Remember that this tech needs to be portable, self-powered, (and in the case of those things left on-site) easy to use, easy to fix, eco-friendly, reliable, resilient, and if all possible, inexpensive.
It's a tall order. I know this. NGOs know this. But think about simple solutions like Plumpy'Nut.
Many of the solutions are already out there, like algae farms. They can use salt or freshwater. It produces oil, oxygen, biomass that can be used in human and animal food (it's even a superfood) or used to make ethanol, and it eats Nitric oxide (NO), Nitrogen dioxide (NO2), and CO2 which are all greenhouse gasses from the burning of fossil fuels. But the solutions haven't been made in a way that is useful to the people who really need it: the folks in non-developed countries.
Make it cheap. (Not US cheap... Haiti cheap, Sub-Saharan Africa cheap, etc.)
Make it portable. (Assume no roads, no airstrip, no nothing)
Make it reliable. (If your iPhone breaks 200-miles from anywhere, it's just junk)
Make it simple. (If construction requires a back-hoe, forgetaboutit)
Make it repairable. (If they can't fix it with what they have, forgetaboutit)
Welcome, and have at it!!!