Anaerobic Digestion Of Wastewater Sludge Research Articles

Petroleum coke supplementation for enhanced biogas production and phosphate removal under mesophilic conditions.

The use of carbon-based conductive materials has been shown to lead to an increase in biogas and methane yields during anaerobic digestion (AD). The effect of these additives on AD using synthetic substrates has been extensively studied, yet their significance for wastewater sludge digestion has not been adequately investigated. Therefore, the aim of this research was to optimize the concentration of petroleum coke (PC) that is a waste by-product of oil refineries, for the anaerobic digestion of wastewater sludge and investigation of phosphate removal in the AD process in the mesophilic temperature range. According to the results of the experiments, supplementing reactors with PC could significantly improve biogas and methane production. Supplementation of reactors with 1.5 g/L PC led to 23.40 ± 0.26% and 42.55 ± 3.97% increase in biogas production and methane generation, respectively. Moreover, the average volatile solids (VS), phosphate, and chemical oxygen demand (COD) removals were 43.43 ± 0.73, 46.74 ± 0.77%, and 60.40 ± 0.38%, respectively.

Anaerobic Digestion of Wastewater Sludge for Improved Energy Recovery: Alkaline Pretreatment Impact, Digestate Quality Assessment, and Reactor Design

Anaerobic Digestion of Wastewater Sludge for Improved Energy Recovery: Alkaline Pretreatment Impact, Digestate Quality Assessment, and Reactor Design

Kinetic Modeling of Methane Production from the Anaerobic Digestion of Wastewater Sludge from a Treatment Plant in Kenitra, Morocco

Kinetic Modeling of Methane Production from the Anaerobic Digestion of Wastewater Sludge from a Treatment Plant in Kenitra, Morocco

Variations in the extraction and clean-up efficiency of DNA during anaerobic digestion of wastewater sludge at mesophilic and thermophilic temperatures

The effect of anaerobic sludge digestion on the isolation of DNA and clean-up efficiency of obtained DNA isolates was studied. Influent and digestate of a continuous full-scale reactor were sampled twice. Also, batch digesters were operated at 35, 45 and 55 °C, namely, AnD.35, AnD.45 and AnD.55. The batch reactors were sampled at 3-to-4-day time intervals during the digestion. A sharp decline in the extraction yield was observed over time for the batch reactors. In AnD.55, the extraction yield declined by 96 % in 12 days. The purity of the DNA isolates was assessed by purity ratios, which included absorption at 260 nm to that at 280 nm (Abs260/280) and to that at 230 nm (Abs260/230). Abs260/230 before clean-up declined as a result of digestion in all reactors, the most severe case being AnD.55 decreasing from 1.91 to 0.47 in nine days. The efficiency of clean-up was evaluated by analysing DNA recovery and changes in the purity ratios. AnD.35 and the full-scale reactor had the highest recoveries (≥ 80 %). Furthermore, a 4.1-fold increase was observed in Abs260/230. Additionally, non-linear correlations were found between purity parameters and the DNA concentration (R2 > 0.7). The observations of the current investigation strongly indicate that the change in DNA extraction yield was at least partly due to the change in DNA extraction efficiency and not just the change in the actual DNA content. This is an important consideration for the quantification of biomass, specific genes, and microbial populations, where DNA isolation plays a key role.

Development of non-linear empirical models to estimate the abundance of carbapenem resistance genes during anaerobic digestion of wastewater sludge at mesophilic and thermophilic temperatures

Antibiotic resistance genes (ARGs) encode resistance against antibiotic drugs in bacteria. Carbapenem antibiotics are one of the last lines of defence against bacterial infections and carbapenem resistance genes (CRGs) accumulate in sludge during wastewater treatment. Anaerobic digestion is the most used method for sludge treatment, and it is important to know the abundance and removal of CRGs in anaerobically digested sludge before land application due to their public health relevance. However, gene quantification is time-consuming, costly, and sensitive to contamination. Previous research indicates a nexus between ARGs, solids, and nutrients in sludge. This study aimed to develop multiple regression models to estimate the abundance of CRGs from solids and nutrient concentrations in sludge, which are routinely measured for the monitoring and design of anaerobic digesters. Batch and continuous reactors were operated at mesophilic and thermophilic temperatures. Sludge samples were taken during digestion. CRGs (blaGES, blaOXA-48 and blaIMP-27), 16S rRNA and integron class 1 were quantified. Solids, chemical oxygen demand, orthophosphate and ammonia were measured as well. Anaerobic digestion reduced the quantity of the genes. First and second-order Kinetic rate constants (K1 and K2) were affected by temperature, and K1 absolute value followed the order blaOXA-48 > blaGES > Int 1. Multiple regression modelling was conducted in linear (LM) and non-linear (NLM) modes. This study presents the first NLM models to simulate the quantities of CRGs during anaerobic digestion of wastewater sludge at mesophilic and thermophilic temperatures. Adjusted R2 ranged 0.352–0.988 and 0.916–0.996 for the LM and NLM models, respectively. NLM models estimated the gene abundance more accurately than LM models, and they may be useful for researchers and practitioners to estimate the removal rate of target genes under different operational conditions.

