Controlling biological growth in rainwater tanks is one of the most important aspects of rainwater harvesting if the water is to be used for human consumption or stored for more than a week. There are a few ways of doing this, the two main methods are dosing a sanitizing agent like Chlorine via circulating system or sterilizing the water through a UV light placed in the tank. The problem with dosing is that you need to remove what you added, for example, chlorine before the water is consumed. This is where a UV light system is nice; it leaves no residual traces in the water. Often a combination of both works best.