Abstract: In the treatment of a waste water containing formaldehyde by a method which involves introducing the waste water into a biological digestor into which there is also introduced another stream comprising a nitrogen-containing compound, especially ammonia or an ammonium salt, process difficulty caused by the formation of refractory adducts of the formaldehyde with the nitrogen compound (especially hexamethylenetetramine) is minimized by introducing the waste water and the nitrogen compound into the digestor in two separate streams instead of allowing the two streams to mix in the digestor feed line or in a preliminary holding pond. The method is especially applicable in systems in which ammonia or some other source of ammonium ions is introduced into the digestor as a nutrient for the contained biomass.