Example of disinfecting water in plastic PET bottles by using the SODIS method in the Mukuru-Kayaba slum. Rose is a member of the Hope and Joy Self Help Group which was established to help people affected by HIV/AIDS. They are running a toilet block, sell water and promote better hygiene and sanitation. Rose is the organising secretary of the group and a proud member. She says that people sometimes give nasty remarks because of the members seropositive status. But this is not demotivating her from educating other people about good water and sanitation practices. She is a SODIS promotor and has a collection of over thirty water bottles on her tiny metal roof. Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. The piped water in this slum has to be purified because it's often not well treated or polluted by sewage due to bad water infrastructure. Mukuru-Kayaba is the second biggest slum in Nairobi with an estimated population of nearly one million.