This study shows that the leaching caprolactam (CPL) in PA6 microplastics significantly enhanced methane production in anaerobic digestion of WAS.
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•PA6 microplastics motivate methane ...production in WAS anaerobic digestion.•PA6 microplastics offer marginal effect on solubilization and hydrolysis.•PA6 microplastics promote acidification and methanogenesis.•PA6 microplastics mainly affect anaerobic digestion by leaching caprolactam.
Polyamide 6 (PA6) as a typical emerging microplastic pollutant has frequently been featured in wastewater treatment plants (WWTPs), yet its associated impact on waste activated sludge (WAS) anaerobic digestion process have not been fully understood. This study identified the key role of PA6 microplastics (5–50 particles/g TS) in methane production during anaerobic digestion through biochemical methane production testing and model-based analysis, and explored the mechanism involved by monitoring the conversion of metabolic intermediates, the activity of key enzymes, and the effects of leachate caprolactam (CPL). Unlike other microplastics that normally inhibit anaerobic digestion, PA6 significantly enhanced methane production due to the effect of CPL. The presence of 10 particles PA6/g TS improved methane production by 39.5% (from 124 ± 6 to 173 ± 8 L CH4/kg VS). Model-based analysis showed that PA6 promoted methane production potential and volatile solids destruction. PA6 offered marginal effect on the solubilization and hydrolysis of WAS, the leaching of CPL improved acidification and methanogenesis due to the promotion of key enzyme activities, which is the main reason why PA6 increased methane production. These findings indicate that PA6 is less toxic to WAS anaerobic digestion than other microplastics, which prompts an understanding of the dual role of microplastics in WWTPs.
Anaerobic digestion (AD) for biogas production is affected by many factors that includes organic loading rate (OLR). This OLR appears to be closely linked to various other factors and understanding ...these linkages would therefore allow the sole use of OLR for process performance monitoring, control, as well as reactor design. This review's objective is to collate the various AD factor specific studies, then relate these factors' role in OLR fluctuations. By further analyzing the influence of OLR on the AD performance, it would then be possible, once all the other factors have been determined and fixed, to manage an AD plant by monitoring and controlling OLR only. Decisions on reactor design, process kinetics, biogas yield and process stability can then be made much more quickly and with minimal troubleshooting steps.
► Solid state anaerobic digestion (SS-AD) of eight types of lignocellulosic biomass. ► Liquid anaerobic digestion (L-AD) of eight biomass feedstocks was compared with SS-AD. ► No significant ...difference in methane yield between SS-AD and L-AD. ► Volumetric biogas productivity of SS-AD was 2- to 7-fold greater than that with L-AD. ► Methane yields from crop residues were higher than those from woody biomass.
Lignocellulosic biomass feedstocks (switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, maple, and pine) were evaluated for methane production under liquid anaerobic digestion (L-AD) and solid-state anaerobic digestion (SS-AD). No significant difference in methane yield between L-AD and SS-AD, except for waste paper and pine, were found. However, the volumetric productivity was 2- to 7-fold greater in the SS-AD system compared with the L-AD system, except for paper. Methane yields from corn stover, wheat straw, and switchgrass were 2–5 times higher than those from yard waste, maple, and pine biomass. Waste paper had a methane yield of only 15L/kg VS caused by souring during SS-AD due to organic overloading. Pine also had very low biogas yield of 17L/kg VS, indicating the need for pretreatment prior to SS-AD. The findings of this study can guide future studies to improve the efficiency and stability of SS-AD of lignocellulosic biomass.
Solid-state anaerobic digestion (SS-AD) generally occurs at solid concentrations higher than 15%. In contrast, liquid anaerobic digestion (AD) handles feedstocks with solid concentrations between ...0.5% and 15%. Animal manure, sewage sludge, and food waste are generally treated by liquid AD, while organic fractions of municipal solid waste (OFMSW) and lignocellulosic biomass such as crop residues and energy crops can be processed through SS-AD. Some advantages of SS-AD include smaller reactor capacity requirements, less energy used for heating, and no processing energy needed for stirring. Due to its lower water content, the digestate of SS-AD is much easier to handle than the effluent of liquid AD. However, SS-AD systems also have disadvantages such as larger amounts of required inocula and much longer retention time.
The principles and applications of the SS-AD process are reviewed in this paper. The variation in biogas production yields of different feedstocks is discussed as well as the need for pretreatment of lignocellulosic biomass to enhance biogas production. The effects of major operational parameters, including C/N ratio, solids content, temperature, and inoculation on the performance of SS-AD are summarized. While an increase in operating temperature can improve both the biogas yield and the production efficiency, other practices such as using AD digestate or leachate as an inoculant or decreasing the solid content, may increase the biogas yield but have negative impact on production efficiency. Different reactor configurations used in current commercial scale SS-AD systems and the impact of economics on system selection are also discussed.
