Abstract: Processes and apparatus are provided for the nitrification and denitrification of digested biosolids. The rate of nitrification/denitrification is controlled by sensing the amount of oxygen or another parameter in the digested biosolids and adjusting the amount of oxygen supplied to the biosolids, such as through automated adjustment of an aeration device. To promote nitrification, the amount of oxygen available in the digested biosolids is increased. To promote denitrification, the amount of oxygen available in the digested biosolids is decreased. The processes and apparatus are well suited for reducing the concentration of ammonium in biosolids or off-gas resulting from aerobic thermophilic digestion of wastewater sludges. The processes and apparatus are well suited for reducing the dosage of chemicals required for dewatering operations of biosolids.