This report describes a mission by the UN Environment/OCHA Joint Unit (JEU) to Tanzania following a power outage which affected the quality of local water supply. In May 2008, Zanzibar experienced a power outage, when the national power grid of Tanzania temporarily collapsed. For a month the power grid on the island of Unguja remained dysfunctional and people were dependent on alternative methods of electricity generation (mainly diesel generators). A United Nations Disaster Assessment and Coordination (UNDAC) team, including an environmental expert deployed by the JEU, carried out an assessment of the impact and provided a risk analysis of the power outage. The environmental assessment focused on drinking water supply, waste water treatment, municipal waste management and industrial installations. The supply as well as the quality of drinking water was seriously hampered by the lack of electricity, and emergency generators were installed at critical points. Short-term recommendations from the environmental expert focused on the provision of chlorine tablets and undertaking an information campaign at emergency public water points, installation of additional generators and increased monitoring of water quality by the authorities. Medium and long-term recommendations included flushing the entire water supply system once the electricity grid was fully functional and retaining the emergency generators as a back-up for possible future emergencies.