Regeneration of zeolites used for ammonium removal from anaerobic groundwater

Abstract

Research performed of the last decades in the waste water treatment field has shown the ability of zeolites to adsorb ammonium via ion-exchange. Recent research has proven that zeolites might also be capable to remove the lower concentrations of ammonium present in groundwater to the for drinking water required standards. This makes it a promising technique as replacement for the traditional biological treatment. Still there is not much known about the effect of multiple regenerations with the different types of regenerant on the adsorption capacity and the durability of the zeolites. This research focuses on the chemical regeneration of zeolites which have been used to remove ammonium from anaerobic groundwater. The main goals hereby have been: - The determination of the effect of multiple regeneration on the ion -exchange capacity of the zeolites - The testing of the effectiveness of two types of regenerant: NaOH and NaCl - To see whether it is possible to reduce the amount of chemicals used for regeneration by reusing the regenerant. Because the adsorption experiments have been done with anaerobic water, an extra objective has been to investigate whether ammonium adsorption is hindered by the presence of Fe2+. A 4 column set-up has been designed in which two types of zeolite, clinoptilolite and aqualite, have been used. At a drinking water treatment plant a side stream of the influent of their rapid sand filters was used as influent for the columns. Two columns (one of each material) have been regenerated with 0,1 M NaOH, the other two with 1,8 M NaCl that was being recirculated.By monitoring the influent and effluent concentration of both ammonia and Fe2+, breakthrough curves could be determined. By comparing these, the impact of the regenerations on the adsorption capacity could be established. Extensive analyses of the used regenerant showed which ions were affected by regeneration and how much regenerant was actually used per regeneration. In the end the zeolites have been loaded and regenerated 11 times. The NaCl also has been used 11 times. Hereby it has been determined that the zeolites do loose some of their ion-exchange capacity by regeneration. This loss is the biggest after the first regeneration. After that the adsorption capacity fluctuates around 80% (NaOH) and 70% (reused NaCl) of it original capacity. The fluctuations are caused by changing lengths of the adsorption phase while the regeneration period stays constant. The reused NaCl shows a higher loss in capacity than the NaOH in combination with a higher chemical usage. From economical point of view NaOH is more feasible. The effect of the extreme pH on the zeolites has not been investigated though. It has been possible to reuse the regenerant and still obtain reasonable results. Whether that really is a reduction of the used chemicals, is not completely clear because no comparative study with single use NaCl has been performed. There has been no evidence that the presence of Fe2+ in the source water interferes with the ammonium adsorption. The positive effect that was found is that the iron is removed by the zeolites to great extent as well.