A thermoelectric Peltier heat transfer device is used to treat a fluid, such as a photographic processing solution to reduce or prevent biogrowth. The device include both hot and cold sides that are in direct fluid flow communication so that the same fluid is treated on both sides of the device. The treated fluid is subjected to flash pasteurization on the hot side of the device, and immediately and rapidly cooled on the cold side without loss of volume. Thus, the same fluid is treated on both sides of the heat transfer device in a closed system before being returned to desired chemical, photographic or other processes.