Sludge Anaerobic Digestion Research Articles

Evaluation of traditional and machine learning approaches for modeling volatile fatty acid concentrations in anaerobic digestion of sludge: potential and challenges.

This study evaluates models for predicting volatile fatty acid (VFA) concentrations in sludge processing, ranging from classical statistical methods (Gaussian and Surge) to diverse machine learning algorithms (MLAs) such as Decision Tree, XGBoost, CatBoost, LightGBM, Multiple linear regression (MLR), Support vector regression (SVR), AdaBoost, and GradientBoosting. Anaerobic bio-methane potential tests were carried out using domestic wastewater treatment primary and secondary sludge. The tests were monitored over 40days for variations in pH and VFA concentrations under different experimental conditions. The data observed was compared to predictions from the Gaussian and Surge models, and the MLAs. Based on correlation analysis using basic statistics and regression, the Gaussian model appears to be a consistent performer, with high R2 values and low RMSE, favoring precision in forecasting VFA concentrations. The Surge model, on the other hand, albeit having a high R2, has high prediction errors, especially in dynamic VFA concentration settings. Among the MLAs, Decision Tree and XGBoost excel at predicting complicated patterns, albeit with overfitting issues. This study provides insights underlining the need for context-specific considerations when selecting models for accurate VFA forecasts. Real-time data monitoring and collaborative data sharing are required to improve the reliability of VFA prediction models in AD processes, opening the way for breakthroughs in environmental sustainability and bioprocessing applications.

Auto-tuning data-driven model for biogas yield prediction from anaerobic digestion of sewage sludge at the south-tehran wastewater treatment plant: Feature selection and hyperparameter population-based optimization

In this study, two data-driven models, artificial neural networks and support vector regression (SVR), have been trained and optimized to predict the biogas yield from anaerobic digesters at the South-Tehran municipal wastewater treatment plant. The auto-tuning approach, including feature selection and hyperparameter population-based optimization, was applied through the genetic algorithm (GA) and particle swarm optimization to improve the training models' performance and help them obtain the best input parameters. The Shapley Additive Explanations (SHAP) analysis was also done to interpret models effectively and assign credit for a model's prediction to each feature. The findings demonstrated that biogas prediction using SVR-GA achieved the highest accuracy, with R2 values of 0.725 and 0.773, and RMSE values (regarding normalized datasets) of 0.477 and 0.492 for the train and test, respectively, while requiring the least computational time compared to other models. The auto-tuning technique, by removing the less important inputs, was able to show that temperature, pH, effluent and influent dry solids, effluent volatile solids (VS), and influent VS of waste sludge were the best input parameters for optimal biogas production modeling. The SHAP analysis revealed that VSeff and temperature were two of the most important features affecting biogas production, exhibiting an inverse impact.

Ammonium-based bioleaching of toxic metals from sewage sludge in a continuous bioreactor

The broader reuse of sewage sludge as a soil fertilizer or conditioner is impeded by the presence of toxic metals. Bioleaching, a process that leverages microbial metabolisms and metabolites for metal extraction, is viewed as an economically and environmentally feasible approach for metal removal. This study presents an innovative bioleaching process based on microbial oxidation of ammonia released from sludge hydrolysis, mediated by a novel acid tolerant ammonia-oxidizing bacteria (AOB), Ca. Nitrosoglobus. Over a span of 1024 days, a laboratory-scale bioleaching reactor processing anaerobically digested (AD) sludge achieved an in-situ pH of 2.5 ± 0.3. This acidic environment facilitated efficient leaching of toxic metals from AD sludge, upgrading its quality from Grade C to Grade A (qualified for unrestricted use), according to both stabilization and contaminants criteria. The improved quality of AD sludge could potentially reduce sludge disposal expenses and enable a broader reuse of biosolids. Furthermore, this study revealed a pH-dependent total ammonia affinity of Ca. Nitrosoglobus, with a higher affinity constant at pH 3.5 (67.3 ± 20.7 mg N/L) compared to pH 4.5–7.5 (7.6 – 9.6 mg N/L). This finding indicates that by optimizing ammonium concentrations, the efficiency of this novel ammonium-based bioleaching process could be significantly increased.

