Microwave discharge electrodeless lamps (MDELs) have been examined for continuous on-site field treatment of contaminated water using solar cells to provide the necessary electrical power. The equipment aimed to decontaminate water in areas impacted by natural disasters such as the extensive earthquake that hit Eastern Japan in 2011. An energy-saving semiconductor microwave generator provided the microwaves to activate the MDELs in a microwave/UVreactor setup operated using a single-mode applicator and an E/H tuner. The herbicide 2,4-dichlorophenoxyacetic acid degraded within a shorter time compared to methylene blue at a defined flow rate through the microwave/UV reactor. Sterilization of natural water contaminated with the bacterial microorganism Escherichia coli was complete within 20 min.