**1. Introduction**

Hydrothermal liquefaction (HTL) of biomass presents a promising procedure [1,2] for overcoming dependency on fossil fuels and advancing toward sustainable decarburization of the transportation sector. Hydrothermal liquefaction enables the conversion of wet biomass or waste materials into bio-crude oil by using hot-compressed water (287–375 ◦C) [3,4] that refines downstream to liquid fuel [5]. It is of special interest in jet fuel. In biorefinery, sustainability has an important role in achieving the viability of a large-scale bio-crude facility. Nutrient recovery presents an attractive option for improving sustainability and adding value to the production chain. After HTL, the macro (P, K, N) and micro (S, Mg, Ca, Fe) nutrients that might be recovered for biomass production systems are distributed between its products, which include the HTL oil (also known as bio-crude), liquid, and solid phases. This distribution of nutrients among HTL phases depends on not only process conditions but also the nature and composition of the biomass feedstocks [6]. Furthermore, it presupposes variations in nutrient recovery strategies among various biomass feedstocks.

Significant research on HTL, and by association, nutrient recovery, has been performed in small-scale batch systems and mainly utilized algae as feedstock [7–9]. The next step toward the commercialization of biofuel production, which involves an HTL pilot-scale plant that operates in a continuous mode, has received considerably less attention [10]. Only a few published reports relate

to nutrient recovery following bio-crude production via continuous HTL. One study by Edmundson et al. [11] has demonstrated that soluble phosphate, which was recovered through acid extraction from the HTL solid phase that originated from continuous HTL of algae feedstock (total volume of system ~1.6 L), could be recycled for algae production. However, replacing nitrogen in a growth medium by using the HTL liquid phase can have a negative e ffect on the growth rate of algae [11]. McGinn's study [12] has considered the HTL liquid phase that results from continuous HTL of algae (total volume of system ~0.45 L). He has noted that the complete decoupling of phosphate (via struvite MgNH4PO4·6H2O precipitation) and nitrogen as ammonia (through ammonia stripping) from the HTL liquid phase suggests a flexible method of recycling them for algae cultivation. To the author's knowledge, no study has previously reported on the feedstock-related fate of nutrients in a pilot-scale HTL unit or discussed nutrient recovery in a large-scale HTL scenario in relation to the distribution of nutrients. This study provides such data, which were gathered in the framework of the HyFlexFuel project. It assesses two campaigns of a continuous HTL pilot plant. Each campaign used a di fferent feedstock—primary sewage sludge (PSS) and the microalgae *Spirulina* (*SPR*)—both of which are highly relevant to nutrient recovery.

Interest in converting sewage sludge to bio-crude via HTL has increased in the past few years [13–15] because of its availability in large volumes as the main byproduct of wastewater treatment plants, its high water content, and its embedded energy potential. In addition, it is an attractive secondary phosphate source [16]. Previous studies have found the phosphate from sewage sludge primarily in the HTL solid phase as a result of a substantial content of metal ions, such as calcium, iron, and aluminum, and the low solubility of their phosphate salts [6]. While some studies have focused on the immobilization of heavy metals from sewage sludge in the HTL solid phase [17,18], no publication has been found that targets the study of phosphate recovery from HTL solid residue of sewage sludge. Phosphate recovery from the HTL solid phase may be possible in the form of struvite (MgNH4PO4·6H2O), as previously demonstrated for the related process of hydrothermal carbonization (HTC). Becker et al. [19] have successfully performed phosphate and nitrogen recovery based on HTC of digested sewage sludge via precipitation of struvite from a mixture of the HTC liquid phase and acid leachate from hydrochar. Other studies have also revealed the potential of struvite production on the basis of hydrothermal treatment of various kinds of biomass [20–22]. Bauer et al. [23] have evaluated a possible approach to HTL liquid phase managemen<sup>t</sup> by way of struvite precipitation for a wide range of biomass that includes pre-digested and digested sludge. Moreover, they have suggested that struvite precipitation from the HTL liquid phase may be economically appealing.

Microalgae are another promising feedstock for HTL. Their advantages include high annual biomass productivity, an ability to grow in poor-quality water, high water, and energy content. Phosphate is significant for various aspects of cellular metabolism of microalgae [24] and is essential for their growth. Prior studies have demonstrated that the HTL of algae biomass results predominantly in phosphate as well as nitrogen and other nutrients, such as K, Na, S, and Mg, in the HTL liquid phase [6,25–27]. The values of nutrients that are recovered in the HTL liquid phase depend heavily on the process parameter and initial algae composition (e.g., marine or freshwater species). Several studies that have been driven by the development of scalable algae-based biofuel production have already addressed nutrient recovery [28]. However, most research has focused on the feasibility of reusing the HTL liquid phase for algae cultivation to reduce the consumption of fertilizers for their growth [29]. Leng et al. [30] and Gu et al. [31] have provided a detailed overview of studies that concern the recycling of nutrients for algae cultivation. Generally, the HTL liquid phase has to be heavily diluted to avoid the inhibitory e ffect of organic compounds, such as phenols, furans, aromatic hydrocarbons, and nitrogenous compounds [26,32], on algal growth rate. Alba et al. [33] have indicated that the lack of essential nutrients (besides N and P) in the HTL liquid phase, which is enhanced by dilution, can lead to a reduction in the growth rate. Only a few studies have applied an integrated approach and precipitated the nutrients from the HTL liquid phase in the form of high-value products, such as struvite. As noted above, McGinn et al. [12] have decoupled phosphate in the form

of struvite from the HTL liquid phase to minimize the inhibition of algae growth through the presence of organic compounds. In addition, Shanmugam et al. [34] have used phosphate recovery via struvite precipitation as a pretreatment for anaerobic digestion of the HTL liquid phase and discovered that the biogas production from the struvite-recovered HTL liquid phase was 3.5x higher than that from the non-struvite-recovered HTL liquid phase.

Struvite precipitation is a well-known technology in wastewater treatment for the recovery of phosphate in the form of slow-release fertilizer [35]. This technique can be coupled with other processes. Some examples of coupling of struvite precipitation with HTL/HTC have been explained above, and another example can be found in reference [36]. The major benefit of struvite crystallization is the production of slow-release fertilizer that is established in the market, transportable, and suitable for long-term storage. Moreover, struvite production does not require severe process conditions; it is precipitated through pH adjustments to approximately pH 9, and the addition of a magnesium source, an ammonium source, or both, if necessary [37]. The production of struvite from HTL byproducts (HTL solid and HTL liquid phases) and its commercialization present an advantage in terms of the economic performance of fuel production and sustainability.

The aim of this study is to examine the feedstock-related application of HTL byproducts for nutrient extraction and evaluate the potential of phosphate recovery in the form of struvite in pilot-scale HTL bio-crude production. The specific research questions of this investigation are as follows: (1) To determine the relationship between feedstock and nutrient occurrence in HTL products at the pilot-scale; (2) to outline perspectives on the use of HTL byproduct streams to produce marketable fertilizer; and (3) to verify the possibility of efficiently recovering phosphate in the form of struvite from HTL byproducts by means of a laboratory-scale study.