Improving anaerobic digestion sludge properties via heterogeneous activation of peroxydisulfate by CuMgAl-layered double hydroxide

Improving anaerobic digestion sludge (ADS) properties by conditioning technology is necessary for subsequent treatment and disposal. In this study, CuMgAl-layered double hydroxide (CuMgAl-LDH) was prepared by co-precipitation method and applied to activate peroxydisulfate (PDS) for ADS conditioning. Experimental results indicated that under the optimal treatment condition (0.15 g LDH/g TS and 0.2 g PDS/g TS addition), the water content of sludge cake and the time to filter decreased from initial 90.83 % to 82.91 % and 75 min to 39.5 min, respectively, and ortho-P as well as non-biodegradable organics contents in the AD liquor (ADL) also decreased remarkably, which was beneficial for ADS dewatering and subsequent ADL backflow treatment. CuMgAl-LDH activating PDS could produce abundant reactive oxygen species (ROS), resulting in microbial cell disruption, bound water release, sludge particle size decrease and non-biodegradable organics degradation. Mechanisms investigation based on the X-ray photoelectron spectroscopy, electron paramagnetic resonance and ROS quenching experiments results indicated that Cu(I)/Cu(II) redox cycle was responsible for PDS activation, and SO4−· made primary contribution to sludge flocs and cells disruption. This research provided preliminary guideline to design copper-based LDH/PDS system for ADS conditioning in wastewater treatment plants.

Mitigated CH4 release of anaerobic waste fermentation is enabled through effluent degassing system equipped with a polydimethylsiloxane membrane contactor

In this study, first, a fed-batch biogas fermenter was established using anaerobic digester sludge treating secondary sludge from a municipal wastewater treatment plant and operated for 120 days on glycerol as the sole substrate. Then, the prefiltered effluent of the anaerobic digester unit was loaded subsequently into a stirred-tank coupled with a hollow-fibre, polydimethylsiloxane (PDMS) gas-liquid membrane contactor and a dissolved methane sensor for studying the gas recovery process under continuous biogas supply, consisting of CH4 and CO2 in different proportions (70/30 CH4/CO2 vol.%; 50/50 CH4/CO2 vol.%; 30/70 CH4/CO2 vol.%.). Experiments showed that besides the actual composition of the internal biogas, the ratio (0.5–2) of sweep gas (N2) and effluent (liquid) volumetric flow rates (G/L) could be a crucial operating factor with influence on the degassing efficiency attainable by the 1 m2 PDMS membrane module. Results were compared to the performance of the same PDMS membrane module working with synthetic anaerobic digester effluents, indicating the dissolved methane recoveries observed with the synthetic effluents (>50 %) considerably surpassed those with the real effluent (<20 %) where the dissolved methane concentrations, at G/L of 1, were in the range of 12.4 to 17.3 mg L−1.

Red seaweed as an abundant, natural methanogenesis inhibitor for industrial biorefinery

Acidogenic fermentation offers a means to renewably refine biomass to volatile fatty acids from which industrial chemicals can be sustainably derived. Small molecules (bromoethane sulfonate) are used in the literature to inhibit methanogenesis and improve fatty acid yields but are impractical at industrial scale due to cost. The red seaweed Asparagopsis taxiformis has been shown to provide methanogenesis inhibition in the microbiome of the ruminant gut over short retention times (up to 72 h), but its suitability for bioreactor control has not been previously characterised. This study investigated the responses of both rumen fluid and anaerobic digester sludge to various Asparagopsis doses. In rumen fluid, doses of ≥5% Asparagopsis showed equivalent inhibition to bromoethane sulfonate at retention times under 30 days, producing VFA of both similar yields and qualities to this standard inhibitor. Digester sludge was not found to respond to Asparagopsis treatment at the levels tested, suggesting a specificity of treatment to microbial community. These data support the application of Asparagopsis as a scalable reactor control agent for rumen-driven fermenter configurations.

