Ion exchange for the desalination and treatment of wastewater

Abstract

Ion exchange for the desalination and tertiary treatment of waste water has involved the development of both the theory and practice of continuous countercurrent ion exchange (CCIX). The need for process development arose after a comprehensive review of the principles and properties of ion exchange as these applied to the treatment of saline sewage plant discharge streams. Initial cost studies also indicated that regenerant chemicals constituted the major cost item in an IX plant. On the basis, therefore, of the IX literature review and the cost exercise, the decision was made. to use CCIX and to develop a specific process for the treatment of secondary sewage effluent. A study was made of the kinetic, equilibrium and organic absorption properties of cation and anion resins and how these properties related to the operation of a CCIX unit. Simultaneously, a novel regenerant chemical recovery scheme based on the use of nitric acid and ammonia was formulated and tested. The various ion exchange reactions involved in the overall desalination and tertiary treatment process were monitored in a CCIX column and from these tests, a steady state theory of the operation of such a column incorporating the previously determined resin exchange properties was proposed and verified. The next phase of the investigation involved the design, construction and commissioning of an interlinked five column CCIX pilot plant. This was also a new development in that this complexity of CCIX plant had not formerly been operated. Initial test work involved synthetic saline solutions and an hydraulic investigation. This was followed by treatment of secondary sewage effluent during which it was shown that combined desalination and organic removal could be achieved, regenerant chemicals could be recovered and feed water dissolved components concentrated for discharge.