Well, that's it: I hope you enjoyed looking into energy in water reuse and desalination using membrane technology.
For the purposes of this course we've considered energy as the key contributor to operating expenditure (OPEX). There are other contributors, but energy (quantified as specific energy consumption, SEC) is the main one.
Pressure has the same units as SEC. So, if a process runs at a high pressure and a low conversion of the feed to product water - as with RO for seawater desalination - the SEC is high.
Recycling sewage uses biological processes, which are slow and so require large land areas, but the overall SEC for reuse of sewage is less than half that of seawater desalination by RO
Of course, there are huge social and political barriers to recycling sewage for drinking water. It can still be used for other purposes, and is already used quite extensively for irrigation in some parts of the world. And, in just a few places, it is reused for drinking as well.