Well, polution, hunger, traffic, stress, this and many others are only symptoms, the true problem, overpopulation, Underlings must be diminished, I have stated it quite clearly, before.
But yet the war against underlinginess is only starting, and resources are already scarce for us People.
Purifying water is most important in this time, it is expensive, residual waters have a lot of stuff in them, organic and innorganic, well, for this, I have devised a simple process:

With the help of a stirling engine, electricity is generated, this electricity is used to move an electric waterpump, the electric motor in this produces ozone, remember that funny smell toys and electric powertools have when used, thats ozone, well, the pump pushes residual waters through a filter, which traps most solids and stuff, organic stuff is used to produce methane, which can be used to produce heat, electricity again might come into action, water is evaporated, leaving more unwished substances behind, then, you condense the vapor, and then with the ozone you have produced, you kill bacteria in the water... some tweaks are needed for this process, and some research as well, but it might just work nicely.