Finham gas-to-grid plant, UK: delivering a multi-disciplinary renewable energy scheme

Finham ‘gas-to-grid’ project near Coventry, UK produces renewable bio-methane from the biogas generated by anaerobic digestion of wastewater sludge. Completed in 2021, the plant now exports bio-methane to the gas grid, contributing to ‘decarbonisation’ of gas supply and helping to deliver water utility Seven Trent’s 2030 net-zero-emissions goals. This paper focuses on the innovative solutions adopted to maximise production of bio-methane, the plant procurement and delivery process, and the challenges of Covid-19, Britain’s exit from the European Union and site-specific constraints.

A mathematical model for the kinetics of concurrent volatile solids destruction and water evaporation during aerobic and anaerobic digestion of wastewater sludge under mesophilic and thermophilic temperatures

Experimental and computational approaches were employed to model kinetics of volatile solids destruction in the presence of water evaporation during biological digestion of wastewater sludge. Water evaporation occurred in both anaerobic and aerobic laboratory digesters with a much higher intensity in the latter. The water evaporation followed a linear trend over time. By inclusion of the linear equation for water evaporation rate, new kinetic models were developed to predict volatile solids (VS) and volatile suspended solids (VSS) concentrations of batch biological digesters. Modelled results using a first-order model were in good agreement with the measured VS concentrations of the anaerobic reactors operated at 35 °C and 55 °C (AnD.55) with coefficients of determination (R2) 0.92 and 0.84, respectively. VS degradation of AnD.55 was slightly more consistent with zero-order kinetics (R2 = 0.86). VS and VSS concentrations measured for batch aerobic digesters also agreed with the first-order model. R2 values were 0.91, 0.75 and 0.99 for reactors operated at 35 °C (AD.35), 45 °C and 55 °C, respectively. As for the progression in terms of VSS concentration, the highest R2 and lowest sum of squared error values belonged to AD.35. This is the first study that developed kinetic models for simultaneous VS degradation and water evaporation under both mesophilic and thermophilic conditions. The outcomes of this study indicate that water evaporation needs to be considered for accurate determination of the kinetic rate constant and solids reduction in laboratory and full-scale sludge digesters.

Variations in the Extraction and Clean-Up Efficiency of DNA Isolates During Anaerobic Digestion of Wastewater Sludge at Mesophilic and Thermophilic Temperatures

Variations in the Extraction and Clean-Up Efficiency of DNA Isolates During Anaerobic Digestion of Wastewater Sludge at Mesophilic and Thermophilic Temperatures

Anaerobic Digestion of Wastewater Sludge and Alkaline-Pretreated Wheat Straw at Semi-Continuous Pilot Scale: Performances and Energy Assessment

During the last decade, the application of pretreatment has been investigated to enhance methane production from lignocellulosic biomass such as wheat straw (WS). Nonetheless, most of these studies were conducted in laboratory batch tests, potentially hiding instability problems or inhibition, which may fail in truly predicting full-scale reactor performance. For this purpose, the effect of an alkaline pretreatment on process performance and methane yields from WS (0.10 g NaOH g−1 WS at 90 °C for 1 h) co-digested with fresh wastewater sludge was evaluated in a pilot-scale reactor (20 L). Results showed that alkaline pretreatment resulted in better delignification (44%) and hemicellulose solubilization (62%) compared to untreated WS. Pilot-scale study showed that the alkaline pretreatment improved the methane production (261 ± 3 Nm3 CH4 t−1 VS) compared to untreated WS (201 ± 6 Nm3 CH4 t−1 VS). Stable process without any inhibition was observed and a high alkalinity was maintained in the reactor due to the NaOH used for pretreatment. The study thus confirms that alkaline pretreatment is a promising technology for full-scale application and could improve the overall economic benefits for biogas plant at 24 EUR t−1 VS treated, improve the energy recovery per unit organic matter, reduce the digestate volume and its disposal costs.

Modeling the anaerobic digestion of wastewater sludge under sulfate-rich conditions.

