Comparison of the photocatalytic degradability of PFOA, PFOS and GenX using Fe-zeolite in water.

Knowledge on the photocatalytic degradability of the emerging poly- and perfluorinated alkyl substances (PFAS) in water, specifically GenX, is limited. GenX has been detected globally in river water and is considered potentially more toxic than legacy PFAS. In this study, we compared the photocatalytic degradability of GenX with the legacy compounds perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) using Fe-zeolite photocatalysts. After 7 h of irradiation, GenX showed lower removal (79%) and defluorination (33%) as compared to PFOA (100% removal and 69% defluorination) and PFOS (100% removal and 51% defluorination). The quasi-first-order degradation rate of GenX (1.5 h1) was 12 and 1.2 times lower than PFOA (18.4 h-1) and PFOS (1.8 h-1), respectively. Additionally, PFOA's defluorination rate (0.9 h-1) was approximately 2.6 and 9 times higher than GenX (0.35 h-1) and PFOS (0.1 h-1), respectively. These outcomes correlate with GenX's lower hydrophobicity, leading to reduced adsorption (40%) compared to PFOA (99%) and PFOS (87%). Based on identified transformation products, we proposed a GenX degradation pathway, resulting in ultra-short-chain PFASs with a chain length of 2 and 3 carbon atoms, while PFOA and PFOS degraded stepwise, losing 1 carbon-fluorine bond at a time, leading to gradually shorter chain lengths (from 7 to 2 carbon atoms). In conclusion, GenX is more challenging to remove and degrade due to its lower adsorption on the photocatalyst, potential steric hindrance, and higher production of persistent ultra-short-chain transformation products through photocatalysis.

Enhanced process control of trickle-bed reactors for biomethanation by vertical profiling directed by hydrogen microsensor monitoring.

Biomethanation is an emerging Power-to-X technology enabling CO2 valorisation to produce biomethane using renewable H2. A promising reactor for facilitating biomethanation is the trickle bed reactor (TBR), however, these bioreactors are conventionally operated with a black-box approach, where the system is solely described by the input and output characteristics. This study employed a novel approach for process surveillance of internal dynamics in TBRs by installing multiple H2 microsensors along its vertical axis. The H2 microsensor monitoring was demonstrated for 135 days in a TBR integrated into a full-scale biogas plant. Despite achieving an overall CH4 productivity of 12.6 L L-1 d-1, the vertical positioning of microsensors revealed a clear zonation with CH4 productivity zones reaching 54.8 L L-1 d-1 and enabled early warning detection of deteriorating process performance days before detecting it in the product gas. Thus, vertically positioned microsensors present a promising solution for securing process stability.

Cellulolytic and Xylanolytic Microbial Communities Associated With Lignocellulose-Rich Wheat Straw Degradation in Anaerobic Digestion.

The enzymatic hydrolysis of lignocellulosic polymers is generally considered the rate-limiting step to methane production in anaerobic digestion of lignocellulosic biomass. The present study aimed to investigate how the hydrolytic microbial communities of three different types of anaerobic digesters adapted to lignocellulose-rich wheat straw in continuous stirred tank reactors operated for 134 days. Cellulase and xylanase activities were monitored weekly using fluorescently-labeled model substrates and the enzymatic profiles were correlated with changes in microbial community compositions based on 16S rRNA gene amplicon sequencing to identify key species involved in lignocellulose degradation. The enzymatic activity profiles and microbial community changes revealed reactor-specific adaption of phylogenetically different hydrolytic communities. The enzymatic activities correlated significantly with changes in specific taxonomic groups, including representatives of Ruminiclostridium, Caldicoprobacter, Ruminofilibacter, Ruminococcaceae, Treponema, and Clostridia order MBA03, all of which have been linked to cellulolytic and xylanolytic activity in the literature. By identifying microorganisms with similar development as the cellulase and xylanase activities, the proposed correlation method constitutes a promising approach for deciphering essential cellulolytic and xylanolytic microbial groups for anaerobic digestion of lignocellulosic biomass.

