Combining granular UASB and biochar adsorption as two-step sustainable domestic wastewater treatment for resource recovery and micropollutant removal

Combining granular UASB and biochar adsorption as two-step sustainable domestic wastewater treatment for resource recovery and micropollutant removal D. Mosca Angelucci*, N. Pennazza*, E. Donati**, M. Brienza***, O. Baaloudj*** and M. C. Tomei* * Water Research Institute, National Research Council (CNR-IRSA), Via Salaria km 29.300, CP 10, 00015 Monterotondo Stazione (Rome), Italy (E-mail: [email protected]; nicolas[email protected]; [email protected] ) ** Institute for Biological Systems - National Research Council (CNR-ISB), Via Salaria km 29.300, CP 10, 00015 Monterotondo Stazione (Rome), Italy (E-mail: enr[email protected]) *** Dipartimento di Scienze di Base ed Applicate, Università degli Studi della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy (E-mail: [email protected]; [email protected]) Abstract The anaerobic process is a promising solution, given its potentiality to recover energy and nutrients, which is advantageous for water reuse. Nevertheless, resource recovery from domestic wastewater in biological processes is challenging due to its low substrate concentrations resulting in reduced process kinetics. This drawback can be faced by high-rate anaerobic bioreactors, able to achieve high biomass concentration and increase process performance. We investigated the feasibility of a granular biomass Upflow Anaerobic Sludge Blanket (UASB) to treat domestic wastewater at low temperatures and different HRTs to enhance energy and resource recovery. Relevant to water reuse, the fate of a target contaminant of emerging concern, sulphamethoxazole (SMX), and its potential impact on process performance have been evaluated. In addition, to further improve treated water quality, a post-treatment has been proposed for the anaerobically treated effluent. This method is based on a cost-effective approach that utilizes biochar derived from a green material i.e. forest wastes. Keywords Adsorption; Anaerobic wastewater treatment; Biochar; Granular UASB; Nutrient and energy recovery; Sulphamethoxazole. INTRODUCTION Anaerobic treatment of domestic wastewater, owing to significant advantages of energy savings, biogas recovery, and lower sludge production, is a promising sustainable technology (Saba et al. 2017). The feasibility of an anaerobic process has been demonstrated only for high-strength wastewater at temperatures above 20-25 °C. In recent decades, the growing interest in sustainable wastewater treatment has driven research efforts toward high-performance technologies that extend the applicability of anaerobic treatment to low-strength wastewater in temperate climates. Anaerobic treatment in high-rate bioreactors is a cutting-edge technology that can be applied as a stand-alone technology or integrated with other treatments as aerobic processes and physical or chemical pre/posttreatments. The peculiarity of high rate bioreactors is the uncoupling of Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT): since operating with highly concentrated biomass, low HRTs can be applied, thus reducing the bioreactor volume. Among high-rate bioreactors, the Up-flow Anaerobic Sludge Bioreactor (UASB) stands as one of the most effective technologies: the operation with granular biomass, allowing a significant increase in biomass concentration while notably reducing sludge production (Stazi et al. 2022). Higher biomass concentrations allows reaching high biodegradation kinetics, which allow operating with reduced bioreactor volumes and lower effluent concentrations. Besides the recovery of the energy content of wastewater, an additional advantage of UASB technology is the production of treated wastewater rich in nutrients that can be recovered and/or reused for agricultural purposes (Hamza et al. 2016). However, the potential reuse of treated effluents could be highly limited by the presence of contaminants of emerging concerns (CECs), such as pharmaceuticals, which are not always effectively removed in conventional wastewater treatment plants (WWTP). Indeed, their removal is a necessary condition for the safe reuse of reclaimed water. Moreover, a post-treatment has been proposed to improve water quality by removing CECs at trace levels. This treatment utilizes biochar, a green and cost-effective adsorbent, which has demonstrated excellent performance in removing various pollutants in a previous study (Baaloudj et al. 2025). In this work, the feasibility of a granular biomass UASB to treat domestic wastewater has been investigated at 20 °C and different HRTs to enhance energy and resource recovery. Relevant to water reuse, the fate of a target CEC, sulphamethoxazole (SMX), and its potential impact on process performance have been evaluated. The combination of a post-treatment using biochar in a downflow fixed bed adsorption system for removing SMX from the anaerobically treated effluent has been also investigated. MATERIALS AND METHODS UASB bioreactor: configuration and operation The lab-scale UASB bioreactor is a glass vessel with a working volume of 4.2 L (Figure 1a) equipped with three peristaltic pumps for influent feeding, effluent discharging, internal liquid recirculation, and gas output connected to a biogas meter. UASB bioreactor is also equipped with a water jacket connected to a circulation thermo-cryostat to control the temperature at a set point of 20 °C. The bioreactor was inoculated with anaerobic granular biomass (originated from a real brewery WWTP) and operated in continuous mode by feeding synthetic domestic wastewater (COD 500 mg/L, Ntot 40-50 mg/L, Ptot 5-6 mg/L) whose composition is reported by Stazi et al. (2022). Internal effluent recycle was applied to ensure hydrodynamic conditions for effective biomasswastewater