This report details the design of a decentralized water treatment system for use in Nepal, which currently lacks improved sanitation for 10.8 million people and a water contamination rate of 71 percent. This contributes to the approximately 140,000 deaths per year from diarrheal illnesses caused by such bacterial contamination. To combat this, the Phutung Research Institute (PRI) in Nepal has developed a low-cost optical sensor that can detect pathogenic bacteria in water. This sensor is a vast improvement over existing tools used to assess water quality, such as consumable test kits or laboratory analysis. However, detecting contaminated water is only one of two steps necessary to provide safe water to those who need it - the water must be cleaned. Such is the purpose of this project: to integrate this sensor in a design that both detects contaminated water and purifies it for people in Nepal. In addition to providing a practical application for PRI’s technology, this project applies other areas of design to create a holistic product intended to operate through its complete life-cycle within Nepal. User ethnography was researched to identify a ubiquitous water tank system as the implementation point, allowing for a single design to be applicable throughout Nepal’s diverse population. This also yielded additional pain points that are addressed to increase the acceptability of this product design in Nepal. Manufacturing and maintenance research yielded a modular architecture that can be constructed in Nepal as much as possible, thereby shortening supply lines and reducing costs while stimulating the local economy. Such a system also allows for the maintenance of sensitive components in the field without specialist intervention. These elements were combined to create a TRL 6 prototype designed to detect bacteria within a home’s water supply and automatically eliminate it with chlorine treatment. It is intended to both demonstrate PRI’s technology and to facilitate field testing in Nepal. This was done while improvising a design method called ‘Who, Why, How into What’. A new method was necessary to organize a project in which multiple diverged areas of design development had to occur simultaneously over a short period of time, barring the use of more traditional and better-defined design methods.