Before water treatment became common, waterborne diseases could spread quickly through a population, killing or harming hundreds of people, but the primary goal of water treatment is to ensure that the water is safe to drink and does not contain any disease-causing microorganisms. The best way to ensure pathogen-free drinking water is to make sure that the pathogens never enter the water in the first place, however, this may be a difficult matter in a surface water supply which is fed by a large watershed. The goal of disinfection is to remove or inactivate all disease-causing organisms in water, however, testing for each type of pathogen individually would be costly and inefficient. Coliform bacteria are often found in the guts of warm-blooded animals such as humans, but can also be found in plants, soil, water, or air and it is relatively simple to test for the number of coliform bacteria found in water, and their presence indicates that other pathogenic bacteria are also likely to be present. During prechlorination, chlorine is usually added to raw water after screening and before flash mixing. Postchlorination, in contrast, is often the last stage in the treatment process. After flowing through the filter, water is chlorinated and then pumped to the clearwell to allow a sufficient contact time for the chlorine to act. From the clearwell, the water may be pumped into a large, outdoor storage tank such as the one shown below and then finally, the water is released to the customer.