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Diniz, B. C., Wilfert, P., Sorokin, D. Y., & Loosdrecht, M. C. M. van. (2025). Anaerobic digestion at high-pH and alkalinity for biomethane production: Insights into methane yield, biomethane purity, and process performance. Bioresource Technology, 429, 132505.
Abstract: The role of high-pH conditions in anaerobic digestion (AD) has traditionally been confined to it’s use in pre-treatment processes. However, operating AD at elevated pH and alkalinity offers significant advantages, including in-situ upgrading of biogas to biomethane. This study examines the potential and scalability of AD under these conditions (pH ∼ 9.3; alkalinity ∼ 0.5 eq/L). The substrate used was the alkaline waste generated from the extraction of extracellular polymeric substances (EPS) from aerobic granular sludge (AGS), and the inoculum used was a haloalkaliphile microbial community from soda lake sediments. To evaluate the system’s performance, the organic loading rate (OLR) was incrementally increased. The highest methane production obtained was 8.4 ± 0.1 mL/day/gVSadded at a hydraulic retention time (HRT) of 15 days and an OLR of 1 kgVS/day/m3. At this loading rate, methanogenesis became the rate limiting conversion. The maximum volatile solids conversion was 48.1 ± 1.1 %. Throughout the reactor operation, methane purity in the biogas consistently exceeded 90 % peaking at 96.0 ± 0.2 %, showcasing the potential for in-situ biogas purification under these conditions. In addition, no ammonia inhibition was observed, even with free-ammonia (NH3) concentrations reaching up to 14 mM. This study underscores the potential of high-pH anaerobic digestion as a sustainable method for both waste treatment and energy recovery.
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Bahgat, N. T., Wilfert, P., Picken, S. J., Sorin, L., Lin, Y., Korving, L., et al. (2025). Impact of phosphorus on the functional properties of extracellular polymeric substances recovered from sludge. Water Research, 274, 123019.
Abstract: Extracellular Polymeric Substances (EPS) are ubiquitous in biological wastewater treatment (WWT) technologies like activated sludge systems, biofilm reactors, and granular sludge systems. EPS recovery from sludge potentially offers a high-value material for the industry. It can be utilized as a coating in slow-release fertilizers, as a bio-stimulant, as a binding agent in building materials, for the production of flame retarding materials, and more. P recovered within the extracted EPS is an intrinsic part of the recovered material that potentially influences its properties and industrial applications. P is present in EPS in different speciation (e.g., P esters, poly-P, ortho-P, etc.). Such P species are already intensively used in the chemical industry to enhance thermal stability, viscoelasticity, emulsification, water-holding capacity, and many other properties of some natural and petroleum-derived polymers. The translation of this knowledge to EPS is missing which prevents the full utilization of phosphorus in EPS. This knowledge could allow us to engineer EPS via phosphorus for specific target properties and applications. In this review, we discuss how P could affect EPS properties based on experiences from other industries and reflect on how these P species could be influenced during the EPS extraction process or in the WWTPs.
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Bahgat, N. T., Wilfert, P., Korving, L., & Loosdrecht, M. van. (2023). Integrated resource recovery from aerobic granular sludge plants. Water Research, 234, 119819.
Abstract: The study evaluated the combined phosphorus, nitrogen, methane, and extracellular polymeric substances (EPS) recovery from aerobic granular sludge (AGS) wastewater treatment plants. About 30% of sludge organics are recovered as EPS and 25–30% as methane (≈260 ml methane/g VS) by integrating alkaline anaerobic digestion (AD). It was shown that 20% of excess sludge total phosphorus (TP) ends in the EPS. Further, 20–30% ends in an acidic liquid waste stream (≈600 mg PO4-P/L), and 15% in the AD centrate (≈800 mg PO4-P/L) as ortho-phosphates in both streams and is recoverable via chemical precipitation. 30% of sludge total nitrogen (TN) is recovered as organic nitrogen in the EPS. Ammonium recovery from the alkaline high-temperature liquid stream is attractive, but it is not feasible for existing large-scale technologies because of low ammonium concentration. However, ammonium concentration in the AD centrate was calculated to be 2600 mg NH4-N/L and ≈20% of TN, making it feasible for recovery. The methodology used in this study consisted of three main steps. The first step was to develop a laboratory protocol mimicking demonstration-scale EPS extraction conditions. The second step was to establish mass balances over the EPS extraction process on laboratory and demonstration scales within a full-scale AGS WWTP. Finally, the feasibility of resource recovery was evaluated based on concentrations, loads, and integration of existing technologies for resource recovery.
