Abstract: The present disclosure provides methods and systems for purifying liquids. In a particular implementation, the system includes a forward-osmosis unit for diluting a water source for a downstream desalination unit. A pretreatment unit may be located hydraulically upstream of the desalination unit, such as upstream or downstream of the forward-osmosis unit. In certain embodiments, the system includes an extraction unit for extracting a relatively easily extractable osmotic agent from an osmotic draw solution. The system may include one or more forward-osmosis units downstream of the desalination unit for diluting a concentrated brine stream produced by the desalination unit. In particular embodiments, a downstream forward-osmosis unit uses the concentrated brine stream as an osmotic agent, such as to extract water from seawater or brackish water. Another downstream forward-osmosis unit may use impaired water as a feed stream.