You can build a large under ground storage tank out of cement blocks just plaster it on both sides and use rebar in the construction, just add some chlorine to the water to keep the algae down should not be that expensive with local labor and it will last a very long time.
Tom / Roxas City
We built one like that a couple of years ago.. It was OK until the water went above half way up and the pressure pushed it through an inch of cement render and through the hollow blocks.
We have just built a steel reinforced 4" cement poured water tank.. The pour was -2-1 sieved sand/cement with 2 bags of sahara waterproofer and let it cure for 3 weeks..
It seems incredible but we filled it with 10 tons of water and it started seeping through outside after about 10 minutes.
The problem is that the tank is narrow,but long and high.. Its the height that is causing problems.. We are tiling the tank interior next week and grouting with a hydraulic water barrier paste..
We have a 20 tonne underground storage tank and although it doesnt seem to lose water,its hard to be completely sure.

Our local Barangay built a huge one on stilts made from CHB and render.. The water was in there for about 20 minutes..Leaked exactly like a sieve.. Abandoned now.. No budget to fix.