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•Helical impeller improves mixing & reduces stratification vs. pitched-blade impeller.•Pitched-blade impeller diminishes bacterial diversity, no effect on methanogens.•Pitched-blade ...impeller enhances digestate dewaterability in lab reactors.•Impeller geometry – a new perspective to enhance digestate dewaterability.
The efficiency of anaerobic digestion (AD) processes is intricately tied to mixing quality. This research investigates the influence of two impeller types, namely a helical ribbon impeller (HRI) and a pitched-blade impeller (PBI), on key aspects of AD. The investigation encompassed mixing dynamics, methane production, microbial communities, and the previously unexplored impact on digestate dewaterability. Results show that agitation with the PBI exhibited stratification, with bottom layer total solids (TS) values of 3.1% for the PBI and 2.6% for the HRI. Nevertheless, methane yield remained unchanged, averaging 286 LN/kg volatile solids (VS)added. Slower mixing with the HRI achieved more uniform mixing and reduced energy requirements. Additionally, impeller type significantly affected digestate dewaterability, leading to a 3.8% increase in TS of the dewatered sludge when using the PBI. These findings highlight the importance of considering mixing not only for methane production and reduced maintenance but also for achieving optimal digestate dewaterability.
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•Biochar simultaneously promoted methane production and phosphorus transformation.•300 °C and 0.075 g∙g−1 of biochar were the optimal temperature and dosage.•BC300 promoted an 18% ...increase in the proportion of active phosphorus.•Biochar may facilitate the hydrogenotrophic methanogenic pathway.•Synergistic effect of microorganisms increased the proportion of active phosphorus.
Biochar has been proved to improve methane production in high solids anaerobic co-digestion (HS-AcoD) of dewatered sludge (DS) and food waste (FW), but its potential mechanism for simultaneous methane production and phosphorus (P) transformation has not been sufficiently revealed. Results showed that the optimal preparation temperature and dosage of sludge-based biochar were selected as 300 °C and 0.075 g·g−1, respectively. Under this optimized condition, the methane production of the semi-continuous reactor increased by 54%, and the active phosphorus increased by 18%. The functional microorganisms, such as Methanosarcina, hydrogen-producing, sulfate-reducing, and iron-reducing bacteria, were increased. Metabolic pathways associated with sulfate reduction and methanogenesis, especially hydrogenotrophic methanogenesis, were enhanced, which in turn promoted methanogenesis and phosphorus transformation and release. This study provides theoretical support for simultaneously recovery of carbon and phosphorus resources from DS and FW using biochar.
This study examined the effects of the microbial electrolysis cell (MEC) reactions on anaerobic digestion of waste activated sludge from municipal wastewater treatment under ambient temperature ...conditions (22–23 °C). Two lab-scale digesters, a control anaerobic digester and an electrically-assisted digester (EAD – equipped with a MEC bioanode and cathode) were operated under three solids retention times (SRT = 7, 10 and 14 days) at 22.5 ± 0.5 °C. A numerical model was also built by including the MEC electrode reactions in Anaerobic Digestion Model No.1. In experiments, the EAD showed reduced concentration of acetic acid, propionic acid, n-butyric acid and iso-butyric acid. This improved performance of the EAD is thought to be achieved by direct oxidation of the short-chain fatty acids at the bioanode as well as indirect contribution of low acetic acid concentration to enhancing beta-oxidation. The VSS and COD removal was consistently higher in the EAD by 5–10% compared to the control digester for all SRT conditions at 22.5 ± 0.5 °C. When compared to mathematical model results, this additional COD removal in the EAD was equivalent to that which would be achieved with conventional digesters at mesophilic temperatures. The magnitude of electric current in the EAD was governed by the organic loading rate while conductivity and acetic acid concentration showed negligible effects on current generation. Very high methane content (∼95%) in the biogas from both the EAD and control digester implies that the waste activated sludge contained large amounts of lipids and other complex polymeric substances compared to primary sludge.
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•MEC (microbial electrolysis cell) reactions lowered organic acids concentration.•MEC reactions improved VSS and COD removal by 5–10%.•Psychrophilic digestion with MEC is equivalent to mesophilic conventional digester.•Electric current governed by organic loading rate not by conductivity or acetate.•High purity methane (∼95%) from waste activated sludge regardless of MEC presence.
The long-term performance of anaerobic digestion (AD) often decreases substantially when treating swine wastewater contaminated with heavy metals. However, the toxicological characteristics and ...mechanisms of continuous exposure to heavy metals under different organic loading rates (OLR) are still poorly understood. In these semi-continuous AD experiments, it was found that zinc concentrations of 40 mg/L only deteriorated the reductive environments of AD. In comparison, a concentration of 2.0 mg/L probably facilitated the reproduction of microorganisms in the operating digesters with a constant OLR of 0.51 g COD/(L·d). Nevertheless, when the OLR was increased to 2.30 g COD/(L·d), 2.0 mg/L zinc inhibited various life activities of microorganisms at the molecular level within only 10 days. Hence, even though 2.0 mg/L zinc could promote AD performances from a macroscopic perspective, it had potential inhibitory effects on AD. Therefore, this study deepens the understanding of the inhibitions caused by heavy metals on AD and the metabolic laws of anaerobic microorganisms in swine wastewater treatment. These results could be referred to for enhancing AD in the presence of zinc in practical swine wastewater treatment.
