Re: refine sewage
As someone who occasionally writes about both landfills and wastewater treatment plants in my job, I can tel you that both of these are FAR more interesting than a casual observer would ever guess.
It would be so much more environmentally efficient to separate out your gray water (sink, shower) from your black water (toilet) water so we don't have to spend so much money treating sort-of-dirty water the same as our really-dirty water, and so much more efficient to treat the water to two levels, a drinking-water quality that can be returned to surface waters (or to reservoirs) and a non-potable-quality that can be used for irrigation, landscaping, or to flush toilets. Communities in the future will have to be designed with much greater sensitivity to water constraints!
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.