Pretreated Sludge Research Articles

Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing

The treatment and resource utilization of sludge from municipal sewage treatment plants is an important environmental issue. Cement kiln co-processing offers a promising solution, but challenges remain, particularly regarding sludge properties and feasibility in kiln systems. This study analyzes the characteristics of three pretreated sludges: mechanically dewatered sludge, deeply dewatered sludge, and lime-dried sludge. Using techniques such as thermogravimetric analysis (TGA) and X-ray diffraction (XRD), this study investigates their calorific values and raw material utilizability in co-processing. As the sludge moisture content decreases from interstitial to bound water, energy consumption per ton of evaporated water rises, particularly below 30%. At 10 °C/min heating, energy consumption for mechanically dewatered sludge at 80%, 30%, and 10% moisture was 3573, 8220, and 34,751 kJ/kg, respectively; for deeply dewatered sludge at 60%, 30%, and 10%, the values were 4398, 7550, and 11,504 kJ/kg. Keeping moisture content above 30% before kiln entry reduces energy use and enhances calorific value. Sludge utilizability as a raw material depends on its pretreatment. The ash composition of deeply and mechanically dewatered sludge resembles iron-rich raw materials, while lime-dried sludge aligns more with limestone. The utilizable ash content was 23.3%, 8.1%, and 46.3%, respectively, with lime-dried sludge showing the highest potential. This study provides insights into sludge properties and their co-processing potential in cement kilns, offering scientific and technical support for practical applications.

Application of machine learning in ultrasonic pretreatment of sewage sludge: Prediction and optimization

In this research, typical industrial scenarios were analyzed optimized by machine learning algorithms, which fills the gap of massive data and industrial requirements in ultrasonic sludge treatment. Principal component analysis showed that the ultrasonic density and ultrasonic time were positively correlated with soluble chemical oxygen demand (SCOD), total nitrogen (TN), and total phosphorus (TP). Within five machine learning models, the best model for SCOD prediction was XG-boost (R2 = 0.855), while RF was the best for TN and TP (R2 = 0.974 and 0.957, respectively). In addition, SHAP indicated that the importance feature for SCOD, TN, and TP was ultrasonic time, and sludge concentration, respectively. Finally, the typical industrial scenario of ultrasonic pretreatment of sludge was analyzed. In the secondary sludge, treatment volume at 0.6 L, the pH at 7.0, and the ultrasonic time at 20 min was best to improve the SCOD. In the ultrasonic pretreatment primary sludge, treatment volume of 0.3 L, pH of 7.0, and ultrasonic time of 15 min was best to improve the SCOD. Furthermore, the ultrasonic power at 700 W and ultrasonic time at 20 min were best to improve the C/N and C/P in the secondary sludge. In the primary sludge, the ultrasonic power at 600 W, and the ultrasonic time at 15 min were best to improve C/N and C/P. This study lays a foundation for the practical application of ultrasonic pretreatment of sludge and provides basic information for typical industrial scenarios.

Enhancing organic matter dissolution and microbial community structure in waste activated sludge using thermal-alkali-rhamnolipid pretreatment: Focus on thermal optimization

This study investigated the impact of pretreatment temperature on the dissolution of organic matter in sludge pretreated with alkaline rhamnolipid and its subsequent effect on the microbial community structure during anaerobic fermentation. The findings revealed that when the pretreatment temperature reached 80 °C, the maximum concentration of SCOD is 5008.89 mg/L, which is 4 times higher than that achieved at room temperature. Three-dimensional fluorescence parallel factor analysis indicated that increasing the pretreatment temperature significantly enhanced the degree of humification of sludge DOM and reduced the ratio of protein-like and fulvic acid in the sludge. The 16S rRNA gene-based analyses revealed that the relative abundance of Bacteroidetes was highest at 80 ℃ (11.14 %), and continued to increase as fermentation time progressed. This study highlights the critical role of temperature in the three-factor pretreatment of sludge, demonstrating that higher pretreatment temperatures not only accelerate the acid production rate during anaerobic fermentation but also enhance overall acid production.

