Valorization of sludge pre-treated with hydrothermal carbonization via fermentation: influence of reactor configuration and process parameters

1 Valorization of sludge pre-treated with hydrothermal carbonization via fermentation: influence of reactor configuration and process parameters Marco Pesenti*, Maria Jose Hernandez Sanchez*, Alisar Kiwan**, Daniele Pirini**, Cristian Torri***, Matteo Grana*, Elena Ficara*, Andrea Turolla* * Politecnico di Milano, Dipartimento di Ingegneria Civile e Ambientale (DICA), Piazza Leonardo da Vinci 32, 20133 MI, Italy (E-mail: marco1.[email protected], maria[email protected], [email protected], [email protected], [email protected]) ** B-Plas S.b.r.l, Via Lanfranco Gessi, 16, 48022 Lugo RA, Italy (E-mail: [email protected], daniele.pirin[email protected]) *** Dipartimento di Chimica “Giacomo Ciamician”, Università di Bologna, Laboratori “R. Sartori”, Via Sant’Alberto 163, 48123 RA, Italy (E-mail: cristian.tor[email protected]) Abstract Sewage sludge from wastewater treatment plants poses considerable challenges for treatment and disposal. Hydrothermal carbonization (HTC) offers a promising sludge treatment method, producing process liquid and hydrochar with notable properties such as better filterability and fermentability. This study examines the potential of HTC to enhance volatile fatty acids (VFAs) production from sewage sludge. Four continuous fermentation reactor configurations were evaluated: a sequencing batch reactor (SBR) and three continuously stirred tank reactors (CSTRs). The tests aimed to evaluate VFA yields under varying operational conditions, including hydraulic retention times (HRT) of 4 days vs 8 days, as well as the effects of upstream solid-liquid separation. The results showed that HTC significantly enhanced the potential for methane production from the treated sludge. CSTR reactors operating at an 8-day HRT consistently achieved the highest VFA conversion rates. Although the SBR configuration exhibited the greatest VFA productivity, overall acidification yields showed no significant differences between the reactor types. Keywords Carbon management, resource recovery, sewage sludge, thermochemical processes, volatile fatty acids, wastewater treatment. INTRODUCTION Sewage sludge from wastewater treatment plants (WWTP) poses significant management challenges due to high treatment and disposal costs. Resource recovery is essential for achieving environmental and economic sustainability, with common approaches including biological treatments, thermochemical conversion, and the land application of stabilized biosolids (Bora et al., 2020). Recently, anaerobic fermentation for volatile fatty acids (VFAs) production has gained attention as a carbon recovery strategy. VFAs have valuable applications in WWTPs for enhancing nutrient removal and in the production of polyhydroxyalkanoates, which could be used in the future as sustainable alternatives to fossil-based plastics (Atasoy et al., 2018). Hydrothermal carbonization (HTC) has emerged as a promising thermochemical technology for sludge treatment, improving its filterability and fermentability (Grana et al., 2024). In HTC reactors, sludge is processed at elevated temperatures (170–350 °C) and pressures (5–25 MPa) for durations ranging from minutes to hours (Chen et al., 2021). HTC generates two primary products: a process liquid (HTC PL) rich in dissolved organic macromolecules and a solid carbonaceous product known as hydrochar. So far, few studies have explored HTC as a pre-treatment for enhancing sludge fermentability, with outcomes varying across investigations (Chen et al., 2021; Grana et al., 2024). This research work, in which the combination of HTC and acidogenic fermentation were studied at the laboratory scale, 2 aims to further clarify the potential benefits of HTC for sustainable sewage sludge management, focusing on valorization pathways. MATERIALS AND METHODS Inoculum and sludge pre-treatments Microbial inoculum and sewage sludge were collected from a WWTP in northern Italy. The microbial inoculum used for fermentation was sourced from an anaerobic digester. To suppress methanogenic activity, the inoculum was pre-treated at 80 °C for 1 hour, and 2bromoethanesulfonic acid was added at a concentration of 0.2 g/L during the acclimation phase. The sludge was thickened to 5% solids before undergoing HTC pre-treatment in a high-pressure laboratory reactor (5 L capacity) at 180 °C for 1 hour. Following the HTC treatment, the resulting slurry was subjected to pressure filtration to separate the HTC PL from the hydrochar. Biomethane potential tests Mesophilic biomethane potential (BMP) tests