In previous study sessions you have learned about sources of water, how they can become contaminated and about ways of protecting them. Even with source protection it is often necessary to treat water to ensure it is safe. This is the case at household level, which is discussed in Study Session 10, and when supplying water for towns and cities. In this study session, you consider the need for large-scale water treatment and the stages of treatment for urban water supply. You will also learn about the management of wastes produced in the process of water treatment, and consider the issues of sustainability and resilience in relation to large-scale water treatment.