Dual Effect of Solution pH on Ammonia Recovery in Membrane Distillation - Influence on pH Partitioning and Mass...

Recovering ammonia from wastewater by membrane distillation (MD) is a sustainable approach to remediating environmental issues while simultaneously conserving energy both in wastewater treatment and in the Haber-Bosch process. MD leverages the volatility of ammonia to enhance ammonia transport, and hence its performance is impacted by the pH of the solution. We comprehensively investigated the effect of pH on ammonia transport and recovery efficiency using both experimental and simulation approaches. Our analyses provide new insights into how solution pH significantly impacts ammonia recovery through two primary mechanisms: it both governs the ammonia-ammonium equilibrium and influences the ammonia mass transfer coefficient. When changing MD feed solution pH from 9 to 10, ammonia flux is enhanced by 177% and ammonia mass transfer coefficient increases from 2.64 × 10^-6 m·s^-1 to 6.14 × 10^-6 m·s^-1. Notably, solution pH adjustment has a more significant effect than increasing solution temperature on enhancing the ammonia mass transfer coefficient and improving recovery efficiency, making it a more feasible and effective approach for improving ammonia transport and recovery. Additionally, our explicit simulations of ammonia recovery efficiency provide valuable insights for optimizing MD performance by adjusting solution pH values and operation time, and enable a maximum profit estimation of $598,000 for operating MD to recover ammonia in a dairy farm with 2000 cows.