## R

### Rainwater harvesting

Process of collecting and storing rainwater (3)

### Raw water

Water that has not yet been treated (1)

### Representative sample

Sample that has all the characteristics of the matter that it was taken from (12)

### Residual chlorine

(in water treatment) the amount of chlorine left in water after all the pollutants have reacted with it (5)

### Resilience

Ability to withstand shock, stress or a natural hazard (5)

### Retention time

(in wastewater treatment) the time that effluent spends in a given environment, e.g. in a sedimentation tank or a stabilisation pond (11)

### Risk

The chance of a hazard occurring (8)

### Risk assessment

Process by which the likelihood of harm from a potential hazard is assessed (8)

### Run-off

Rainwater that ‘runs off’ the land i.e. flows over the land surface (3)