After watching the magic school bus and travelling with my family around the world, I find the differences in water systems interesting.
What is an overview (diagram?) of how water flows for cities? Consider where it comes from, how it gets there, how it is cleaned, how it is taken away, how it is cleaned before we put it back.
http://www.thameswater.co.uk/cycles/accessible/sewage_treatment.html
The Real Scientific Paper on Sewage Treatment
Pick 5 completely different cities around the world.
- how do they get water to the people?
- how much water do they need?
- how do they treat their waste?
- what are some special needs of that location?