Clarifying the mechanism of peroxydisulfate and sulfite activated by Fe2+ for waste activated sludge pretreatment: Enhancement of dewaterabiliy, removal of antibiotic resistance genes and pathogenic microorganisms

The remarkable performance of SO4•— based advanced oxidation processes in improving waste activated sludge (WAS) dewatering has recently received considerable attention. However, its potential effect on the removal of emerging pollutants is often overlooked. In this study, two types of pretreatment methods (S2O82-/Fe2+ and SO32-/Fe2+) were employed to comprehensively evaluate their performance in improving the dewatering of WAS and the removal of antibiotic resistance genes (ARGs) and pathogenic microorganisms. At the optimal dosage of 1.2/1.5 mmol-S2O82-/Fe2+/g-VS, the capillary suction time reduction rate of WAS after S2O82-/Fe2+ was 1.8 times than that of SO32-/Fe2+. Notably, both S2O82-/Fe2+ and SO32-/Fe2+ could remove the 12 types of ARGs identified in WAS. Nonetheless, S2O82-/Fe2+ was more effective in removing ARGs, with a range of removal rate (0.96–2.17 log-units), and reduced the propagation and proliferation potential of residual ARGs. Furthermore, the coliforms content in WAS after S2O82-/Fe2+ decreased significantly with a minimum of 1.84 ± 0.24 log (MPN/g-TS) compared to SO32-/Fe2+. Further analysis indicated that the redistribution of bound extracellular polymeric substances (EPS) in WAS, especially the variations in protein, was the key factor affecting the dewaterability. In contrast to SO32-/Fe2+, the S2O82-/Fe2+ could quickly destroy the EPS structure to release bound water, accelerate the dissociation of WAS flocs, and cause cell lysis and DNA dissolution because of the continuous production and accumulation of SO4•— in the hostile conditions inside. Therefore, S2O82-/Fe2+ is an efficient, economical, and green method to improve WAS dewatering and the removal of pollutants.

Green cement production using sludge from wastewater treatment plants

This study explores the potential of reusing sludge from wastewater treatment plants (WWTPs) as a valuable resource. By calcining the sludge, the resulting ashes are incorporated into cement, leading to the production of eco-friendly or green cement. This method offers a sustainable solution for managing solid waste generated by WWTPs, particularly in Algeria, where waste disposal poses significant environmental challenges. The research focused on evaluating the performance of the green cement by examining the development of mechanical properties, such as compressive strength and flexural strength, over time. The results demonstrate the effectiveness of the proposed valorization process for the sludge studied in this context. Additionally, this approach contributes to reducing the carbon emissions and energy consumption associated with conventional cement production. The incorporation of sludge ash as a partial replacement for clinker also highlights the potential for resource conservation and cost reduction in the construction industry. Overall, this research not only addresses waste management issues but also opens the door for further innovation in the use of alternative materials in cement production. Future studies could explore the optimization of sludge pre-treatment and the long-term environmental benefits of large-scale implementation.

Comparative Studies on Methyl Ester Production from Pretreated Sludge Palm Oil Using Homogeneous and Heterogeneous Base Catalysts

A heterogeneous base catalyst transesterification process with a calcium oxide (CaO) catalyst was performed to produce high-purity methyl ester (ME) from pretreated sludge palm oil (PSPO) derived from sludge palm oil (SPO). Additionally, a comparative analysis was conducted with potassium hydroxide (KOH) as a homogeneous base catalyst to assess the distinctions between heterogeneous and homogeneous base catalysts. The response surface methodology (RSM) was utilized to determine the optimal and recommended conditions for both transesterification processes. For heterogeneous transesterification, a varying CaO catalyst loading (10–60 wt.%), methanol (25–65 wt.%), and reaction time (60–180 min) were essential parameters. Meanwhile, homogeneous transesterification involved investigating the KOH catalyst loading (1–3 wt.%), methanol (1.8–5.5 wt.%), and reaction time (20–60 min). For the heterogeneous-base-catalyzed reaction, the recommended conditions were as follows: a molar ratio of methanol to oil of 5.83:1 (41.61 wt.%), 31.3 wt.% CaO, and a reaction time of 119.0 min, which resulted in a ME purity of 96.51 wt.%. The optimal conditions for homogeneous transesterification were a molar ratio of methanol to oil of 0.49:1 (3.45 wt.%), a 40 min reaction time, and a 1.39 wt.% KOH concentration, which achieved 96.59 wt.% ME and met the standard.

Enhancing waste sludge solubilization through radio frequency treatment perforating bacterial cells

Sludge solubilization is known as a rate-limiting step of anaerobic digestion. Although radio frequency (RF) has been applied for sludge pretreatment due to its similar thermal effect as microwave, the potential non-thermal effects of RF treatment remain controversial. In this study, we demonstrate that RF pretreatment enhances the solubilization and lysis of sludge by 8.02%–19.69% through both thermal and non-thermal mechanisms with less energy input. Scanning electron microscope images provide direct evidence that RF-induced microcurrents penetrated bacterial cells, leading to the release of intracellular substances through formed pores. Additionally, the non-thermal effect of RF treatment which could weaken the cell protection and accelerate the lysis rate involves the disruption of binding forces between extracellular polymeric substances and microbial cells. On average, the utilization of RF at a frequency of 27.12 MHz demonstrates its efficacy as a sludge pretreatment technique, as evidenced by a 13.39% reduction in energy consumption and a 16.9% improvement in treatment performance compared to conductive heating (CH). The findings of this study elucidate the possible mechanism of RF treatment of sludge and could establish a theoretical basis for the practical application of RF treatment in sludge management.

