Components   Cistern Use In order to use rainwater collected in a cistern safely, a number of filtration and purification devices should be installed. The main concern is to protect the user from harmful substances that can enter the water via the rooftop. These hazards include dirt, rust, scale, silt and other suspended particles, bird and rodent feces, and airborne bacteria. To safeguard against these substances, filtration needs to occur at key points in the rainwater harvesting system. The first step of filtration occurs with a screen installed on the gutter to prevent leaves and other debris from entering the cistern (see maintenance section for concerns regarding screen buildup). The first flush of rainwater collected from the roof will contain high levels of contaminates, and must therefore be diverted from the main cistern. A “roof wash” device should therefore be installed to collect and isolate the first 10 gallons of rainwater from the main cistern. There are a number of “roof wash” systems that operate with a float (similar to the float device in toilet tanks) that controls a bypass valve. This step is not required if the collected water is intended for irrigation purposes only. Next, the cistern should contain a particulate filtration system. This can include one or a series of in-line filter to sieve sediment of five microns or larger. The cost of using two particulate filters in series, rated at 20 and 5 micron particle size, is about $40 with replaceable filter cartridges costing $3-5 each. Other filter options include mixed media or slow sand filtration systems. Activated charcoal is often used in conjunction with one of these other filtration devices and should always be used before, rather than after, disinfection. Disinfecting the water once it is collected kills microorganisms that have survived the filtration process. This step is only necessary under certain conditions (see regulations section). Ultra-violet light is the method of choice in the United States. Ozone, used mainly in Europe, kills microorganisms and turns organic material into carbon dioxide and water. Ozone remaining in the water is harmless. Other forms of disinfectant include iodine, bleach, chlorine (a carcinogen), and boiling before use. Further treatment via membrane processes such as reverse osmosis or nano-filtration can be used if there is a need to remove salts and metals from the water. This is usually not necessary due to low levels of these pollutants in rainwater from roofs.