Why remove the nitrogen? Leave it in, sell the water to farmers to spread on crops, cheaper than buying fertilizer.
You clearly have not been exposed to the joys of the Clean Water Act and ammendments. The biggest problem with your suggestion is the daily volume of water involved. It would be impractical to transport several hundred thousand gallons per day. Also, there are strict regulations in place for land irrigation in order to avoid runoff. Second, its illegal to irrigate crops for human or animal consumption with treated wastewater without jumping through mucho hoops first (if even then).