Separation of Cellulose from Wastewater and Valorisation via Pyrolysis: A Case Study in the Czech Republic

Currently, the recovery of resources from urban wastewater (WW) represents a priority. On this topic, the potential recovery of cellulose for its subsequent reuse in different sectors is gaining interest. In this work, a large-size conventional wastewater treatment plant (WWTP) was selected as a case study. A preliminary mechanical treatment was used, with the aim of separating, quantifying, and characterizing cellulose in WW. The results suggest that the per-capita production of dry primary cellulosic sludge (D-PCS) is equal to 1.46 ± 0.13 kgD-PCS PE−1 y−1, with an average calorific value of 21.04 MJ kg−1DM. Cellulosic fibres have an average length of &gt;100 µm and a thickness of 2–5 µm. The D-PCS was subsequently treated via medium-temperature pyrolysis; a total of 29.5% of the initial D-PCS was converted into pyrolyzed primary cellulosic sludge (P-PCS) and only 26% into pyrolytic gas. More than 44.5% of the dried cellulose can be converted into pyrolytic oil. Moreover, three different scenarios of recovery have been considered, and the impact of cellulose separation in terms of COD fluxes entering the WWTP and potential energy recovery has been studied. The results suggested that, in this case study, the potential separation of the primary cellulosic sludge from the influent water flux would have no significant impact on COD load entering the biological treatments and biogas production in the anaerobic digestion of the secondary sludge.

Impact Evaluation of Wastewater Treatment Based on the Anaerobic Digestion of Sewage Sludge Using the Life Cycle Assessment Method

All the inputs and outputs of a technical system can be interpreted from an environmental point of view. Using the life cycle assessment (LCA) approach, some changes that are less harmful to the environment can be included in the system. This research aims to evaluate the environmental effects of the wastewater treatment plant (WWTP) in South Tehran, and the LCA method was used in this study. Based on the data of qualitative parameters obtained from the measurement of Tehran province’s water and sewage company, the environmental emissions were calculated and analyzed using SimaPro software (9.0.0) and the standards defined under the ReCiPe 2016-midpoint method. In the ReCiPe 2016 method, the results were expressed in two intermediate levels (including three classes of influence) and final (including 18). The results showed that the treated wastewater and chlorine factors had the most adverse environmental effects. Among the 18 effect classes, the treated wastewater in the class of marine environmental toxicity with the amount of 101.1531 kg 1,4-DCB had the most environmental impacts among other classes. The power consumed by the biogas-burning combined heat and power (CHP) unit in the wastewater treatment (WWT) process reduced the environmental effects in most impact classes. The most adverse environmental effects of the WWT process are related to damage to human health and the ecosystem. According to the findings, the use of CHP systems is suggested for energy saving and also for reducing harmful effects on the environment.

Recovery of organic matters by activated sludge from municipal wastewater: Performance and characterization

Municipal wastewater treatment processes consume a significant amount of energy and generate substantial carbon emissions. However, organic matters existing in municipal wastewater hold the potential as a valuable carbon source. Activated sludge has the potential to capture and recover the organic matters, thereby enriching carbon sources and facilitating subsequent sludge anaerobic digestion as well as in line with the concept of sustainable development. Based on above, this study investigated the enrichment and recovery characteristics and mechanisms of activated sludge adsorption on carbon sources in municipal wastewater, while optimizing the recovery conditions. The results indicated that insoluble organic matters, as well as a fraction of dissolved organic matters, can be effective recovered within approximately 40 min. Specifically, 74.1% of insoluble organic matters and 25.8% of soluble organic matters were successfully captured by the activated sludge, resulting in a 5.0% increase in sludge organic matter content. Moreover, activated sludge demonstrated remarkable recovery of particulate organic matters across various particle sizes, particularly larger particles (>5 μm) with high protein content. Notably, the dissolved biodegradable organics such as tryptophan and tyrosine protein-like substances according to 3D-EEM and lipids, proteins/amino sugars, and carbohydrates according to FT-ICR MS can be effectively recovered. Finally, the study revealed that the recovery of organic matters from the wastewater by activated sludge followed the pseudo-second-order kinetics model, with surface binding, hydrogen bonding and interparticle diffusion in sludge flocs as the primary adsorption mechanisms. This approach had abroad application prospects for improving the profitability of wastewater treatment plants.

