Despite STEM education is highly advocated in Hong Kong schools, teachers and students are still lack of the relevant personal experience and practical training, especially in an informal learning environment. This paper reports on a new event of STEM Olympiad 2016 organized by The Education University of Hong Kong and supported by the Education Bureau of the HKSAR Government and other institutions, aiming at providing senior secondary school students with informal learning opportunities to deepen their understanding of STEM and reflecting on their interest towards pursuing further study in STEM subjects and career; and creating an atmosphere conducive to the teaching of STEM in secondary schools and heighten teachers' and students' awareness of the importance of STEM education. Students participating in the STEM Olympiad completed a “Knowledge Challenge” on both individual and team bases, and competed in the “Innovations in Practice Challenge” to solve the local wastewater treatment problems. Each team had to design, assemble and test a treatment system prototype to meet the design criteria and to demonstrate their knowledge and skills in STEM without prior training and preparation. Before the final challenge, a mass expert lecture was delivered by an experienced engineer to enrich students' knowledge in the wastewater treatment and its underlying scientific principles. A series of 11 parallel education forum and workshops were also held to raise participants' awareness of the importance of STEM education as attended by hundreds of teachers and students. The competition yielded a total of 36 gold, silver and bronze medals awarded to the competition winners. We conducted student and teacher questionnaire survey and interviews after the competition to collect data on their perceptions and opinions of the event. As expected, the feedback is generally positive in many aspects. In particular, the students widened their vision in STEM and the teachers also obtained a holistic