Where does the sand come ...

Where does the sand come from?? Are these open top tanks? Algae is understandable with no on-going disinfection. How often are the tanks filled? Is the water supply only turned on periodically? In many countries, the distribution system has so many breaks that the utility cannot leave the water on for very long or they will lose too much water to leakage. And when the water is turned off, the water then leaks back into the piping and the water that enters the piping when the pressure is turned off is typically contaminated. When the water is turned back on, contaminated water is delivered to the tanks. When the water from the roof top tank enters their house, do they drink it without further treatment? Do they only drink bottled water? Do they filter the water and then drink it? Do they boil the water and drink it? What is the normal use for this water? I can understand individual tanks vs a community tank. With a community tank, if no one has a finanacial motive to keep the water treated and safe, it will not be done. Is there a community building association? Is there a person in the building that can be trained to take care of the water tanks - and paid from a community fund (tenants of the building)? With individual tanks, the people living in the apartment, etc. will be much more inclined to take care of the tanks. What is the typical size of the tanks? For initial cleaning, there is probably no shortcut to manual cleaning. As I have stated before, if the tank is big enough to get in to for cleaning, make sure the tank is adequately ventilated. Many people have died inside unventilated tanks! Maintaining disinfection in the tank is the best solution in the long term, but this will require continual treatment and monitoring. If my guess about breaks in the main water lines is true, the on-going disinfection is important. Chlorine (bleach) is cheap. Are these tanks easy to get to on the roof? Chlorine test strips (even from a pool suppl;y store) are a relatively easy way to check for chlorine residual at the sink in the house. Have you written a report describing the details of your problem and the issues you are facing? That would help in determining the appropriate solution. Are the people in the apartments being educated on the importance of disinfection? As an example, I was surveying small communities in the north of Brazil. They have many rain water collelction tanks. The government provides them bottles of bleach to disinfect the tanks. Many of the people use the bleach to clean their clothes rather than treat the water!! The solution isn't always simple.