to Hybrid Homes