Energy efficiency and treatment performance of BPM-ED for ammonium recovery from ammonium sulphate and ammonium citrate scrubbing effluents

Abstract

Excess nitrogen disposal into the environment increased with the discovery of an artificial way of nitrogen gas conversion to ammonia, and its implementation to industrial scale. The excess wasted fixed nitrogen ended up in rivers, lakes, oceans which resulted in degrading air, water and soil due to exceedance of the amount of nitrogen that can be absorbed by soil and plants. Currently, the global wastewater treatment objective has been adapting sustainable solutions by shifting from contaminant removal to resource recovery. Nutrients that were once considered as waste are now being considered as resources. Bipolar membrane electrodialysis (BPM-ED) in combination with stripper/scrubber is one of the combined technologies that aim at contaminant removal and resource recovery.

This study looks into energy efficiency and treatment performance from ammonium sulphate and ammonium citrate scrubber effluents by BPM-ED technology. A three-compartment BPM-ED was used to investigate the use of salt mixture in the feed solution and factors affecting ammonium yield.

Salt mixture of ammonium sulphate and tri-ammonium citrate were used. It was found that use of a salt mixture of ammonium sulphate and tri-ammonium citrate decreased the energy consumption and increased the current efficiency for both of the salts. Use of a mixture of ammonium sulphate and tri-ammonium citrate resulted in better energy performance than either of the salts when they were used alone. Ammonium recovery performance was higher when tri-ammonium citrate concentration was higher in the initial feed solution. This was due to increase in buffer capacity and basic pH in feed solution due to OH- leakages from base compartment. Sulphate ions were favored over citrate ions in their transfer through anion exchange membranes due to higher mobility of sulphate ions. As a further investigation a BPM-ED stack with different combinations, perhaps a two-compartment stack, can be tested to get a deeper understanding to optimize the operation.

Factors affecting ammonium yield that were investigated were H+ leakages and ammonia diffusion. It was found that H+ leakages were same for ammonium sulphate and potassium sulphate feed solutions and H+ leakages were not affected by ammonia diffusion. Ammonia diffusion resulted in higher energy consumption and loss of ammonium recovery potential. Ammonia diffusion increased even with experimental time of 60 minutes. Ammonia diffusion rates also increased through experimental time of 60 minutes. Ammonium and potassium transfer rates from diluate compartment did not follow a specific trend within the first 30 minutes. Clear reasoning behind this was unknown and the results were not completely satisfactory. In current study, it was assumed that ion cross-over and back diffusion were identical for ammonium and potassium. Investigating ion cross-over and back diffusion of the ions in BPM-ED could bring a deeper understanding to the energy efficiency and treatment performance.