The suitability of recycling ...

The suitability of recycling (i.e. application without treatment) greywater for irrigation purposes depends on many factors.  In a single-family residential application, using greywater collected from bath, shower, lavatory, and laundry drainage, the greywater could be applied or poured onto the soil with little health or environmental risk.  While it is possible the water could contain disease causing microorganisms shed from the body or soiled laundry garments, there is a greater probability of transferring disease within the family due to personal contact and food preparation.  In North America, where Health agencies permit greywater to be used for irrigation without treatment, untreated greywater is normally only allowed to be used for landscape irrigation (i.e. non-food vegetation) if applied below the surface of the soil.  The rationale is to prevent the greywater from coming into direct contact with humans (kids, gardeners, etc.).

In developing national water reuse guidelines (2012), Health Canada initially considered only greywater sources - under the premise that greywater was somehow inherently  a lower health risk than mixed wastewater.  However, their risk assessment analysis concluded there was no significant benefit to considering only reclaiming greywater.  The types of contaminants present - including potential for disease causing bacteria, parasites and viruses - were similar for both greywater and mixed wastewater, and the technologies used to treat both were identical.  

Back to your question, the least expensive method of irrigation with greywater is surface distribution, taking into consideration the source of greywater and the potential for disease transmission due to human contact with the water before it enters the soil.  Bacteria naturally present in the soil will rapidly biodegrade the organics present, and natural processes will remove any pathogens that may be present in the greywater.  If the soil is subject to at least seasonal fresh water infiltration as a result of rainfall, any salts that otherwise may accumulate and adversely affect soil structure will be flushed out.  Some plants may be sensitive to the soaps, detergents in greywater, and/or the pH of the water - so that should be a consideration in assessing suitability for a specific application. 

Constructed wetlands are commonly used to treat domestic wastewater, which typically consists of more than 60 percent greywater without adversely affecting the plants.  The plant roots are in constant contact with the wastewater, including greywater components, and the plants thrive on the nutrients present.

http://nepis.epa.gov/Exe/ZyPDF.cgi/30005UPS.PDF?Dockey=30005UPS.PDF