Three Birds with One Stone: N/P Recovery, Dewaterability Improvement, and Facilitating Liquid Digestate Treatment of Anaerobically Digested Sludge

This study developed a novel dewatering process coupled with nutrient recovery to raise the resource value of anaerobically digested sludge. The hydration of multicharged anions was proposed to compete with the hydration of hydrophilic organic components (i.e., salting-out effects). Accordingly, phosphate was used as the salting-out agent to improve sludge dewaterability, and simultaneously, it balanced the molar ratio of NH4+-N and PO43–-P in the aqueous phase, which could facilitate the recovery of N/P as struvite from the liquid digestate. The addition of phosphate (70% of its solubility) at the initial sludge pH of 5 could obtain the lowest centrifugally dewatering equilibrium height (h∞) of 0.036 cm and reduce the bound water fraction by 63.5%. For the first time, time-of-flight secondary ion mass spectrometry (ToF-SIMS) in situ identified the phosphorus speciation and microdistribution in anaerobically digested sludge. Additionally, the quartz crystal microbalance with dissipation (QCM-D) analysis confirmed that the organics-phosphate interactions led to a dense water-repelling system of sludge flocs with an increasing rigidity, which mainly contributed to the improved centrifugal dewatering performance. All the above contributions are expected to suggest a promising way for simultaneously recovering N/P, improving dewaterability, and facilitating liquid digestate treatment of anaerobically digested sludge.