Temporal Dynamics and Contribution of Phage Community to the Prevalence of Antibiotic Resistance Genes in a Full-Scale Sludge Anaerobic Digestion Plant.

Anaerobic digestion (AD) is an important biological resource recovery process, where microorganisms play key roles for material transformation. There has been some knowledge about the prokaryotic community and antibiotic resistance genes (ARGs) in AD, but there has been very limited knowledge of phages. In this study, samples from a full-scale AD plant were collected over 13 months, sequenced, and analyzed for viral and prokaryotic metagenomes. Totally, 3015 viral operational taxonomic units (vOTUs) were detected, mostly assigned to Caudoviricetes. The phage community had faster temporal variation than the prokaryotic community. Warm seasons harbored a higher abundance of both temperate phages and broad host-range phages. Seven ARGs of 6 subtypes were carried by 20 vOTUs, a representative ermT gene was synthesized and expressed, and the resistance activity in the host was examined, confirming the real activity of virus-carried ARGs in the AD process. Some of the ARGs were horizontally transferred between the phage and prokaryotic genomes. However, phage infection was not found to contribute to ARG transfer. This study provided an insight into the ecological patterns of the phage community, confirmed the antibiotic resistance activity of virus-carried ARGs, evaluated the contribution of phages on the ARG prevalence, and laid the foundation for the control strategies of the community and antibiotic resistance in the AD process.

Calibration of a sewage sludge anaerobic digestion model with multiple mineral precipitation for two case studies

Including multiple mineral precipitation in anaerobic digestion models is important to accurately assess the quantity of phosphates (PO4-P) in the liquid phase of digested sludge that can be recovered as a fertilizer in struvite precipitation reactors or by emerging processes such as ion exchange membranes. However, the performance of such processes is highly impacted by the presence of potassium (K+) or calcium (Ca2+) ions, which are most often neglected when validating precipitation models. The novelty of this work was to provide a calibration procedure of an anaerobic digestion model with multiple mineral precipitation that can be used for different case studies and do not focus only on PO4-P but also on K+, Ca2+ Mg2+. To draw this calibration procedure, data from two anaerobic digesters located near Lyon (France) and in the Navarra region (Spain), treating sludge with different biodegradability and concentration ranges of phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K) and soluble inorganic carbon (SIC), have been used. This procedure includes the calibration of the inert COD fraction (fXI,COD) and kinetic precipitation constants (Kr) using Bayesian Monte Carlo techniques. However, a unique parameter set for both anaerobic digesters cannot be identified, leading to site-specific parameters for fXI,COD (0.30 and 0.43), Kr,MgCO3 (274 and 0.4), Kr,CaCO3 (21 and 0.12) and Kr,ACP (270 and 58). The calibrated model was able to estimate PO4-P, Ca2+, Mg2+ and K+ concentration in digested sludge with a relative error below 10 %.

Comparing the effects of Al-based coagulants in waste activated sludge anaerobic digestion: Methane yield, kinetics and sludge implications

Due to its effectiveness and ease of application, the process of flocculation and coagulation is often used for pollution removal in wastewater treatment. Most of these coagulants precipitate and accumulate in waste activated sludge (WAS), and could negatively affect sludge treatments, as observed for anaerobic digestion. Nowadays, wastewater treatment plants (WWTPs) are widely discussed because of the current paradigm shift from linear to circular economy, and the treatments performed at the facility should be planned to avoid or reduce adverse effects on other processes. The aim of this study was to compare the impact of poly aluminum chloride (PAC) and aluminum sulfate (AS) on WAS anaerobic digestion, by feeding replicate serum reactors with different levels of coagulant (5, 10 and 20 mg Al/g TS). Reactors without the addition of any coagulants represented the control group. Results revealed that Al-based coagulants inhibited methane production, which decreased as the coagulant addition increased. The inhibition was much more severe in AS-conditioned reactors, showing average reductions in methane yield from 14.4 to 31.7%, compared to the control (167.76 ± 1.88 mL CH4/g VS). Analytical analysis, FTIR and SEM investigations revealed that the addition of coagulants affected the initial conditions of the anaerobic reactors, penalizing the solubilization, hydrolysis and acidogenesis phases. Furthermore, the massive formation of H2S in AS-conditioned reactors played a key role in the suppression of methane phase. On the other hand, the use of coagulant can promote the accumulation and recovery of nutrient in WAS, especially in terms of phosphorus. Our findings will expand research knowledge in this field and guide stakeholders in the choice of coagulants at full scale plant. Future research should focus on reducing the effect of coagulants on methane production by modifying or testing new types of flocculants.

