If you are in the US, then you must follow US EPA design manual for wastewater reuse for irrigation/agriculture purposes.  Caution - there are differing degrees of water quality for reuse based upon the type of crop - human consumption, etc.

Todd
www.oxilume.com

Very general question. There is no standard treatment method. You need to contact the regulatory agency where you are located. A general guideline for water reuse can be found in the Title 22 California regulations.

Treatment would generally consist of secondary treatment, tertiary filtration, and disinfection.

60304. Use of recycled water for irrigation
(a) Recycled water used for the surface irrigation of the following shall be a disinfected
tertiary recycled water, except that for filtration pursuant to Section 60301.320(a)
coagulation need not be used as part of the treatment process provided that the filter
effluent turbidity does not exceed 2 NTU, the turbidity of the influent to the filters is
continuously measured, the influent turbidity does not exceed 5 NTU for more than 15
minutes and never exceeds 10 NTU, and that there is the capability to automatically
activate chemical addition or divert the wastewater should the filter influent turbidity
exceed 5 NTU for more than 15 minutes:
(1) Food crops, including all edible root crops, where the recycled water comes
into contact with the edible portion of the crop,
(2) Parks and playgrounds,
(3) School yards,
(4) Residential landscaping,
(5) Unrestricted access golf courses, and
(6) Any other irrigation use not specified in this section and not prohibited by
other sections of the California Code of Regulations.
(b) Recycled water used for the surface irrigation of food crops where the edible portion
is produced above ground and not contacted by the recycled water shall be at least
disinfected secondary-2.2 recycled water.

http://www.dhs.ca.gov/ps/ddwem/publications/waterrecycling/index.htm