Ammonia recovery from anaerobic-fermentation liquid digestate with vacuum membrane distillation

Vacuum Membrane Distillation (VMD) is systematically investigated herein, for recovering ammonia after phosphates precipitation, towards development of a sustainable process to valorize anaerobic-fermentation liquid-digestates (separated from the sludge). Initially, batch tests were performed, with representative synthetic solutions, to identify a narrow range of conditions favoring optimal ammonia (i.e. total ammonia nitrogen-TAN) separation, mainly regarding selectivity and recovery in H2SO4 scrubbing-solution. Next, guided by the preceding test-results, targeted batch experiments with digestate samples from an industrial plant, led to determination of near-optimum conditions for ammonia recovery. Specifically, for the small TAN concentration (<200 mg/L) encountered in the digestate feeds, VMD at modest temperature (T< similar to 45 degrees C) and driving force Delta p (<20 mbar) allowed high ammonia removal fraction r (greater than 0.85) with fair selectivity. Under such conditions, the (ammonia-water) distillate flux J exhibited linear dependence on Delta p; moreover, J and pH of feed solution remained practically constant for a significant distillate removal-fraction R (under constant Delta p and T). Based on theoretically supported correlation of ammonia removal-fraction r with distillate removal-fraction R, and other results, an approach is proposed for realistic preliminary design of a continuous VMD-based process, thus facilitating further steps towards large-scale process development.

Automated summary

This study systematically investigates the application of Vacuum Membrane Distillation (VMD) for recovering ammonia after phosphates precipitation, aiming to develop a sustainable process for utilizing anaerobic-fermentation liquid-digestates. Batch tests and targeted experiments with industrial digestate samples were conducted to determine the optimal conditions for ammonia recovery. Under specific conditions of low TAN concentration, modest temperature and driving force, VMD achieved high ammonia removal fraction with fair selectivity. The study also proposes an approach for realistic preliminary design of a continuous VMD-based process.