Selective Transport of NH₃ by Silica-Based Ceramic Pervaporation Membrane

Abstract

Nitrogen (N) removal is one of the key tasks for every wastewater treatment plant under the growing pressure of environmental protection. Prevailing applied N treatment technologies are mostly energy-intensive. This dilemma triggers a rethinking of the way to approach N management. “N2kWh-From pollutant to power” is a project that aims at creating an energy-positive 푁 treatment system by exploiting the energy potential stored in reduced N compounds. Both thermal and electrical energy needed in this scheme will be provided by Solid Oxide Fuel Cell (SOFC) using the ammonia recovered from wastewater. To guarantee a proper function of the SOFC a minimum of 5 wt% of ammonia in the gas mixture is a prerequisite, whereas, the reject water of anaerobic digester only typically contains 0.15 wt%. Therefore, selectively stripping NH3 in waste streams is the main technical bottleneck of this project.This study aims at contributing to the understanding of selective transport of NH3 by pervaporation. A comprehensive literature study indicates silica-based ceramic membrane is able to perform this selective transport. Therefore, the task of this research was to test the feasibility of selective transport of NH3 by commercially available silica-based pervaporation membranes (hydrophobic PDMS and hydrophilic Hybrid Si AR). The objective was to find the optimum operating condition for maximizing the selective transport of NH3 by silica-based ceramic pervaporation membrane. To this end, the impact of flow regime (laminar and turbulent flow), temperature (35 oC and 45 oC), presence of additional salt (NaCl and Na2CO3) and TAN concentrations (total ammonium nitrogen; 1.5, 12.0 and 20.0 g TAN·L-1, respectively) was assessed through a series of systematic experiments.It was found that the PDMS membrane was unable to selectively transport NH3 from liquid solution because both H2O and NH3 are polar molecules while the hydrophobic PDMS was nonpolar. Besides, the PDMS membrane tested was not stable under solutions used in this study.The results of Hybrid Si pervaporation membrane showed that the optimum operating conditions for selective transport of NH3 was 35 oC, Re=2,400, 20.0 g TAN·L-1 ammonium bicarbonate solution (pH adjusted to 10). For these conditions, the highest perm-selectivity (ratio of mass transfer coefficients of NH3 and H2O) was 0.5 indicting NH3 was less selectively transported than H2O.The impact of flow regime on selective transport of NH3 was related to polarization effects and depended on TAN concentration in feed solution. In the tested temperature and TAN concentration range, perm-selectivity was independent on both parameters. In addition, the effect of both temperature and TAN concentration on the ammonia was mainly due to the driving force, so their influence on the perm-selectivity was limited. Presence of salt seemed to have a positive impact on the perm-selectivity. Although salt has little impact on the mass transfer coefficient of NH3, it decreased the mass transfer of H2O resulting in a better selective transport of NH3.As for energy consumption, it was inversely related to TAN content. At the optimum operating condition 7 MJ·kg-1 - N was consumed by pervaporation in this study. Compared with the energy consumption of air stripping, pervaporation is a promising technology for NH3 recovery.