Nitrate Removal by Anion‐Exchange Resins

Abstract

Reported here are the results of a project that was intended to determine to what degree interferences would decrease nitrate-removal capacity of a resin when treating various waters.Nitrate in drinking water was first associated in 1945 with a temporary blood disorder in infants called methemoglobinemia, which causes serious illness and, on occasion, death when infants consume water containing concentrations greater than 10 mg nitrate nitrogen/1. Because this has occurred with sufficient frequency and widespread geographic distribution, the hazard is recogni/ed, and a limit has been assigned to the concentration of nitrate in drinking water of 10 mg/1 as nitro gen. This is about 45 mg/1 of the nitrate ion.To treat water containing nitrate exceeding the proposed EPA Drinking Water Standards (DWS), an economical nitrate removal method is needed. An ion-exchange process has been proposed and is currently being used on a low TDS concentra tion water. The purpose of this study was to determine whether or not this process would also be successful and economical for treating high nitrate concentration water when high concentra tions of other anions are also present.Most of the high nitrate concentrations in water are found in ground-water supplies. Ground waters are also likely to have other anions that compete with nitrate-exchange sites when ion-exchange resins are used for treatment. Strong anion-exchange resins will remove nitrates, but the degree of competi tion from other anions during the removal of nitrates is not clearly defined. Some of these competing anions include sulfates, chlorides, silicates, and phosphates. Iron, turbidity, and colloidal matter can cause caking or clogging problems.Eighty-two ground-water supplies having above-average nitrate concentrations were checked for variations of total dis solved solids (TDS), chlorides, sulfates, and iron. These sup plies were scattered throughout the US, and the wide range of 528 RESEARCH concentrations of these other parameters (Table 1) demon strates that the presence of nitrate is not related to concentra tions of the other parameters listed. Therefore, high nitrate levels could occur with either high or low concentrations of the other parameters. A check of some state water-supply invento ries revealed the same pattern. High TDS concentrations are related to geographical and geological location but not in any ratio pattern of one ion to another. Although no analytical results for silica concentrations were available in the previously mentioned supplies, silica is present in many ground waters. BackgroundIn 1965 a Tennessee firm 1 proposed a continuously regener ated ion-exchange process, operated with the resin moving through a closed loop, using a strong anion-exchange resin to remove nitrates from drinking water. They claim at least two advantages for their process. First, with continuous regenera tion, the waste brine is continuously produced, thereby avoid ing slug discharges to the sewer, if that is the disposal method. Second, they also claimed that...