I am trying to find if making energy from human waste (excrement, sewage) is profitable? Lets say I live in a city of 16 million people which does not put waste into use. What could be the profitability?
There are certainly viable ways to generate energy from all sorts of biological waste. Typically involving bio-reactors which use microorganisms to produce eg. ethanol or methane. some types of waste can also be burned directly.
Profitability is another matter and depends on a whole host of factors. On one hand there is the cost of building the initial infrastructure, it certainly seems likely that most urban sewerage systems would need a certain amount of modification and there are potential issues with separating non-biodegradable materials and industrial waste which can find their way into sewage.
Initial outlay and running costs will depend on the scale and design of the plant as well as how centralised it is in respect to the overall layout of the city and existing infrastructure as well as the availability of suitable sites.
There is also a reasonable possibility that any sort of novel human waste processing facility may meet with suspicion and resistance from residents in the planing stages.
Economic and regulatory factors such as taxation, land prices and long term energy costs in terms of estimating financial viability. Depending on jurisdiction this sort of project may also have complex licensing requirements.
Indeed any sort of large scale energy project is financially risky as the capital costs tend to be large and it usually takes a significant amount of time to bring a return on investment.
There are also a range of different technologies and infrastructure models to consider from fully centralised collection and processing to distributed systems which have micro-generators in individual households, buildings or city blocks all of which have their own pros and cons.
Clearly a distributed system is more of a retail business model like central heating or roof mounted solar panels where you are selling the plant while a centralised system is closer to the traditional utilities model where you are selling the energy.
While all of the above considerations are clearly highly relevant to the engineering side something like this goes far beyond being purely an engineering or technological problem. Indeed one of the difficulties with this field is that the engineering, economic, political and social aspects are so closely intertwined and it is hard to come up with a practical engineering solution without knowing what will work economically and vice versa.
Sounds like a thesis. The chemical reactions for decomposition of detritus and waste give energy to certain eukaryotes, so it's possible but out of the scope of a forum to determine financial factors without a process or business plan. That being said, carbon nanotubes (3D) can catalyze reactions unlike anything we've seen. Harness that and give me a call.