Regrowth of Fecal Coliforms in Class A Biosolids

Eight types of Class A biosolids were tested for fecal coliform (FC) reactivation and/or regrowth at 20, 35, and 50°C for 21 days. Growth of FC did not occur at 20 or 50°C, but it was observed in two samples incubated at 35°C after a lag period of 48 hours. In undigested biosolids, final FC concentration exceeded $10^{4}$ MPN/g, whereas in thermophilically digested biosolids, the final FC concentration remained below $10^{3}$ MPN/g, as FC regrowth may have been affected by the presence of the anaerobic bacterial consortium responsible for the digestion process. Fecal-coliform reactivation and regrowth within treatment plant operations seem unlikely but can occur in land application of biosolids.