Biological ion exchange capable of sulphate-based secondary ion exchange during long-term DOC removal

Biological ion exchange (BIEX) offers removal of dissolved organic carbon (DOC) with greatly reduced regeneration frequency. In the present work, a strong base anionic exchange resin was operated without regeneration and using inlet water with either Low (12 mg L^-1) or High (60 mg L^-1) sulphate and DOC of 2.75 or 5.0 mg L^-1. Filters operated continuously for 226 days (16,500 bed volumes) and achieved DOC removal varying from 32% to 50%. Initially, sulphate and DOC were retained by the resin with chloride being released. During this period, DOC removal occurred due to traditional mechanisms, referred to as primary ion exchange. Following this initial period, DOC removal continued even though the conventionally defined resin capacity was exhausted (based on chloride loading). During the later period, no chloride release was observed, but instead sulphate was released. Although suggested by others, the present study is the first to confirm the direct exchange in charge equivalence of anions removed (DOC and nitrate) to released (sulphate) during the secondary ion exchange mechanism. Further, increasing inlet sulphate from 12 to 60 mg L^-1 resulted in a 19% decrease in DOC removal. Finally, percent DOC removal was affected only by an increase of inlet DOC but not changes to the counter ion or after DOC loading on the resin increased to 1/3 of total capacity. This work promotes BIEX as a viable alternative to biological activated carbon and a leading solution for low-maintenance DOC removal.