thanks for your questions.
Question 1) "What are you trying to filter/purify? Bacteria? Viruses? Chemical contamination?"
yes, originally I had a fourth design-criterion in it (but I took it out because I thought it would get too complicated). But you are right, I am not only worried about micro-organisms, but also about chemical contamination. which is completely neglected with most filtration-systems. Generally, this criterion is "kind-of" implied when speaking about "water-filtration", which means that it makes water safe to drink. I do not subscribe to the theory that a little bit of chemical contamination is harmless, but obviously it is difficult to set a high standard in this regards, unless one is using reverse osmosis or distillation. Because chemical contamination can be quite hazardous in industrial areas, even if it is "diluted", I'd say the filtration system should protect one from chemical contamination, and not merely from the (viral, bacterial, fungal) pathogens.
To make it specific enough for this challenge, lets put the fourth criterion back in (also added into opening post):
Criterion 4) Filtration. The system integrates all of these three steps:
a) pre-filtering to get the dirt particles out,
b) pathogen filtering and
c) chemicals filtering (oil, heavy metals, radioactive isotopes, poisons etc.).
since it is difficult to set a quantitative standard for c), let's simply say it has to have at least one way of chemical purification.
Question 2) "Are you saying every component of the system has to be able to be sourced around the globe? I am not sure anything can be sourced around the globe that could be reliably trusted to filter even bacteria. How could you trust an item you purchase in a third world country with the required fidelity/precision."
Almost. What I mean is that those parts that are likely to break down and that can not be repaired would render the user helpless. Sturdy parts that can take normal levels of abuse can be considered items that may only be sourced by the original manufacturer. Parts that cannot be maintained as they are too high-tech, fragile parts and consumables like filter-cartridges are considered impossible to replace on the go. If items can be improvised, that's acceptable too.
How to trust an item in the 3rd world, is a really great question. I think I would trust a regular coffee-filter, or some chemicals from the pharmacy. But it depends. Do you have any specific items in mind?