A wastewater treatment plant and related method comprise a treatment stage including a biological-process substage configured for growing unicellular organisms adapted to reduce contaminants in the wastewater which are dissolved, including at least one of organic matter and nitrogenous matter, by digestion thereof, and which are adapted to floc after digestion and a floc-removal substage downstream from the biological-process substage, relative to the flow of wastewater, and configured for substantially removing the unicellular organisms that have flocked. The treatment stage is configured to form majority and minority flows of treated wastewater, and the minority flow is configured to be recycled upstream of the biological-process treatment substage. The plant includes a heat transfer assembly configured for transferring heat from the majority flow of treated wastewater to the minority flow thereof to increase temperature of wastewater to be treated.