Valorization of Byproducts from Hydrothermal Liquefaction of Sewage Sludge and Manure: the Development of a Struvite-Producing Unit for Nutrient Recovery

Hydrothermal liquefaction (HTL) presents a promising technology to recover energy from sewage sludge and manure as bio-crude oil. The liquid and solid phases after HTL are considered as byproducts; however, the latter is rich in nutrients, which offers a possibility for nutrient recovery and manufacturing of valuable products. This study focuses on the simultaneous recovery of phosphate and ammoniacal nitrogen as struvite by combining the liquid HTL byproduct and leachate from the solid HTL byproduct. To implement a flexible struvite-producing unit, the effect of an inlet-mixed solution composition on the struvite precipitation was investigated on a laboratory scale and preferable operating conditions were identified. These operating conditions were used afterward to evaluate and adapt an air-agitated system for struvite precipitation subsequent to the HTL production chain. Moreover, the precipitation process in the air-agitated system was evaluated using synthetic model solutions compared to real HTL byproducts to offer insight into the effect of impurities on the product quality. The product, which has a struvite content of more than 75 wt %, was manufactured from the HTL byproducts of manure and sewage sludge as well. It has potential for use as a fertilizer based on its elemental composition but the use of citric acid was found to be effective to improve the quality of the product, especially of sewage sludge. The results showed that the air-agitated system could be effectively implemented in the HTL production chain. It has the potential to enhance features of the product, leading to a ready-to-use fertilizer in shape (particle size) and quality (struvite content).

Automated summary

HTL technology is a promising method to recover energy from sewage sludge and manure, producing bio-crude oil. This study focuses on recovering phosphate and ammoniacal nitrogen by producing struvite from the byproducts of HTL. The air-agitated system tested in the laboratory showed potential for enhancing the quality of the product, making it suitable for fertilizer use.