Strategies for improving economic and environmental sustainability of wet chemical phosphorus recovery from sewage sludge ash

1 Strategies for improving economic and environmental sustainability of wet chemical phosphorus recovery from sewage sludge ash L. Esposito*, G. Boniardi**, D. El Chami***, R. Canziani* and A. Turolla* * Politecnico di Milano, Dipartimento di Ingegneria Civile e Ambientale (DICA), Piazza Leonardo da Vinci 32, 20133 Milano, Italy (E-mail: [email protected], [email protected], [email protected]) ** The University of Queensland, School of Chemical Engineering, 4072 St. Lucia, Queensland, Australia (E-mail: [email protected]) *** TIMAC AGRO International, 27 av. Franklin Roosevelt, 35408 Saint-Malo, France (E-mail: [email protected]) Abstract Phosphorus (P) recovery from sewage sludge ash (SSA) represents a promising solution to P supply concerns, with a key challenge consisting of the poor circularity of current technologies. This research explores the potential of an innovative two-step (TS) wet chemical P recovery process from SSA, assessing the use of industrial by-products as secondary reagents and investigating the reuse of the leachate from a conventional one-step (OS) process as an extractant for a subsequent P extraction stage. Lab-scale TS tests were conducted by simultaneously employing secondary extractants (H2SO4 and HCl) and precipitants (lime milk), comparing process performances to a conventional OS process performed with primary reagents. Secondary reagents were effective in recovering P, with average extraction and precipitation efficiencies of 86% and 99%, respectively. The second extraction step successfully increased P concentration in the leachate by 33% to 43%, indicating its potential for reuse. The TS process generated high-value P-rich products, with a P content up to 18% when using H2SO4. However, the overall recovery efficiency of the TS process was up to 27% lower than that of the OS process, indicating the need for further optimization of the second extraction step. Keywords Acid leaching extraction; Bio-based fertilizers; Process optimization; Secondary reagents INTRODUCTION Phosphorus (P) plays a pivotal role in sustaining human society, with around 89% of global P production dedicated to fertilizers manufacturing (Desmidt et al., 2015). Currently, P is primarily extracted from phosphate rocks (PR), a non-renewable resource. In the last decades, intensive agriculture practices, population growth and rapid urbanization have led to overexploitation and quality decline of PR reserves. In this scenario, recovering P from alternative and renewable sources became crucial to address the future P demand (Carrillo et al., 2024). Sewage sludge ash (SSA) emerged as a promising alternative P source due to its high P content (4 – 15.7%), comparable to medium/low-grade PR (Esposito et al., 2024). Between the various P recovery processes from SSA, wet chemical leaching gained significant attention in the literature due to its flexibility, low energy consumption and scalability (Canziani et al., 2023). Despite their potential, wet chemical processes remain economically unviable, being characterized by estimated operating expense (5 – 6 €/kg P recovered) significantly higher than the average price for PR (1.1 €/kg P) and conventional P-based fertilizer (2.2 €/kg P; Esposito et al., 2024). Moreover, the production of primary reagents, especially H2SO4, significantly contributes to process environmental impact, in terms of CO2 footprint and embodied energy (Fahimi et al., 2021). Therefore, identifying strategies to enhance the circularity of wet chemical P recovery processes is crucial to improve their competitiveness against traditional P production methods. 2 Previous works have primarily focused on optimizing the P extraction step, assessing the performance of different extractants – including inorganic acids, organic acids, alkaline and chelating agents – in terms of P and metals/heavy metals/metalloids extraction efficiency (Boniardi et al., 2024a; Luyckx et al., 2020). Selective P precipitation was rarely investigated, with only one study exploring the use of an innovative precipitant derived from a low-grade magnesium oxide mining by-product (Boniardi et al., 2024b). However, to the best of the authors’ knowledge, no research investigated the feasibility of performing wet chemical P recovery from SSA using solely secondary reagents, such as wastes or by-products from other industrial processes. Building upon this gap, this research evaluates the potential of an innovative two-step wet chemical P recovery process, assessing the simultaneous use of industrial by-products as secondary extractants (H2SO4 and HCl) and precipitants (lime milk – LM) and exploring the potential reuse of the leachate from a conventional one-step process as an extractant for a subsequent P extraction stage. These approaches aim to decrease reagent consumption and increase P concentration in the leachate, enhancing the circularity of wet chemical P recovery processes. MATERIALS AND METHODS Wet chemical phosphorus recovery The two-step (TS) P recovery process investigated in this work was tested at the lab-scale following the configuration illustrated in Figure 1. Figure 1. Two-step (TS) P recovery process tested in this work. The tests employed fly ash sampled from the electro-filters of the Werdhölzli mono-incineration fullscale plant (Zurich, Switzerland). Details on the physical and chemical characterization of the employed SSA are provided in Boniardi