The article discusses high molecular weight polymers, which are natural anionic polyelectrolytes bearing an excess of negative charges. They are present in the effluents of waste water treatment plants employing biological processes. These colloids are derived principally from the capsular slimes of bacteria and other microorganisms. Although the absolute quanitity of polymers is less than 10 mg. l., the effects produced by these hydrocolloids have a large influence on physiocochemical processes used to purify waste water. The polymers offer great resistance to filtration and interfere with other membrane processes such as electrodialysis and heat transfer.