A review on the recovery of humic substances from anaerobic digestate of sludge as a potential fertilizer: Quantification, efficiency and interaction with pollutants

This study reviewed the recovery of humic substances (HS) from anaerobic digestate of sludge as a potential fertilizer, focusing on the quantification of HS, the efficiency of HS recovery, and its interaction with pollutants. The potential pitfalls of current misunderstanding for HS quantification in sludge were pointed out. HS present in sludge showed potential to be used as a fertilizer, which solubilized insoluble phosphates for enhanced soil fertility. Pretreatment prior to anaerobic sludge digestion and composting of anaerobic digestate of sludge (ADS) improved the amount of HS in anaerobic digestate. But the amount of HS extracted from the retentate of ADS was much lower than the level required for the common HS fertilizer. Therefore, an additional concentration was required to process the retentate of ADS into HS liquid fertilizer. The quinone moieties in HS accepted electrons from anaerobic microbial respiration and their role in the degradation of organic pollutants were summarized. By binding with HS, the speciation of metals in sludge was changed from water-soluble and exchangeable to organic- and sulfide-bound fractions. Future research should focus on the novel application of machine learning for quantifying HS within sludge, offering a practical approach to interpret complex fluorescence data and enhance understanding of HS characteristics and distribution. Further studies should explore the application of hydrothermal humification to enhance HS content in ADS, offering an energy-efficient method for rapid fertilization in sustainable agriculture. This study offers a window into HS recovery from ADS.