Evaluation of urea removal by persulfate with UV irradiation in an ultrapure water production system.

The removal of urea by persulfate with UV irradiation in an ultrapure water (UPW) production system was examined using a continuously operated column reactor. Urea is a substance that is not properly removed by the unit processes in UPW production systems. Based on our monitoring of urea concentration over 1 year of the operation of a UPW production facility, a relatively high concentration of urea was introduced between February and May in 2016 and the total organic carbon (TOC) concentration of the UPW was increased in that period. Approximately 50% of the urea in the raw water was removed by the UPW production process, for which double-pass reverse osmosis units were instrumental. The addition of persulfate to the TOC reduction UV unit in the UPW production process was examined to improve the efficiency of urea removal. As a result, supplying over 20 μmol/L persulfate to the TOC reduction UV reactor improved the efficiency of urea removal from 9% to 90%. UV dose was an important factor of urea removal. In the ion analysis, nitrate and sulfate were detected in the effluent, and the ratio of produced nitrate to the removed urea was approximately 2.