The water passes through the media layers to the underdrain, the standpipe, and is collected in a bucket placed below. The biological layer forms as organisms are captured and collect on top of the filter media (in this case, sand). The organisms do not get along and predate on each other. The accumulation of organisms and organism parts enables the biofilm to get better and better at captuing disease-causing bacteria and viruses (up to 99%!). next