Membrane fouling during nutrient recovery from digestate using electrodialysis: Impacts of the molecular size of dissolved organic matter

Electrodialysis (ED) has shown application potential in nutrient recovery from digestate thanks to the frequent electrode reversal practice that can clean the ion-exchange membranes regularly. However, the fouling caused by dissolved organic matter (DOM) is still a challenge to the long-term operation of ion-exchange membranes in ED, and the mechanism of this fouling has not been well studied regarding these compounds’ molecular size. Taking pig manure digestate as an example, this study utilized an ultrafiltration-ED design to fractionate the DOM into many different ranges of molecular size and then assessed the molecule size-specific DOM-fouling to ED membranes. The results showed that the fouling to the anion-exchange membrane (AM) was significantly influenced by molecular size, while the cation-exchange membrane was antifouling over full molecular size ranges. The DOM with a molecular size of smaller than 10 kDa, accounting for 9.1% of the total DOM in digestate, was found to mainly cause AM deterioration. In particular, the DOM smaller than 1 kDa migrated into the interior of AM, thus increasing the membrane resistance by 25.7% and decreasing the ion exchange capacity by 3.5%. The DOM with molecular size in the range of 1–10 kDa caused the fouling in the zone of 0–100 μm depth from the membrane surface. Meanwhile, the DOM with molecular size larger than 10 kDa did not cause obvious irreversible fouling. For instance, the DOM being larger than 0.1 μm, accounting for 75.2% of the total organic matter in the digestate, did not cause irreversible membrane fouling. These results indicate that attentions should be paid to the fouling caused by DOM with small molecular size of less than 10 kDa.