A wastewater treatment system includes wastewater having nitrogen-containing compounds, an anoxic zone having denitrifying bacteria, and an aerobic zone having nitrifying bacteria. The anoxic zone is coupled to the aerobic zone, and wastewater flows from the anoxic to the aerobic zone or vice versa. A fluidized bed heat exchanger configured to accept heat from a heat engine and to transfer the heat from the heat engine to the wastewater is positioned in the aerobic zone or the anoxic zone. The fluidized bed heat exchanger includes particulate media, and fluidization of the particulate media scrubs bacterial growth from portions of the fluidized bed heat exchanger. Treating wastewater can include flowing wastewater having nitrogen-containing compounds into a biological reactor having an anoxic zone and an aerobic zone, and heating the wastewater with heat from a heat engine to facilitate denitrification reactions in the anoxic zone and nitrification reactions in the aerobic zone.