Synergistic effect of biological pre-treatment on co digestion of rice straw and sewage sludge: Process optimization and microbial interactions

The increasing demand for renewable energy sources has prompted a search for innovative and efficient approaches for biogas production. This study investigates the potential of achieving sustainable biogas production through a synergistic combination of biological pre-treatment and co-digestion of rice straw (RS) and sewage sludge (SS). The goal is to enhance the overall methane yield, mitigate substrate limitations, and optimize the biogas production process. In this study, rice straw and sewage sludge were anaerobically co-digested in ratios of 100:0, 70:30, 50:50, 30:70, and 30:70. It was observed that the co-digestion ratio of 70:30 is optimal to achieve maximum methane yield of 0.3 L CH4/(g VS added). Biological pre-treatment with five different organisms Sphingobium sp., Paenibacillus sp., Microbacterium sp., Pseudomonas sp., and Stenotrophomonas sp. was also examined for RS and co-digested with SS at the optimized ratio to improve the degradability of RS. This pre-treatment strategy is anticipated to enhance the accessibility of microorganisms to substrates and accelerate the rates of hydrolysis. The biological pre-treatment resulted in a 20–23% improvement in methane yield compared to untreated substrates. Metagenomic studies revealed the dominance of Bacteroidetes, Firmicutes, and Proteobacteria, which are capable of utilizing lignocellulosic biomass and ultimately converting it to methane. The acetoclastic methanogensis is the major methane generating pathway which is supported by the complete dominance of genus Methanosaeta.

Thermal chemical pretreatment of waste-activated sludge for enhanced solubilization and biogas production: a review

ABSTRACT Waste-activated sludge (WAS) generation is increasing due to the increased generation of wastewater owing to industrialization and urbanization. The disposal of WAS is a significant environmental and financial concern that can be offset by stabilization and valorization via anaerobic digestion (AD). However, the biodegradation is limited due to microbial cells and extracellular polymeric substances (EPS). Physical, chemical, biological, and hybrid pretreatments and the addition of accelerant materials for enhancing direct interspecies electron transfer (DIET) are among the strategies to overcome the poor biodegradation of WAS. Alkaline pretreatment disintegrates the floc structure of WAS by increasing the osmotic pressure, while microwave irradiation has thermal and a-thermal effects and increases the availability of organics for biodegradation. Moreover, the combination of alkaline and thermal pretreatments has synergic effects on solubilization, biogas production, and dewaterability. The disintegration of WAS is recognized by alteration in volatile solid (VS) content, sCOD, BOD/sCOD, turbidity, nutrients solubilization, dewaterability, particle size, specific surface area, change functional group, alteration in microorganism community, microorganism abundance, color, moisture content, lag phase, and biogas production. Higher doses of pretreatment increase COD solubilization but not biodegradable COD. Maximum COD solubilization ranged from 2 to 37% in alkaline, 21 to 260% in the microwave (MW), and 28 to 624% in hybrid pretreatments.

Elevating clean energy through sludge: A comprehensive study of hydrothermal carbonization and co-gasification technologies

This study explores the recycling challenges of industrial sludge, owing to its non-recyclable properties and associated environmental problems. To promote sustainable energy utilization, a novel approach combining hydrothermal carbonization and co-gasification was employed to facilitate the conversion from waste to energy. The industrial sludge was pretreated in the batch-type hydrothermal treatment unit at 180–220 °C, followed by co-gasification. The experimental results indicate that pretreating the sludge at the hydrothermal temperature of 200 °C maximized its thermal decomposition, leading to a rougher structure with obvious cracks, eventually transforming into numerous fragmented small particles. At 1100 °C with a blending mass ratio of 1:1, the sludge hydrochar at 200 °C significantly enhanced the reactivity of coal char, exhibiting the gasification reactivity index R0.9 of 1.57 times higher than that of untreated char. Using the in-situ technique with the heating stage microscope, it was first observed that the addition of pretreated sludge coal chars underwent gasification in the shrinking core mode, displaying a significant ash melt flow phenomenon. Based on the in-situ X-ray diffraction, it was discovered that more amorphous structures were formed by the reaction of Fe with other minerals in the sludge-coal blended char after hydrothermal carbonization at 200 °C. With pretreatment at the hydrothermal temperature of 200 °C, the sludge can increase the specific surface area of the blended char and facilitate the cracking of carbon crystals during co-gasification. Its specific surface area and the Raman spectroscopic ratio ID1/IG were 1.76 and 1.17 times that of coal char, respectively. Collectively, this study highlights the potential for energy recovery from industrial sludge, contributing to sustainable waste management in the chemical industry.

