I would like to first appreciate everyone in this forum.
I have been given a task to come up with the best possible option for water purification system in an emergency situation that will be cost-effective and sustainable. Target population: 2000.
Addendum: if you have more time, you can construct a wetlands.
We have had the experience of a wetland on the roof (!) of a building in Germany. All water from showers and toilets (!) was running through. At the end of the approx. 15 m wetlands the water has a high drinking quality, outperforming the German standards for drinking water.
The lowest low-cost system: for a population of 2.000 @ 3 l of water per person per day you must clean 6.000 l of water.
The following could be done with buckets, cans or with drums.
1. Gravel filter: fill vessel with gravel, let the water go in from the top, take the water out from the bottom.
2. Sand filter: same as step 1. If you put the vessel from step one higher than the sand filter, you need no pump.
After 100 l (approximation) you should back-flush both filters. Discard the back-flush water.
3. Put the water in empty, transparent PP bottles and expose to sun for 6 hours (recommendation of WHO). Also suitable are glas containers/flasks or PP bags.
In case, there is no sun, you need an UV flow through debacterisation (could be manufactured with low cost by yourself).
Attention: this method cleans and sterilizes only slightly dirty water. It removes not oil or potentially hazardous chemicals.
Take a look at the website for a company called MECO who makes water filtration systems for the US Army.
They make a portable system that will even process salt water into potable quality water. They are portable and could be set up for an emergency supply. Their LWP system is pretty robust and cost effective.
In an emergency situation it will still depend whether there is any access to water and of what source. Usually I would recommend a point of use system such as a water filter. We work with a company called basic water needs: basicwaterneeds.com . You can also do this in combination with drilling a borehole/ setting up a waterpump for example. Again, I do not know what the exact situation or the region or country is. Good luck. Mark