This paper is based on an academic work conducted by a group of students of the Engineering Project course within the Chemical Engineering Degree at the Engineering School of Barcelona. The objective of the exercise was to design a rainwater harvesting and greywater recycling system for a detached house and calculate the number of people that could be self-sufficient. Local rainfall, roof area for collecting rainwater and daily water consumption per inhabitant were considered. The effective amount of rainwater and purified greywater was also obtained. In this design, the rainwater is filtered, stored and preserved in a tank, and disinfected with UV light. A small quantity can be made drinkable. The greywater is filtered, treated in a biological reactor, flocculated, sedimented and finally disinfected with UV light.