Microalgae (Spirulina) and primary sewage sludge are considerable feedstocks for future fuel-producing biorefinery. These feedstocks have either a high fuel production potential (algae) or a particularly high appearance as waste (sludge). Both feedstocks bring high loads of nutrients (P, N) that must be addressed in sound biorefinery concepts that primarily target specific hydrocarbons, such as liquid fuels. Hydrothermal liquefaction (HTL), which produces bio-crude oil that is ready for catalytic upgrading (e.g., for jet fuel), is a useful starting point for such an approach. As technology advances from small-scale batches to pilot-scale continuous operations, the aspect of nutrient recovery must be reconsidered. This research presents a full analysis of relevant nutrient flows between the product phases of HTL for the two aforementioned feedstocks on the basis of pilot-scale data. From a partial experimentally derived mass balance, initial strategies for recovering the most relevant nutrients (P, N) were developed and proofed in laboratory-scale. The experimental and theoretical data from the pilot and laboratory scales are combined to present the proof of concept and provide the first mass balances of an HTL-based biorefinery modular operation for producing fertilizer (struvite) as a value-added product.

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).