Effect of different digestate biochars as promoters via sludge anaerobic digestion on subsequent pyrolysis products: Focusing on the nitrogen, sulfur, and chlorine releasing characteristics

Anaerobic digestion generates a significant amount of byproduct known as digestate, necessitating resourceful disposal. However, the digestate biochar may exert certain impacts on the pyrolysis of sludge, especially regarding the release of N/S/Cl during the pyrolysis process. This may lead to environmental risks and impose restrictions on the recycling of digestate. There is limited existing research on this topic. In this study, the impact and mechanism of digestate biochar on pyrolysis products were analyzed through kinetic analysis, TG-MS, and Py-GC/MS, with a specific focus on the release characteristics of polluting elements during the pyrolysis process. The findings indicate that the Fe-containing digestate biochar, owing to the larger specific surface area, richer functional groups and catalytic activity associated with Fe3+, enhances the production of C/H/O gases during pyrolysis while reducing the release of NH3, NO2, and H2S. Particularly, Fe-containing digestate biochar significantly enhances the release of Cl-containing gases. The decrease in tar content indicates that Fe-containing digestate biochar promotes the catalytic removal of tar. A comprehensive comparison and analysis of the reaction mechanism were conducted in this study, providing a theoretical basis for the release of pollutants from digestate pyrolysis while enhancing the control and management of pollutants containing N/S/Cl.

Phosphorus release and realignment in anaerobic digestion of thermal hydrolysis pretreatment sludge – Masking effects from high ammonium

Waste activated sludge (WAS) is a significant phosphorus (P) repository, and there is a growing interest in P recovery from WAS. Typically, the commercial technology for treating WAS involves thermal hydrolysis pretreatment (THP) coupled with anaerobic digestion (AD). However, there is ongoing debate regarding the transformation and distribution of P throughout this process. To address this, a long-term THP-AD process was operated in this study to comprehensively investigate P transformation and distribution. The results revealed that a substantial biodegradation of dissolved organic nitrogen (DON) raised the pH of the digestate to 8.3 during the AD process. This increased pH facilitated the dissolution of Al, leading to a reduction of 6.92 mg/L of NaOH-P. Simultaneously, sulfate reduction contributed to a decrease of 11.04 mg/L of Bipy-P in the solid. However, the reduction of Bipy-P and NaOH-P in the solid did not result in an improved P release to the supernatant. Conversely, a decrease of 23.60 mg/L P in the aqueous phase was observed after anaerobic digestion. The disappeared P was primarily precipitated with Mg and Ca, driven by the increased pH, and it contributed to the increase of HCl-P in the solid from 107.80 to 144.52 mg/L. These findings were further confirmed by results obtained from scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. This study provides valuable insights into the mechanisms of P transformation during THP-AD process that is nearly opposite from conventional AD system.

Integration of hydrothermal liquefied sludge as wastewater for anaerobic treatment and energy recovery: Aqueous phase characterization, anaerobic digestion performance and energy balance analysis

For the hydrothermal liquefaction (HTL) process as the final method for waste activated sludge, the liquid by-product of the process, i.e., the HTL aqueous solution, poses the greatest bottleneck due to its large volume and contaminant levels. In this study, the effects of hydrothermal liquefaction pretreatment (HTL) on the performance of anaerobic digestion and energy balance of the waste-activated sludge (WAS) aqueous phase were investigated. The results showed that HTL could greatly increase the hydrolysis of WAS and enhance the protein, polysaccharide and volatile fatty acid contents in the aqueous phase. The anaerobic digestion showed that the optimal gas and methane production efficiency was achieved at 160 °C. Positive net energy gain was obtained in each process (ΔE) and the optimal HTL heat recovery rate was obtained at 160 °C. HTL was found to improve the hydrolyzing bacterial community in anaerobic digestion but had little effect on the archaeal community.

