A method of conditioning wastewater includes flowing wastewater into and through a first fluid tube and flowing a heat transfer fluid into and through a second fluid tube. The heat transfer fluid entering the second fluid tube has a different temperature than the wastewater entering the first fluid tube. The first fluid tube and said second fluid tube are positioned within a first casing that is surrounded by insulation. The first casing and the insulation are positioned within a second casing. The wastewater in said first fluid tube and said heat transfer fluid in said second fluid tube are arranged to allow heat transfer between the wastewater in said first fluid tube and the heat transfer fluid in said second fluid tube.