A novel, facile, and cost-effective upcycling treatment of food waste digestate was developed in this study. Food waste digestate after anaerobic digestion was able to be transformed into value-added solid fertilizer using one-step gelation method. Sodium alginate (SA) was used as the main encapsulating material, and the K-rich biochar made from wood chips was used as the core material. The suspension mixture of food waste digestate, SA, and biochar could be simply made into solid beads under gelation in the calcium chloride solution. The nutrient recovery rates from the digestate were low (< 60% for phosphate, < 15% for ammonia or potassium) by using biochar only, while large amount of K+ was released. However, by using SA as coating, over 90% of phosphate, 38.9% of ammonia, and 38.3% of potassium could be recovered from the food waste digestate. Moreover, large amount of Na+ occurring in the food waste digestate was not able to be absorbed, which would also be beneficial to the utilization of this developed solid fertilizer in the crop cultivation application.