Stick or leave - Pushing methanogens to biofilm formation for ex situ biomethanation.

Biomethanation exploits the ability of methanogenic archaea to convert CO2 and renewable H2 from electrolysis to biomethane. Biofilm reactors are promising for biomethanation scale-up due to high CH4 productivity and low energy input for H2 gas-liquid mass transfer. Effects of operational conditions on biofilm dynamics remain largely uncharacterized but may increase reactor potentials further. This study investigated the effect of hydraulic retention time (HRT) on methanogenic biofilm activity and composition. Commercial carriers floating in liquid were exposed to H2/CO2 for 87 days with the liquid phase being subject to either 18 hours, 10 days, or 20 days HRT. Methanogenic biofilms were dominated by hydrogenotrophic methanogens, but biofilm CH4 productivity was enhanced at 18 hours HRT due to wash-out of competing planktonic species, which otherwise hampered proliferation of biofilm biomass at long HRT. It is suggested that high-rate biofilm reactors can increase methanogenic biofilm activity by minimizing the liquid's H2 exposure.

Anammox and partial nitritation in the mainstream of a wastewater treatment plant in a temperate region (Denmark).

The Marselisborg WWTP (Aarhus, Denmark) fed the mainstream nitrification/denitrification tanks with excess sludge from a sidestream DEMON tank for more than three years to investigate if anammox can supplement conventional nitrification/denitrification in a mainstream of a temperate region. To evaluate this long-term attempt, anammox and also denitrification rates were measured in activated sludge from the main- and sidestream at 10, 20 and 30 °C using 15N-labelling (stable isotope) experiments. The results show that anammox contributes by approximately 1% of the total nitrogen removal in the mainstream tanks and that anammox conversion rates there are approximately 800-900 times lower than in the DEMON. A distinct temperature dependence of both anammox and denitrification rates was also confirmed, however, results from different temperatures did not significantly alter relative shares, e.g. anammox rates in activated sludge from the nitrification/denitrification tanks are also negligible at 30 °C. This indicates that the anammox bacteria abundance in the nitrification/denitrification tanks is too low to play an important role and that an adaptation to lower temperatures had not occurred. Additional in situ measurements in the nitrification/denitrification tanks further revealed that full nitrification dominates over partial nitritation. Dominant nitritation-anammox is therefore excluded per se and also nitrite shunt activities are not particularly supported.

How Low Can You Go: Methane Production of Methanobacterium congolense at Low CO2 Concentrations.

Autotrophic hydrogenotrophic methanogens use H2/CO2 as sole carbon and energy source. In contrast to H2, CO2 is present in high concentrations in environments dominated by methanogens e.g., anaerobic digesters (AD), and is therefore rarely considered to be a limiting factor. Nonetheless, potential CO2 limitation can be relevant in the process of biomethanation, a power-to-gas technology, where biogas is upgraded by the addition of H2 and ideally reduce the CO2 concentration in the produced biogas to 0-6%. H2 is effectively utilized by methanogens even at very low concentrations, but little is known about the impact of low CO2 concentrations on methanogenic activity. In this study, CO2 consumption and CH4 production kinetics under low CO2 concentrations were studied, using a hydrogenotrophic methanogen, Methanobacterium congolense, as model organism. We found that both cellular growth and methane production were limited at low CO2 concentrations (here expressed as Dissolved Inorganic Carbon, DIC). Maximum rates (V max) were reached at [DIC] of 100 mM (extrapolated), with a CO2 consumption rate of 69.2 fmol cell-1 d-1 and a CH4 production rate of 48.8 fmol cell-1 d-1. In our experimental setup, 80% of V max was achieved at [DIC] >9 mM. DIC half-saturation concentrations (K m) was about 2.5 mM for CO2 consumption and 2.2 mM for CH4 production. No CH4 production could be detected below 44.4 µM [DIC]. These data revealed that the limiting concentration of DIC may be much higher than that of H2 for a hydrogenotrophic methanogen. However, DIC is not a limiting factor in ADs running under standard operating conditions. For biomethanation, the results are applicable for both in situ and ex situ biomethanation reactors and show that biogas can be upgraded to concentrations of 2% CO2 (98% CH4) while still retaining 80% V max at pH 7.5 evaluated from M. congolense. Since DIC concentration can vary significantly with pH and pCO2 during biomethanation, monitoring DIC concentration through pH and pCO2 is therefore important for keeping optimal operational conditions for the biomethanation process.