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•High organic loading rates increased toxicity of zinc on microorganisms.•20 and 40 mg/L zinc severely damaged the integrity of the cell membrane.•40 mg/L zinc caused almost complete cell membrane damage by day 70.•2.0 mg/L zinc inhibited the activities of various anaerobic microorganisms.•40 mg/L zinc severely impaired sludge structure stability.
•Swine manure anaerobic digestion (AD) at 55 °C raised 47.9% methane than at 35 °C.•More oxytetracycline (OTC) led to less methane from mesophilic and thermophilic ADs.•Adding 400 mg/L OTC reduced ...energy conversion efficiencies of ADs by 21.4–48.4%.•More than 63.0% and 72.5% OTC were removed via mesophilic and thermophilic ADs.•Methanolinea was bearable to high content OTC in mesophilic and thermophilic ADs.
Anaerobic digestion (AD) technology is a valuable method for producing biogas fuels and treating livestock wastes such as swine manures concurrently. However, the effect of emerging antibiotics on the AD process is still undiscovered. In this study, the influence of oxytetracycline (OTC) on the AD process was investigated under mesophilic (35 ± 0.5 °C) and thermophilic (55 ± 0.5 °C) conditions, respectively. The presence of OTC significantly inhibited the production of methane in AD process, where the methane yields decreased by 58.6% and 73.3% in mesophilic and thermophilic ADs when the initial concentration of OTC was 400 mg/L, respectively. Besides, OTC can be markedly degraded by the AD process with a removal efficiency higher than 90% when the OTC initial concentration is lower than 10 mg/L. Furthermore, a higher concentration OTC led to a lower biomethane yield, energy conversion efficiency, and contaminant removal during both mesophilic and thermophilic ADs. With adding of 400 mg/L OTC, Clostridium sensu stricto 1 (32.9%) and Anaerolinea (29.3%) are dominant to biodegrade organic matter during mesophilic and thermophilic AD systems. Correspondingly, Methanosaeta was functional in producing biomethane in both mesophilic (60.8%) and thermophilic (56.4%) AD systems. Additionally, Methanolinea was bearable to high concentrations of OTC during mesophilic and thermophilic AD processes.
•A new time-based yield was developed according to the current challenges in AD efficiency.•The lag, logarithmic and stabilizing phases of biogas production were investigated.•Four modes related to ...maximum biogas production and stabilization time were evaluated.•Kinetic models recruited to determine the maximum biogas rate (Rm) and the lag phase time.•R2 coefficient augmented from 7 to 51 % for time-based yield comparing the current yield.•The significance of the Rm increased from 52 to 94 %, highlighting the logarithmic phase time.
The development of sustainable energy sources including biogas produced from urban and industrial wastes has received much attention over the past few years. Given that biogas can be an alternative to fossil fuels before long, its yield, representing the value of energy production from the substrate, has been of utmost importance. With reference to the current definition, biogas yield simply depends on the maximum biogas produced per unit mass of the substrate. In this line, anaerobic digestion (AD) is a time-consuming process, whose duration affects the biogas production yield as well as the maximum biogas production rate. Against this background, the present study dedicated focus on the time factor to present a new definition of biogas/methane (CH4) production yield in AD. In light of this, the time entailed for the AD process (T95%), including the lag phase time (Tlag), the time in proportion to the logarithmic production of biogas (Tlog), and the biogas stabilization time (reaching 95 % of the maximum biogas production rate, T→95%) were taken into account. In accordance with previous research, four modes, viz., (i) equal maximum biogas-different time, (ii) different maximum biogas-equal time, (iii) different maximum biogas-different time (receding yield) and (iv) different biogas maximum-different time (approaching yield) were assessed. For this purpose, the kinetic models, namely, modified Gompertz (MG) and logistic function (LF), were recruited to determine the maximum biogas production rate (Rm) and the lag phase time (λ). Among the mentioned modes, the yield difference between the samples investigated according to the current and time-based yields was 24 % on average, and reached 85 % in the first and third modes. Evaluating the data from 32 studies, using the Python programming language, correspondingly revealed that the correlation coefficient between the AD process time (T95%) and the current and time-based yields augmented from 7 % to 51 %. This demonstrated the impact of the time factor on the biogas yield. Based on the MG model, the significance level of the Rm parameter increased from 52 % to 94 %, highlighting the value of the logarithmic phase time of biogas in the AD reactor.