Environmental and economic life cycle assessment of emerging sludge treatment routes

Before implementing large-scale emerging sludge treatment methods, it is crucial to conduct a comprehensive analysis of their environmental impacts and economic costs at the system level. This study utilizes standardized Life Cycle Assessment (LCA) and Life Cycle Cost (LCC) accounting methods to evaluate four emerging sludge treatment routes: anaerobic fermentation (AF), pyrolysis (PY), hydrothermal carbonization (HTC), and alkaline thermal hydrolysis (ATH). These routes are compared with two traditional sludge treatment routes, anaerobic digestion (AD) and aerobic composting (AC). The results indicate that the conventional route AD is the optimal choice in terms of environmental pollution, carbon emission control, and life cycle cost. The emerging routes AF and ATH have significant environmental benefits from carbon source recovery and protein recovery. However, they require approximately twice the cumulative energy demand of AD and additional consumption of chemicals, which imposes a greater environmental burden. While PY and HTC outperform the conventional sludge treatment routes in terms of the total environmental impact (1.79E-10, 1.72E-10) due to energy recovery and the use of clean energy sources. AF and PY demonstrate better economic performance (842.11 and 712.00 CNY) compared to the traditional route AC (905.06 CNY). However, both HTC and ATH incur costs approximately 1.3 times higher than AC due to their high energy and material demands. Furthermore, sensitivity analysis reveals that emerging treatment routes are highly influenced by energy and resource recovery and consumption. Therefore, reducing the energy and resource inputs for emerging routes could significantly decrease their environmental impact and cost, maximizing their resource recovery advantages.

Acidogenic Fermentation of Food Waste for the Production of Short-Chain Fatty Acids: The Impact of Inoculum Type and Inoculum Heat Pretreatment

Acidogenic fermentation is an emerging biotechnology that allows for the utilization of food waste as a feedstock to produce high-value products such as short-chain fatty acids (SCFAs), effectively offering a tangible solution for food waste management as well as resource recovery. The objectives of the current study were to identify the ideal inoculum, waste-activated sludge (WAS) or anaerobic digester sludge (AD), for the acidogenic fermentation of food waste at room temperature, as well as to evaluate the impact of heat pretreatment of these inoculums on fermentation performance. The maximum hydrolysis yield of 399 g sCOD/kg VS added was obtained when untreated AD was used as the inoculum, whereas the pretreated AD inoculum provided the highest SCFA yield and conversion efficiency of 238 g sCODSCFA/kg VS added and 71%, respectively. Heat pretreatment had a detrimental impact on the WAS inoculum, leading to lower hydrolysis and SCFA yields, but exerted a positive influence on the AD inoculum. The microbial community showed that heat pretreatment negatively impacted the abundance of non-spore-forming hydrolytic and acidogenic microorganisms. Overall, this study demonstrates the critical role of inoculum type and heat pretreatment in optimizing the acidogenic fermentation process, laying the groundwork for future refinements in SCFA production from food waste through inoculum design.

Temperature-phased anaerobic sludge digestion effectively removes antibiotic resistance genes in a full-scale wastewater treatment plant