Parameters affecting acetate concentrations during in-situ biological hydrogen methanation.

Surplus electricity may be supplied to anaerobic digesters as H2 gas to upgrade the CH4 content of biogas. Acetate accumulation has been observed following H2 injections, but the parameters determining the degree of acetate accumulation are not well understood. The pathways involved during H2 consumption and acetate kinetics were evaluated in continuous lab reactors and parallel batch 13C experiments. Acetate accumulation increased during initial H2 injections as organic loading rate increased and CO2 levels decreased below 7%. The share of CH4 in H2 and 13C mass balances increased after repeated H2 injections, which corresponded with the increase of Methanomicrobiales observed via qPCR. The organic loading rate, the inorganic carbon level and level of methanogen adaption hence determine acetate kinetics during biomethanation of H2. The three identified parameters may form the base of a decision tool to assess acetate accumulation during H2 injections to an anaerobic digester.

Influence on anaerobic digestion by intermediate thermal hydrolysis of waste activated sludge and co-digested wheat straw.

This paper analyses time (30 and 60 min) and temperature (120-190 °C) effects of intermediate thermal hydrolysis (ITHP) in a two-step anaerobic digestion of waste activated sludge (WAS) with and without wheat straw as a co-substrate. Effects were analyzed by measuring biochemical methane potential for 60 days and assessing associated kinetic and chemical data. Compared to non-treatment, ITHP increased the secondary step methane yield from 52 to 222 L CH4 kg VS-1 and from 147 to 224 L CH4 kg VS-1 for pre-digested WAS and pre-co-digested WAS respectively at an optimum of 170 °C and 30 min. The hydrolysis coefficients (khyd) increased by up to 127% following treatment. Increasing ITHP time from 30 to 60 min showed ambiguous results regarding methane yields, whilst temperature had a clear and proportional effect on the concentrations of acetic acid. The energy balances were found to be poor and dewatering to increase total solids above the values tested here is necessary for this process to be energetically feasible.

Primary sewage sludge filtration using biomass filter aids and subsequent hydrothermal co-liquefaction.

Hydrothermal liquefaction (HTL) is a promising technology for biofuel production and treatment of wastewater sludge. The current study investigates a novel utilization of biomass-assisted filtration of primary sludge to obtain high dry matter (DM) content sludge. Drastic improvements in filtration speed are achieved using different types of lignocellulosic biomass filter aids prepared via mechanical pre-treatment. The combined sludge-biomass filter cake is subsequently used as a feedstock for HTL and shows superior bio-crude yields and properties compared to their individual counterparts. The chemical energy recovery to bio-crude is increased to 75% compared to 46% for biomass and 67% for sludge on its own. The increased DM content of filter cakes (∼25%) compared to primary sludge (5%) increases the energy efficiency of HTL of primary sludge by a factor of 4.5. Introducing a biomass filteraid-HTL combination to a wastewater treatment plant would reduce the organic carbon load to treat by 62%. By combining sludge with lignocellulosic biomass the use of alkali catalyst can be avoided entirely which represents a major cost factor in HTL of lignocellulosics.

Small temperature differences can improve the performance of mesophilic sludge-based digesters.