contact and granulation maintenance and to favor the release of biogas bubbles entrapped within the granular bed. Fed/discharged wastewater flow rate depends on the HRT applied to the bioreactor, e.g. 4.2, 6.7, and 11.2 L/d for HRT of 24, 15, and 9 h, respectively, according to the experimental campaign reported in Table 1. Table 1. Operating conditions applied to the UASB bioreactor and experimental results. Experimental phases SMX HRT COD removal Biogas yield (µg/L) (h) (%) (mL/min) I - 24 95.0 (3.0) 0.15 II - 15 96.8 (0.5) 0.30 III - 9 95.5 (0.6) 0.44 IV 3-6 24 95.3 (1.8) 0.13 V 7-15 24 96.3 (0.9) 0.15 This experimental campaign was designed to validate the UASB feasibility without micropollutant presence in the first part (phases I-III) while in the last trials, SMX was added to the feed solution at a concentration range of 3-15 µg/L in two subsequent experimental stages (IV-V) to favor the biomass acclimatization. In the last experimental phases, a precautionary HRT of 24 h has been applied. For each experimental phase, periodic measurements of COD, NH3, Ntot, Ptot (and SMX when present) within both influent and effluent streams have been carried out. Additional analysis of total suspended solids (TSS) and volatile fatty acid (VFA) concentration have been performed in the effluents for each experimental phase. For SMX measurements, chromatographic analysis was carried out by an Acquity™ UPLC H-Class Bio system set up with a solvent mixing system, an autosampler, a thermostatically controlled column and a PDA detector. SMX determination was performed on an Acquity UPLC HSS – T3 column (150 x 2.1 mm id, 1.8 mm) using a mobile phase consisting of water (phase A) and acetonitrile (phase B), both containing 0.1% (v/v) formic acid. Flow rate was 0.4 mL/min, the injection volume was 5 μL, the column temperature was maintained at 40 °C and SMX was detected at a wavelength of 267 nm. Other analytical determinations were performed according to Standard Methods (APHA, 2012). Post-treatment biochar adsorption Tests to check the feasibility of the post-treatment with the biochar system for removing SMX at trace level in a synthetic solution with a concentration of 10 µg/L have been performed. Fixed-bed column experiments were conducted using a glass column (internal diameter 3.5 cm, length 15 cm). The solution circulated downward and it was fed with a peristaltic pump. Effluent samples were analyzed at regular time intervals using a UHPLC-MS/MS to measure the removal efficiency of SMX. The experimental setup used in this study is presented in Figure 1b. Figure 1. Picture of the lab-scale UASB (a) and of the biochar column system (b). RESULTS UASB bioreactor Results of the granular UASB for the experimental phases I, II, and III are summarized in Table 1 and displayed in Fig. 2: increasing Organic Loading Rates (OLRs) due to decreasing HRTs guarantee satisfactory process efficiencies for COD removal (always above 95%) and biogas production (within the range of literature values for urban wastewater). In all tested conditions, influent nitrogen and phosphorus were completely recovered in the treated effluent, thus promoting their potential reuse as fertilizers. No VFA accumulation has been observed during the whole experimental period in the treated effluents, hence confirming the stability of the anaerobic process. After the experimentation of the I, II, and III phases, SMX was added to the influent solution for the subsequent IV and V stages, which are still ongoing. Preliminary results of the SMX experimental trials suggested a limited impact on process efficiency, maintaining both satisfactory COD removals (within the range of 94.5-97.4%) and biogas production with values between 0.1 and 0.4 mL/min with values comparable for the same HRT. Removals of SMX are within the range of 20-60% with greater values in the V phase; further tests are scheduled and underway to discriminate a) b) the amount of SMX biodegraded and that sorbed by the granular biomass. Regarding nutrient recovery and VFA measurements in the treated effluents, no substantial variations have been detected during the IV and V experimental phases in comparison to the previous ones. Figure 2. Results of I, II, and III experimental phases (COD_RE: COD removal efficiency). Post-treatment biochar adsorption The biochar system performed effectively in removing SMX reaching concentration values ≤ 0.5 µg/L, as confimed through regular sampling and analysis. The results demonstrated that biochar is a low-cost and effective post-treatment option. The biochar's performance highlights its ability to complement the UASB bioreactor by enhancing the removal of pharmaceuticals from anaerobically treated effluents, thereby improving overall treatment efficacy. This combined approach has the potentiality of being an effective sustainable solution for domestic wastewater treatment. Acknowledgements This research received funding from MUR under the umbrella of the PRIMA — Partnership for Research & Innovation in the Mediterranean Area through the research project SAFE “Sustainable water reuse practices improving safety in agriculture, food and environment” ID project: 1826. REFERENCES APHA, American Public Health Association (2012). Standards Methods for the Examination of Water and Wastewater, 22 ed. APHA, Washington, DC. Baaloudj, O., Chiron, S., Zizzamia, A. R., Trotta, V., Buono, D. Del, Puglia, D., et al. (2025). Efficient biochar regeneration for a circular economy: Removing emerging contaminants for sustainable water treatment. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 705, 135730. https://doi.org/10.1016/j.colsurfa.2024.135730 Hamza, R. A., Iorhemen, O. T., & Tay, J. H. (2016). Advances in biological systems for the treatment of high-strength wastewater. 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