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< ~45 ◦C) and driving force Δp (<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 Δp; moreover, J and pH of feed solution remained practically constant for a significant distillate removal-fraction R (under constant Δp 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.