Anaerobic digestion (AD) is a biological treatment process to stabilize organic solids and produce biogas. If present, sulfate is reduced to sulfide by anaerobic sulfate-reducing bacteria and the sulfide can be toxic to anaerobic microorganisms. Here, the effect of high initial sulfate concentration on AD of wastewater sludge was investigated using lab-scale batch experiments. Additionally, a systematic mathematical modeling approach was applied for insight into the experimental results. Cumulative biogas and methane production decreased with increasing initial sulfate doses (0-3.300mgSL-1 ). The correlation between the sulfate dose and methane production was consistent with theoretical predictions and model results, indicating no toxic effect of sulfide on methane production. The carbon dioxide content in the biogas decreased linearly with the increasing sulfate dose, which is consistent with the model-predicted behavior of the bicarbonate and hydrogen sulfide buffering system. The examined high sulfate concentrations resulted in no clear negative effects on the COD removal or VSS destruction of the wastewater sludge, indicating negligible inhibition by sulfide toxicity. Even considering the possibility of ferrous sulfide precipitation and the low model estimates of residual sulfide concentration the residual sulfide concentration was higher than reported concentrations that trigger process inhibition. PRACTITIONER POINTS: The effect of sulfate loading on anaerobic digestion of waste activated sludge was characterized. The stoichiometry of sulfate reduction allows accurate prediction of CH4 loss. High sulfate levels (up to 3300mg/L as S) did not affect COD/VSS removal. Sulfide formation increases effluent COD; often misinterpreted as sulfide toxicity. Correcting COD for sulfide's contributions is crucial for results interpretation.

Fluorescence intensity, conductivity, and UV–vis absorbance as surrogate parameters for real-time monitoring of anaerobic digestion of wastewater sludge

Traditional wastewater treatment monitoring techniques are time-intensive and do not allow real-time process control, creating a need for methods capable of rapid assessment. As such, the objective of this research was to develop and test new methods to measure surrogate wastewater and sludge characterization parameters (conductivity, fluorescence intensity, and UV–vis absorbance) and evaluate each parameter’s suitability for real-time assessment of treatment progression and performance. Anaerobic digestion of mixed primary and secondary sludge was carried out in a continuous reactor over 28 days. COD (chemical oxygen demand), TS/VS (total solids/volatile solids), pH, conductivity, UV–vis absorbance, and fluorescence intensity were measured over the treatment period and linear regressions between surrogate parameters and COD and VS were performed to identify potential relationships. Regression analyses yielded strong correlations between COD and conductivity, fluorescence intensity (FI), and UV–vis absorbance (R2 values of 0.84, 0.72, 0.74 respectively), and between VS versus conductivity and UV–vis absorbance (R2 values of 0.68, 0.73 respectively) illustrating their potential as real-time monitoring tools. It was also possible to evaluate the progression of treatment by monitoring the point of changes in the trends of the surrogate parameters.

Importance of 2,4,6-Trichloroanisole (TCA) as an odorant in the emissions from anaerobically stabilized dewatered biosolids

Odours from stabilized biosolids after anaerobic digestion of wastewater sludge can cause local community impact. Apart from the well-known odorants such as sulfur compounds, contributions from other volatile organic compounds (VOCs) to nuisance odours is limited. The presence of compounds with low odour detection thresholds (ODTs) at low concentrations, can present challenges for analytical identification. Thirty-six biosolids samples were taken after anaerobic stabilisation and dewatering at a wastewater treatment plant, Sydney, Australia. Biosolid cake samples were stored outside in loosely covered trays under aerobic conditions, however without interactions with soil microorganisms as it would be in reality. All biosolids cake samples were analysed over a period of 35 days. Emissions were collected onto Tenax TA sorbent tubes using a U.S. EPA flux hood method at storage days 1, 3, 7, 10, 14, 21 and 35. Gas chromatography (GC) coupled with mass spectrometer detector (MSD) and an olfactory detection port (ODP) was used to identify a musty/moldy/earthy type odorant in the biosolids emissions as 2,4,6-trichloroanisole (TCA). Measured odour intensities, classified on a scale from 1 to 4, and odour characters were specified by three ODP assessors. TCA was identified in all biosolid cake emissions. The measured odour intensities of the TCA did not significantly alter as the biosolids were aged, however varied between biosolids cakes. Due to its odour intensity, 85% frequency of detection and its low ODT, which is orders of magnitudes lower than sulfur compounds, TCA should be considered as a potential odorant of concern in biosolids emissions.

