Urban water and food security in this century and beyond: Resource-smart cities and residents.

The urban world population will increase from 3 to 8.5 thousand million in the 21st century. Cities become hot spots of both demand for water and global food and for disposed used water and nutrients. Sustainability requires that resource flows through our cities are co-managed and connected to agriculture. Reduced use of harmful chemicals in consumer products facilitates treatment to a quality that allows reuse/recycling of water and nutrients. A solid and liquid waste hierarchy can assist in ordering measures. A novel flexible water balance can guide city infrastructure and keep toilet water separate. New water-saving equipment can substantially reduce water use without losing personal comfort. The combination of these new approaches ascertains access to safe urban water, and that recovered nutrients from cities can substitute half of chemical fertilisers needed in food production. Now, thousands of new cities and suburbs provide unique opportunities to develop resource-smart and sustainable flows.