Humans - part of the problem and part of a possible solution.
The human body is good at concentrating nutrients into waste, both liquid and solid. Around 80% of the nitrogen entering the human body is concentrated into liquid waste with similar levels for phosphorus, in essence this is a concentrated source of valuable minerals.
We then use water on a one-way trip to transport the waste away and in doing so dilute it down by a factor of up to 200, this is an opportunity for recycling and reuse, but it is not easy to solve.
Efforts to separate the solid and liquid human waste at source have been taking place, the Europeans have a large research effort. This must be both localized and decentralized, but once achieved you will have captured sort of the 80% of the Nitrogen and say 50% of the phosphorus from an urban population. Obviously, it reduces load on the main centralized system, and this is an attractive option
This hybrid solution sits between centralized and decentralized options.
A Chemical Engineering Opportunity.
There are many opportunities in this space for the Chemical Engineering community, can we solve the problem and deliver societal value?
Can Chemical Engineers develop small decentralized systems that can be mass produced sitting in the home managing the separation of waste in a way that eliminate the use of water as a transportation method, protects human health and exports waste streams for alternate transportation and recovery of valuable constituents. We have a range of modern technologies including computer systems, sensors, and networks to help, basically this is almost the opposite of the big centralized systems.
Think of something that has the analogue of a fridge. Plug it in and 20 years later you may buy a new one. You never do maintenance and they run and run and run until they die. But these are obviously not just a simple mechanical device, these are Chemical Engineering systems. I like the phrase Chematronics as it is like Mechatronics (embodies what the mechanical engineers did to exploit the new digital world).
There is nothing stopping really these being rolled out in developing countries situations as well and starting to become part of it of a new business model which embodies the concept of nutrient concentrators. They would take load off waterways because that that nutrient isn't going to be discharged. It may be worth to even go to a higher in high enough water quality so that the water can be reused even to Potable water standard, there's nothing fundamentally stopping this.
So, as we often say wastewater is just wasted water.