et al. (2021). The first extraction step (extraction1) was performed under the following operating conditions: 1 N H2SO4 or HCl; Liquid-to-Solid ratio (L/S1) of 10 mLacid solution/gSSA; contact time (t1) of 30 min. These values were selected based on practical considerations, as explained in Boniardi et al. (2024b). The employed H2SO4 (95%w/w) and HCl (13%w/w) samples resulted from the treatment of Sor Cl-rich wastes and were provided by Nuova Solmine S.p.A (Italy) and Eco92 S.r.l (Italy), respectively. The second extraction step (extraction2) was carried out using a fraction of the leachate from extraction1 (leachate1) and applying the following operating conditions based on the applied extractant: L/S2 of 15 mLleachate1/gSSA (H2SO4) or 6 mLleachate1/gSSA (HCl); t2 of 20 min (H2SO4) or 30 min (HCl). These values were determined based on preliminary titration tests of leachate1 with SSA, targeting a pH of 2. This pH level was chosen to 3 decrease the reagent consumption in the subsequent precipitation step while preventing early precipitation of Aland Fe-phosphates from the leachate, typically occurring in the pH range of 2 – 4 (Boniardi et al., 2024b). Leachates from both the extraction steps (leachate1 and leachate2, respectively) were subjected to precipitation (precipitation1 and precipitation2, respectively) by adding a 2%w/v LM solution till a target pH of 7 was reached. Target LM concentration and pH were selected in line with the available literature (Boniardi et al., 2021; Boniardi et al., 2024b). The LM sample (27%w/v) resulted from the acetylene production process and was supplied by Hydroconsulting S.r.l. (Italy). To provide a baseline for comparison, a conventional one-step (OS) P recovery process was conducted at the lab-scale, applying the same operating conditions as in extraciton1 and precipitation1. For this purpose, commercial-grade H2SO4 (96%w/w) from Carlo Erba Reagents S.r.l. (Italy), HCl (37%w/w) and Ca(OH)2 (powder) from Sigma Aldrich (Germany) were used. Additional details about the methodologies adopted for each P recovery step are provided in Boniardi et al. (2021). Phosphorus recovery performance To comprehensively evaluate the performance of the discussed processes, the following parameters were analysed: P extraction efficiency (PEE, %); P precipitation efficiency (PPE, %); P overall recovery efficiency (PRE, %); P content in the precipitate (Ppr, %). Parameters of the first step, second step and overall TS process are indicated by subscripts 1, 2 and TOT, respectively. PEE and PPE were determined as in Boniardi et al. (2021) and Boniardi et al. (2024b), respectively. PRE was calculated as the ratio between the outcoming and incoming P mass across the boundaries of the considered recovery step. RESULTS AND DISCUSSION The performances of the processes analysed in this work are summarized in Figure 2. Figure 2. P recovery performances of the H2SO4- (a) and HCl-based (b) configurations of the onestep (OS) and two-step (TS) processes tested in this work. No substantial differences were noticed when comparing PEE1 and PEE from TS and OS processes, with average values between 85% and 87%. Moreover, extraction2 successfully enhanced the P content in the leachate, with an increase of 33% and 43% for H2SO4and HCl-based routes, respectively. These outcomes indicated the suitability of the employed secondary acids for P extraction and highlighted the potential of recovering additional P with leachate1, possibly reducing the input of primary extractant and, therefore, enhancing the circularity of the process. In extraction2, PEE2 was significantly lower than PEE1, probably due to the lower extracting potential of leachate1 compared to the initial acid solution. PPE was consistent across all recovery processes configurations, with values exceeding 98%. This finding suggested that the precipitation step may be unaffected by the number of performed extraction (a) (b) 4 steps or the choice of acid extractant. In the TS process, both the acids showed high and comparable PRE1 ranging between 85% and 87%. However, the HCl-based configuration exhibited a PRE₂ 25% lower than that of H2SO4-based one, possibly due to the lower observed PEE2. As a result, the PRETOT for the TS configurations was 74% with H2SO4 and 59% with HCl, highlighting the better performance of H2SO4 when applying the TS process. The PRETOT of the TS processes was consistently lower than that of the OS processes, with differences up to -27% when employing HCl. This outcome, mainly attributed to the lower extracting potential of leachate1 compared to the initial acid solution, underscored the need of further optimization of the extraction2 operating conditions. Finally, the Ppr of each TS process configuration ranged between 14% and 18%, slightly exceeding that of H2SO4- (11%) and HCl-based (17%) OS configurations. This finding indicated the suitability of the TS process for recovering high-value P-rich products. REFERENCES Boniardi, G., Turolla, A., Fiameni, L., Gelmi, E., Malpei, F., Bontempi, E., & Canziani, R. (2021). Assessment of a simple and replicable procedure for selective phosphorus recovery from sewage sludge ashes by wet chemical extraction and precipitation. Chemosphere, 285, 131476. Boniardi, G., Paini, E., Seljak, T., Azzellino, A., Volonterio, A., Canziani, R., & Turolla, A. (2024a). Optimization of phosphorus wet acid extraction from sewage sludge ashes: Detailed process insight via multi-variate statistical techniques. 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