http://westcoastpetroleum.com/how.htm
HUM BUGS (Hydrocarbon Utilizing Micro-Organisms) - These organisms are found everywhere in the environment. They enter the fuel tank by air and water.
They form a layer of scum at the free water-fuel interface, die, and settle on the tank bottom forming a gritty layer that appears like sand or black pepper. The combination of scum, bottom grit, and bad fuel is the major ingredient of BUG MUCK which enters the fuel system causing plugged fuel lines, filters, and gauges; corroded pumps and injectors; swollen and blistered hoses, washers, and connectors; increased fuel costs; increased maintenance costs; and unacceptable air pollution levels.