'Synergistic use of ion exchange and RO technologies in multicontaminant removal applications in drinking water'

摘要

Some drinking water sources have a number of contaminants such as nitrate, sulfate,hardness and total organic carbon (TOC) that need to be reduced either to comply withmaximum contaminant level (MCL) regulations, to meet secondary guidelines, or tocontrol disinfection byproducts on subsequent disinfection of the water. Individualreduction of each contaminant by ion exchange is technically possible but the overallcost is becoming prohibitive, especially for regions where disposal of high concentrationspent brine is regulated. RO technology, because of its broad‐based salt rejectioncapability is an attractive alternative for simultaneous removal of such contaminants,but the challenge of disposing of relatively large volumes of concentrate (typically 25 to30 percent of total) has significantly limited its use. If concentrate volumes can bereduced to more manageable volumes, say 5 to 10% of total or lower, then disposal viadeep well injection and evaporation ponds becomes increasingly attractive. In addition,regions served by rapidly depleting aquifers can conserve more on scarce supplies.Synergistic use of ion exchange and RO technologies has the potential to reduceconcentrate volumes to less than 5 to 10 percent when use is made of a relatively newtechnology called cyclic ion exchange. Ion exchange softening of the RO feed allowshigher recovery rates by minimizing the potential for scaling of the RO membranes. Thehigher recovery rate results in higher brine concentrations in the RO reject, and this isturn is used in the cyclic ion exchange process to periodically regenerate the softeners.Recent advances in the technology now allows regeneration with brine concentrationsas low as 0.2% (2000 mg/l), allowing extended application of the technology to feedwaters with total dissolved solids (TDS) typical of that for drinking water. This paperexamines the technical and economic feasibility of the technology for treating drinkingwater with multiple contaminants, such as hardness, nitrate, sulfate, fluoride and TOC.