Anaerobic digestion and biogas beneficial use at municipal wastewater treatment facilities in Colorado: A case study examining barriers to widespread implementation

In 2004 the state of Colorado became the first state in the U.S. to implement a renewable energy standard. Electrical utilities have met the state's energy goals using a combination of wind, hydroelectric, and solar power, while giving relatively little consideration to biogas generated from domestic wastewater treatment (i.e., biomass accounts for < 0.1% of renewable energy used in Colorado). An examination of wastewater treatment facilities designed to treat flowrates >7570 m3d-1 indicates that energy generated from the anaerobic digestion of wastewater sludge and combined heat and power technologies is limited in Colorado. At the time of this study, only 39% of examined facilities used anaerobic digestion and only 4 of 24 anaerobic digestion facilities actively employed combined heat and power. Barriers to widespread implementation of anaerobic digestion and combined heat and power were primarily associated with costs, e.g., infrastructure or equipment capital costs; however, other barriers were identified, including: effectiveness of available grant and loan programs, state-level regulations, electric utility rate structures, the influence of local decision-making authorities, and a reluctance by wastewater treatment facilities to change sludge digestion practices. To overcome these barriers, recommendations for the state of Colorado include: initiate a comprehensive program focused on promoting current incentives, review the efficacy of available incentive programs and make long-term modifications, enact regulatory updates, increase education and outreach, and establish a program to promote co-digestion of wastewater sludge with other organic waste streams. This state-level case study explores a large number of facilities at a granular level, the results of which may be relevant to other states or regions and can provide insight into barriers elsewhere.

Synthesis of Cr2O3/C3N4 composite for enhancement of visible light photocatalysis and anaerobic digestion of wastewater sludge

Visible light photocatalysts of Cr2O3/C3N4 composites (with different melamine concentrations) were prepared by high temperature calcination method. The composites samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy SEM, energy-dispersive X-ray spectroscopy (EDX), UV–visible spectroscopy and particle size analysis, which clearly indicated the coexistence of both Cr2O3 and C3N4 in the composites. The Cr2O3/C3N4 catalysts were tested for photocatalytic degradation of 2-chlorophenol in wastewater and solubilization of sludge in anaerobic digestion process to enhance biomethane production. The co-catalytic performance of Cr2O3, with 6% of melamine (precursor of C3N4), improved the photocatalytic degradation of 2-chlorophenol (k = 0.0156 min−1) under visible light, where up to 94% removal was achieved at optimum pH 5.0, pollutant concentration of 60 mg/L, and time duration of 180 min. On another hand, application of Cr2O3/C3N4 for photocatalytic pretreatment of sludge released the soluble substances in solution in which sCOD was increased from 431 mg/L to 3666 mg/L after 6 h and VS content decrease by only 9.1%, which indicated that the short time pretreatment could avoid the further mineralization of organic to complete degradation. Thereafter, anaerobic digestion of solubilized sludge was achieved after 30 days with production of 634 ml kg−1VS of methane and 46% of organic matter removal efficiency (OMRE), compared with 472 ml kg−1VS and 402 ml kg−1VS of methane, 35 and 31% of OMRE respectively in photolytic and raw sludge (control) reactors. These results can provide a useful base and reference for the multi applications of visible light Cr2O3/C3N4 photocatalyst in enhancement of degradation of toxic pollutant in wastewater and sludge stabilization with bioenergy production in practice.

Evolution of flow regimes in non-Newtonian liquids under gas sparging

This paper provides experimental evidence supporting the idea that the transition to turbulent flow is governed by the ratio of the specific power input (SPI) and the term G/λ, where G is the viscoelastic modulus of the liquid and λ is the relaxation time, in a vessel containing a fluid agitated by sparged gas (air) at low superficial gas velocities. This finding provides a method for judging the flow regime within a vessel a priori using a nondimensional quantity and can be used as a scale for decision-making in cases where real-time visual analysis is not possible. While the work reported below is motivated by anaerobic digestion of wastewater sludge, the results are obtained using model liquids and should, therefore, have wide application in chemical process engineering, such as CFD simulation of mixing in viscoelastic fluids and mixing in fermentation processes.

Sludge electrooxidation as pre-treatment for anaerobic digestion.

Anaerobic digestion of wastewater sludge is the preferred method for sludge treatment as it produces energy in the form of biogas as well as a stabilised product that may be land applied. Different pre-treatments have been proposed to solubilise organic matter and increase biogas production. Sludge electrooxidation with boron-doped diamond electrodes was used as pre-treatment for waste activated sludge (WAS) and its effect on physicochemical properties and biomethane potential (BMP) was evaluated. WAS with 2 and 3% total solids (TS) achieved 2.1 and 2.8% solubilisation, respectively, with higher solids requiring more energy. After pre-treatment, biodegradable chemical oxygen demand values were close to the maximum theoretical BMP, which makes sludge suitable for energy production. Anaerobic digestion reduced volatile solids (VS) by more than 30% in pre-treated sludge with a food to microorganism ratio of 0.15 g VSfed g-1 VSbiomass. Volatile fatty acids were lower than those for sludge without pre-treatment. Best pre-treatment conditions were 3% TS and 28.6 mA cm-2.