Sludge is a major by-product and the final reservoir of antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs). Temperature-phased anaerobic digestion (TPAD), consisting of thermophilic anaerobic digestion (AD) (55 °C) and mesophilic AD processes (37 °C), has been implemented in WWTPs for sludge reduction while improving the biomethane production. However, the impact of TPAD on the ARGs' fate is still undiscovered in lab-scale experiments and full-scale WWTPs. This study, for the first time, investigated the fate of ARGs during the TPAD process across three seasons in a full-size WWTP. Ten typical ARGs and one integrase gene of class 1 integron (intI1) involving ARGs horizontal gene transfer were examined in sludge before and after each step of the TPAD process. TPAD reduced aac(6′)-Ib-cr, blaTEM, drfA1, sul1, sul2, ermb, mefA, tetA, tetB and tetX by 87.3–100.0 %. TPAD reduced the overall average absolute abundance of targeted ARGs and intI1 by 92.39 % and 92.50 %, respectively. The abundance of targeted ARGs in sludge was higher in winter than in summer and autumn before and after TPAD. During the TPAD processes, thermophilic AD played a major role in the removal of ARGs, contributing to >60 % removal of ARGs, while the subsequent mesophilic AD contributed to a further 31 % removal of ARGs. The microbial community analysis revealed that thermophilic AD reduced the absolute abundance of ARGs hosts, antibiotic resistant bacteria. In addition, thermophilic AD reduced the abundance of the intI1, while the intI1 did not reproduce during the mesophilic AD, also contributing to a decline in the absolute abundance of ARGs in TPAD. This study demonstrates that TPAD can effectively reduce the abundance of ARGs in sludge, which will suppress the transmission of ARGs from sludge into the natural environment and deliver environmental and health benefits to our society.

Enhancing BMP and digestibility of DAF sludge via hydrodynamic cavitation

This study addresses fats, oils, and grease bioconversion challenges in dissolved air floatation (DAF) sludge from dairy processing waste streams. A hydrodynamic cavitation (HC) based pre-treatment method was developed for enhanced anaerobic digestion of DAF sludge. Bench-scale pre-treatment experiments were carried out using a vortex-based HC device at 20 L/min and 250 kPa pressure drop. Influence of severity of the pre-treatment (number of passes through HC device) and substrate concentration on biomethane potential (BMP) was quantified. The study revealed that soluble chemical oxygen demand (sCOD) increases with number of passes (34 % after 80 passes). Higher sludge concentration was found to increase the lag time observed in the BMP data. It was possible to tolerate 3 % VS sludge. The pre-treatment showed a notable increase in BMP, exceeding 82 % of theoretical BMP, with VS removal surpassing 73 %. The highest methane yield achieved was 756 mL/gVS of sludge. The net energy gain (after subtracting energy required for pre-treatment) was found to be more than 100 kWh/ton of sludge. The developed pre-treatment process and presented results provide a basis for the effective valorisation of DAF sludge, promoting a circular economy approach.

Magnetic porous microspheres enhancing the anaerobic digestion of sewage sludge: Synergistic free and attached methanogenic consortia

The addition of exogenous materials is a commonly reported method for promoting the anaerobic digestion (AD) of sludge. However, most exogenous materials are nano-sized and their use encounters problems relating to a need for continuous replenishment, uncontrollability and non-recyclability. Here, magnetic porous microspheres (MPMs), which can be controlled by magnetic forces, were prepared and used to enhance the methanogenesis of sludge. It was observed that the MPMs were spherical particles with diameters of approximately 100 µm and had a stable macroporous hybrid structure of magnetic cores and polymeric shells. Furthermore, the MPMs had good magnetic properties and a strong solid–liquid interfacial electron transfer ability, suggesting that MPMs are excellent carriers for methanogenic consortia. Experimental results showed that the addition of MPMs increased methane production and the proportion of methane in biogas from AD by 100.0 % and 21.2 %, respectively, indicating the MPMs notably enhanced the methanogenesis of sludge. Analyses of variations in key enzyme activities and electron transfer in sludge samples with and without MPMs in AD revealed that the MPMs significantly enhanced the activities of key enzymes involved in hydrolysis, acidification and methanation. This was achieved mainly by enhancing the extracellular electron transfer to strengthen the proton motive force on the cell membrane, which provides more energy generation for methanogenic metabolism. A careful examination of the variations in the morphology, pore structure and magnetism of the MPMs before and after AD revealed that the MPMs increased the prevalence of many highly active anaerobes, and that this did not weaken the magnetic performance. The microbial community structure and metatranscriptomic analysis further indicated that the acetotrophic methanogens (i.e., Methanosaeta) were mainly in a free state and that CO2-reducing methanogens (i.e., Methanolinea and Methanobacterium) mainly adhered to the MPMs. The above synergistic metabolism led to efficient methanogenesis, which indicates that the MPMs optimised the spatial ecological niche of methanogenic consortia. These findings provide an important reference for the development of magnetic porous materials promoting AD.