OBJECTIVE: To assess the effect of small temperature increases in mesophilic sludge-based digesters in order to develop and evaluate strategies for improving the biogas production in full-scale digesters. RESULTS: Methane production was strongly affected by small temperature differences, and this result was consistent across samples from 15 full-scale digesters. The specific methane yield varied between 42 and 97.5 ml g VS-1 after 15 days of incubation at 35 °C, and improved when increasing the digester temperature to 39 °C. Only a limited quantity of additional gas was required to balance out the cost of heating and a positive energy balance was obtained. Further increases in temperature, in some cases, negatively affected the production when operated at 42 °C compared to 39 °C. CONCLUSIONS: Small temperature increases should be applied to mesophilic sludge-based digesters to optimize the biogas production and is applicable to digesters operated in the lower mesophilic temperature range.

Removal of antibiotics during the anaerobic digestion of pig manure.

Antibiotics are frequently used in animals to treat sickness and prevent infection especially in industrial meat production. Some of the antibiotics cannot be completely metabolized and, as an unavoidable result, are excreted and thus end up in manure which is then spread in the environment. Currently increasing amounts of manure is used in biogas production before spreading the residuals on agricultural fields. In this study, the removal patterns of sulfonamides (sulfadiazine, sulfamethizole, sulfamethoxazole) and macrolides (clarithromycin, erythromycin), as well as trimethoprim, were investigated during the anaerobic digestion of pig manure. Batch kinetic tests were conducted both at thermophilic and psychrophilic condition for 40 days. Some of the antibiotics (clarithromycin, sulfadiazine, sulfamethizole) were persistent in all experiments. Thus, no biodegradation was found for sulfadiazine and sulfamethizole in this study. From the studied compounds, only erythromycin was clearly removed and probably degraded during anaerobic digestion with 99% and 20% removal under thermophilic and psychrophilic condition. The removal of erythromycin was fitted to a single first-order kinetic reaction function, giving reaction rate constant of 0.29day-1 and 0.005day-1, respectively.

In-situ biogas upgrading with pulse H2 additions: The relevance of methanogen adaption and inorganic carbon level.

Surplus electricity from fluctuating renewable power sources may be converted to CH4 via biomethanisation in anaerobic digesters. The reactor performance and response of methanogen population of mixed-culture reactors was assessed during pulsed H2 injections. Initial H2 uptake rates increased immediately and linearly during consecutive pulse H2 injections for all tested injection rates (0.3 to 1.7LH2/Lsludge/d), while novel high throughput mcrA sequencing revealed an increased abundance of specific hydrogenotrophic methanogens. These findings illustrate the adaptability of the methanogen population to H2 injections and positively affects the implementation of biomethanisation. Acetate accumulated by a 10-fold following injections exceeding a 4:1 H2:CO2 ratio and may act as temporary storage prior to biomethanisation. Daily methane production decreased for headspace CO2 concentrations below 12% and may indicate a high sensitivity of hydrogenotrophic methanogens to CO2 limitation. This may ultimately decide the biogas upgrading potential which can be achieved by biomethanisation.

Anaerobic digestion of sulfate-acidified cattle slurry: One-stage vs. two-stage.

Two strategies to include acidified cattle manure (AcCM) in co-digestion with normal cattle manure (CM) are presented in this work. The strategies are a single thermophilic (50 °C) continuous stirred tank reactor (CSTR) anaerobic digestion and a two-step (65 °C + 50 °C) CSTR process. In both strategies, two different inclusion levels of H2SO4-acidified CM (10% and 20%) in co-digestion with normal CM were tested and compared with a control CSTR fed only CM. Important enhancement of methane (CH4) yield and solid reductions were observed in the thermophilic one-step CSTR working with 10% AcCM. However, a higher inclusion level of AcCM (20%) caused volatile fatty acid accumulation in the reactor and a more than 30% reduction in CH4 production. In terms of CH4 production, when 10% of AcCM was co-digested with 90% of CM, the two-step anaerobic co-digestion yielded less than the single step. During the first step of the two-step CSTR process, acidogenesis and a partial sulfate reduction were achieved. However, sulfide stripping between the first and the second step must be promoted in order to advance this technology.