혐기소화에서의 바이오가스 생산 증진을 위한 슬러지 전처리 기술

Economic feasibility is one of the most important factors in energy production from regenerative biomass. From the aspect, biogas from anaerobic digestion of wastewater sludge is regarded as the most economical because of its cheap substrate and additional income from the disposal of waste sludge. Sludge hydrolysis has been regarded as the rate limiting step of anaerobic digestion and many sludge pre-treatment technologies have been developed to accelerate anaerobic sludge digestion for enhanced biogas production. Various sludge pre-treatment technologies including biological, thermo hydrolysis, ultrasonic, and mechanical methods have been applied to full-scale systems. Sludge pre-treatment increased the efficiency of anaerobic digestion by enhancing hydrolysis, reducing residual soilds, and increasing biogas production. This paper introduces the characteristics of various sludge pre-treatment technologies and the energy balance and economic feasibility of each technology were compared to prepare a guideline for the selection of feasible pre-treatment technology. It was estimated that thermophilic digestion and thermal hydrolysis were most economical technology followed by Cell rupture TM , OpenCEL TM , MicroSludge TM , and ultrasound. The cost for waste sludge disposal shares the biggest portion in the economic analysis, therefore, water content of the waste sludge was the most important factor to be controlled.

Comparison of different thickening methods for active biomass recycle for anaerobic digestion of wastewater sludge

The effect of returning solids to the digester, after one of three thickening processes, on volatile solids reduction (VSR) and gas production was investigated. Three different thickening methods were compared: centrifugation, flotation and gravitational sedimentation. The amount and activity of retained biomass in thickened recycled sludge affected the efficiency of digestion. Semi-continuous laboratory digesters were used to study the influence of thickening processes on thermophilic sludge digestion efficiency. Centrifugation was the most effective method used and caused an increase of VSR from 43% (control) up to 70% and gas generation from 0.40 to 0.44 L g(-1) VS. Flotation and gravitational sedimentation ways of thickening appeared to be less effective if compared with centrifugation. These methods increased VSR only by up to 65 and 51%, respectively and showed no significant increase of gas production. The dewatering capacity of digested sludge, as measured by its specific resistance to filtration, was essentially better for the sludge digested in the reactors with centrifugated and settled recycle. The VS concentration of recycle (g L(-1)), as reflecting the amount of retained biomass, appeared to be one of the most important factors influencing the efficiency of sludge digestion in the recycling technology.

The rheological behaviour of anaerobic digested sludge

Producing biogas energy from the anaerobic digestion of wastewater sludge is one of the most challenging tasks facing engineers, because they are dealing with vast quantities of fundamentally scientifically poorly understood and unpredictable materials; while digesters need constant flow properties to operate efficiently. An accurate estimate of sludge rheological properties is required for the design and efficient operation of digestion, including mixing and pumping. In this paper, we have determined the rheological behaviour of digested sludge at different concentrations, and highlighted common features. At low shear stress, digested sludge behaves as a linear viscoelastic solid, but shear banding can occur and modify the apparent behaviour. At very high shear stress, the behaviour fits well to the Bingham model. Finally, we show that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations, and depends only on the yield stress and Bingham viscosity, both parameters being closely linked to the solids concentration.

Operation of the SHARON Denitrification Process to Treat Sludge Reject Water Using Hydrolyzed Primary Sludge to Denitrify

An efficient biological treatment to treat reject water from anaerobic digestion of wastewater sludge is the SHARON denitrification process, which takes place in a chemostat reactor, where aerobic/anoxic periods are alternated under specific hydraulic retention time (HRT) and temperature conditions that favor ammonium oxidizers growth and ensure the total washout of nitrite oxidizers, achieving the biological nitrogen removal over nitrite. An optimized performance of this process to treat Spanish reject water was obtained using methanol and working at an HRT of 2 days, 33 degrees C, and cycle length of 2 hours. Supernatant of hydrolyzed primary sludge was tested to denitrify. Because biochemical oxygen demand was not extremely high in the primary sludge, the fluid dynamics of the system were changed, with respect to the strategy with methanol, but maintaining the reject water influent flowrate. The use of hydrolyzed primary sludge improved the process efficiency, because the alkalinity present in the primary sludge buffered the process until an optimum pH range.