were conducted using a volumetric device (AMPTS II, Bioprocess Control). Tests were conducted to optimize the temperature and duration of HTC treatment for enhancing sludge biodegradability. The tests were conducted on untreated sludge (US) and the HTC slurry after the HTC treatment at various temperatures (170, 180, and 200 °C) and at 180 °C for different durations (0, 30, 60, and 120 minutes). Acidogenic fermentation tests Four acidogenic fermentation reactor configurations were tested: one sequencing batch reactor (SBR) and three completed stirred tank reactors (CSTRs). All reactors were operated under thermophilic conditions (50 °C). Table 1 summarizes the specific process parameters for each configuration, including reactor volume, retention time, and the presence or absence of upstream solid-liquid separation for the feeding sludge. Samples of feedstock and fermentate were collected weekly and characterized in terms of total solids (TS), volatile solids (VS), volatile suspended solids (VSS), total suspended solids (TSS), pH, VFAs, sCOD, NH4+, and PO43-. RESULTS AND DISCUSSION Biomethane potential tests Figure 1a shows the results of BMP tests conducted on US and HTC slurry obtained at various temperatures. HTC treatment significantly improved the BMP values of the secondary sludge, with 180 °C emerging as the best temperature for anaerobic treatment. Therefore, this temperature was selected for all subsequent fermentation experiments. Figure 1b illustrates the BMP test results for HTC slurry obtained at 180 °C for different durations. The findings reveal that the highest BMP values were achieved with a treatment duration of 60 minutes and that extending the duration to 120 minutes resulted in a product with reduced biodegradability. Overall, BMP tests revealed that HTC treatment at higher temperatures and longer contact times produces more recalcitrant structures, negatively impacting overall methane yield. Acidogenic fermentation tests Experimental data were processed to compare different reactor configurations by calculating acidification yield (net VFA production to total COD input), acidification efficiency (VFA output to soluble COD output), productivity (net VFA production per unit reactor volume per HRT), and the COD mass balance. Table 2 reports the acidification yield, efficiency, and productivity of the reactors at steady state. The three short HRT reactors showed similar acidification yields, indicating no significant 3 advantage in feeding reactors with filtered versus unfiltered HTC PL. All reactors exhibited relatively low acidification efficiency, suggesting that a substantial fraction of the soluble COD in the effluent comprised non-VFA organic compounds. The SBR configuration, with its decoupled and halved HRT, achieved a higher organic loading rate and greater productivity. Conversely, reactors operating at HRT of 4 days exhibited a lower VFA conversion rate of the influent, as evidenced by the acidification yield in Table 2. Increasing the HRT to 8 days in CSTR3 did not significantly improve the acidification yield. This suggests that a substantial portion of the influent COD was not converted into VFAs. Further insights are provided by the COD mass balance between inlet and outlet COD for all four reactors after the acclimation period, as depicted in Figure 2a. Methanogenesis losses were minimal, but the conversion of influent COD to VFAs remained low, averaging around 25%, with CSTR3 achieving the highest conversion rate of 33%. Finally, Figure 2b presents the average VFA composition across all reactor configurations postacclimation. The VFA mixture was consistently dominated by acetic acid and propionic acid, followed by isovaleric acid, butyric acid, and isobutyric acid. Table 1. Process parameters of the reactors used in the acidogenic fermentation tests. Reactor type Running time [days] Volume [L] Retention time [days] Feed type SBR 108 0.7 HRT=2 and SRT=3.2 HTC PL CSTR1 84 2 HRT=SRT=4 HTC slurry CSTR2 84 2 HRT=SRT=4 HTC PL CSTR3 52 2 HRT=SRT=8 HTC slurry Table 2. Performance indicators (avg. ± st.dev.) of the reactors at steady state: Acidification yield (YA), acidification efficiency (EA) and productivity (P). SBR CSTR1 CSTR2 CSTR3 YA [%] 21.1 ± 4.2 25.7 ± 4.6 23.5 ± 4.4 30.6 ± 2.5 EA [gCOD-VFAOUT /gCODs,OUT] 0.30 ± 0.06 0.29 ± 0.05 0.26 ± 0.04 0.31 ± 0.02 P [gCOD-VFAOUT /(L*d)] 2.3 ± 0.5 1.7 ± 0.4 1.5 ± 0.3 0.9 ± 0.1 a) 4 Figure 1. Effect of HTC temperature (a) and treatment time (b) on biomethane production. Figure 2. Average VFA composition (a) and COD mass balance (b) for all reactors configurations. 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