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Wilfert, P., Krause, S., Liebetrau, V., Schönfeld, J., Haeckel, M., Linke, P., et al. (2015). Response of anaerobic methanotrophs and benthic foraminifera to 20 years of methane emission from a gas blowout in the North Sea. Marine and Petroleum Geology, 68, 731–742.
Abstract: Methane emissions from marine sediments are partly controlled by microbial anaerobic oxidation of methane (AOM). AOM provides a long-term sink for carbon through precipitation of methane-derived authigenic carbonates (MDAC). Estimates on the adaptation time of this benthic methane filter as well as on the establishment of related processes and communities after an onset of methane seepage are rare. In the North Sea, considerable amounts of methane have been released since 20 years from a man-made gas blowout offering an ideal natural laboratory to study the effects of methane seepage on initially “pristine” sediment. Sediment cores were taken from the blowout crater and a reference site (50 m distance) in 2011 and 2012, respectively, to investigate porewater chemistry, the AOM community and activity, the presence of authigenic carbonates, and benthic foraminiferal assemblages. Potential AOM activity (up to 3060 nmol cm−3 sediment d−1 or 375 mmol m−2 d−1) was detected only in the blowout crater up to the maximum sampling depth of 18 cm. CARD-FISH analyzes suggest that monospecific ANME-2 aggregates were the only type of AOM organisms present, showing densities (up to 2.2*107 aggregates cm−3) similar to established methane seeps. No evidence for recent MDAC formation was found using stable isotope analyzes (δ13C and δ18O). In contrast, the carbon isotopic signature of methane was recorded by the epibenthic foraminifer Cibicides lobatulus (δ13C −0.66‰). Surprisingly, the foraminiferal assemblage in the blowout crater was dominated by Cibicides and other species commonly found in the Norwegian Channel and fjords, indicating that these organisms have responded sensitively to the specific environmental conditions at the blowout. The high activity and abundance of AOM organisms only at the blowout site suggests adaptation to a strong increase in methane flux in the order of at most two decades. High gas discharge dynamics in permeable surface sediments facilitate fast sulfate replenishing and stimulation of AOM. The accompanied prevention of total alkalinity build-up in the porewater thereby appears to inhibit the formation of substantial methane-derived authigenic carbonate at least within the given time window.
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Bahgat, N. T., Siddiqui, A., Wilfert, P., Korving, L., & Loosdrecht, M. C. M. van. (2024). FePO4.2H2O recovery from acidic phosphate-rich waste streams. Water Research, 260, 121905.
Abstract: Phosphorous not only needs to be removed to prevent eutrophication of wastewater effluent receiving surface water bodies, but it also has to be recovered as a scarce finite reserve. Phosphorus chemical precipitation as NH4MgPO4·6H2O, Ca3(PO4)2, or Fe3(PO4)2 ·8H2O is the most common method of phosphorus recovery from phosphorus-rich streams. These minerals ideally form under neutral to alkaline pH conditions, making acidic streams problematic for their formation due to the need for pH adjustments. This study proposes FePO4 .2H2O (strengite-like compounds) recovery from acidic streams due to its simplicity and high efficiency, while also avoiding the need for pH-adjusting chemicals. The effect of initial pH, temperature, Fe (III) dosing rates, and Fe (II) dosage under different oxidation conditions (pO2 = 0.2, 1, 1.5 bar, different H2O2 dosing rates) on phosphorus recovery percentage and product settleability were evaluated in this study. The precipitates formed were analyzed using optical microscopy, SEM, XRD, SQUID, Raman, and ICP. Experiments showed that Fe (III) dosing achieved phosphorus recovery of over 95 % at an initial pH of 3 or higher, and the product exhibited poor settleability in all initial pH (1.5-5), and temperature (20–80 °C) tests. On the other hand, Fe (II) dosage instead of Fe (III) resulted in good product settleability but varying phosphorus recovery percentages depending on the oxidation conditions. The novelty of the study lies in revealing that the Fe (II) oxidation rate serves as a crucial process-design parameter, significantly enhancing product settleability without the requirement of carrier materials or crystallizers. The study proposes a novel strategy with controlled Fe2+-H2O2 dosing, identifying an Fe (II) oxidation rate of 4.7 × 10−4 mol/l/min as the optimal rate for achieving over 95 % total phosphorus recovery, along with excellent settleability with a volumetric index equal to only 8 ml/gP.
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