Proposing solutions for potable water supply in developing countries is of great importance. Not only is water shortage an issue, but the poor chemical and biological quality of water sources in these countries cause millions of deaths due to disease every year. Taking advantage of the high levels of solar irradiation in those countries, the use of solar systems for water treatment is possible. For this project, a passive valve-less flow-through solar thermal water pasteurization system was designed, built, and tested for the microbiological decontamination of water. Based on standard elementary school’s water consumption, we designed this system to provide up to 1,000 liters of clean, pasteurized water per day. The system utilizes solar collectors to absorb radiant energy for heating, and it utilizes a heat exchanger to preheat contaminated water entering the system. For this type of system, the heat exchanger may provide up to 70% of the heat transfer contributing to thermal inactivation of waterborne pathogens, so its design and performance is critical for the system’s overall efficiency.