Relationship between temperature and growth of organisms causing Nocardia foams in activated sludge plants

Foaming, an operational problem in activated sludge plants, is commonly caused by actinomycetes of the genera Nocardia, Rhodococcus, Gordona, Tsukamurella and Mycobacterium, and such foams are often referred to as Nocardia foams. This paper addresses conflicting reports about the temperature at which foaming occurs by determining the temperatures at which various nocardioforms will grow in pure culture. Strains tested included foam isolates, type strains of organisms reported in the literature to be involved in foaming, and type strains of related actinomycetes. The data showed that all three categories contained organisms that grew at 5°C, and these were principally Rhodococcus spp. Thus the so-called Nocardia foams at low temperatures are more likely to be caused by Rhodococcus species. Although some foam isolates, especially Nocardia pinensis, grew over a relatively narrow range of temperatures, this growth range is still sufficient for them to cause problems in many plants. A few isolates grew at 40°C or higher, suggesting these were more likely to occur in plants treating warm wastewater or possibly could grow in foam where ambient air temperature is high.