consists of a fixed bed of rocks, lava, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media over which sewage or other wastewater flows downward and causes a layer of microbial slime (biofilm) to grow, covering the bed of media. Aerobic conditions are maintained by splashing, diffusion, and either by forced air flowing through the bed or natural convection of air if the filter medium is porous.
aerobic bacteria degrade sewage as it seeps through a bed of crushed stones covered with bacteria and protozoa
streams: recover rapidly from degradable, oxygen- demanding wastes and excess heat, caused by accidental or deliberate releases of toxic inorganic and organic chemicals by industries or mines, malfunctioning sewage treatment plants, nonpoint runoff of pesticides and nutrients from cropland or animal feedlots
lakes: plant nutrients, oil, pesticides, toxic substances (atmosphere)
groundwater: storage lagoons, septic tanks, landfills, hazardous waste dumps, deep injection wells
Ocean: dumping wastes