Innovative waste activated sludge pre-treatment using a raceway pond Reactor: Integration of Low-Temperature and solar radiation

A novel pre-treatment, using a raceway pond reactor (RPR), combining the use of thermal and solar effects, was applied to enhance anaerobic digestion (AD) of waste activated sludge (WAS) from an urban wastewater treatment plant. Thermal pre-treatment at 25, 50, 60 and 70 °C was carried out for 5 h with (under distinct values of UV radiant power) and without solar radiation and varying WAS depth in the RPR (5 or 7 cm). Higher temperatures (60 and 70 °C) and longer treatment times (3 to 5 h) resulted in increased disintegration of bacterial cells. The WAS pre-treatment significantly (p-value ≤ 0.001) boosted soluble chemical oxygen demand (SCOD) levels, achieving a disintegration degree of 19.7 % at 70 °C with solar radiation (7.2 kJUV L-1WAS), which corresponds to a 15-fold increase of SCOD, compared to the baseline at 25 °C without radiation. Moreover, at the highest temperature (70 °C), there was a significant SCOD increase (p-value ≤ 0.05) of approximately 400 mgO2 L-1 in the presence of solar radiation. This synergetic effect between temperature and solar radiation caused cell damage, membrane rupture, and release of cellular content, shifting extracellular polymeric substances from tightly bound to loosely bound and soluble fractions, consequently improving methane production. Methane production increased by 131 % compared to the control (25 °C) under 70 °C and after 7.2 kJUV L-1WAS, indicating a positive impact on WAS hydrolysis. The technology adopted in this work fosters methane production predominantly using renewable solar energy without chemical residues. This is the first report on RPR for WAS pre-treatment, combining mild temperature and solar radiation.

Combining radio frequency heating and alkaline treatment for enhancement of sludge disintegration and volatile fatty acids production from anaerobic fermentation

Sludge pretreatment plays a crucial role in solubilizing particulate matters to release organic matter for subsequent anaerobic fermentation (AF). This study innovatively combines radio frequency (RF) heating and alkaline treatment, and finds that the combined pretreatment achieved a sludge disintegration rate of 35.11 %, which is 15.19 % and 8.48 % higher than single RF or alkaline pretreatment. The dissociated ions from the alkali are conducive to RF action on sludge. Furthermore, the combined pretreatment significantly benefits the subsequent AF experiments, resulting in a 9-fold increase in volatile fatty acids production. Considering cost-effectiveness, the optimal operating condition is a 10-minute RF treatment at pH 10 with a total cost of 4.35 × 10-3 dollars per kg soluble chemical oxygen demand (SCOD) increased. These findings provide a foundational basis for the development of a novel technology for sludge pretreatment.

Life cycle carbon footprints of alternative sludge pretreatment technologies to recover carbon sources for denitrification

Excess sludge from wastewater treatment holds promise as a carbon source for denitrification. While there have been attempts at technological breakthroughs for carbon recovery from sludge through pretreatment and hydrolysis, a low-carbon footprint solution remains elusive. In this study, we evaluated seven pretreatment technologies for carbon recovery from sludge using life cycle assessment (LCA) modeling. The carbon footprints of technological configurations involving alkaline, ultrasound, thermal, oxidation, mechanical, microwave, and alkali/thermal treatments were quantified based on 123 consistent datasets, and compared by incorporating probability and uncertainty analyses with Monte Carlo and global sensitivity methods. Alkaline treatment achieved carbon footprint savings with a median of −128.71 kg CO2-eq/t TS, and the other technological configurations all show loads in median values. Significant variations in carbon footprints were observed both within and across different technological configurations. Probability distributions reveal overlaps among these configurations. Alkaline treatment has a high probability of obtaining carbon footprints ranging from −300 to 300 kg CO2-eq/t TS, while those for the other configurations showed various distribution with modes ranging from 0 to 3000 kg CO2-eq/t TS. Energy and chemical consumptions during pretreatment emerging were identified as key factors influencing the carbon footprint, with contributing 95–99% of the result uncertainty. Recommendations based on the top 15% probability indicate that technological configurations involving alkali/thermal and alkaline treatments offer a good balance of carbon source quality (180.12 kg/t TS and 142.39 kg/t TS, respectively) and relatively low carbon footprints (517.29 ± 80.06 kg CO2-eq/t TS and 136.99 ± 648.64 kg CO2-eq/t TS, respectively). These findings help identify the most sustainable technologies for carbon recovery from sludge and their potential improvements from a carbon footprint perspective. This study thus underscores the need for further research and technological advancements in sludge treatment to facilitate carbon recovery through denitrification.

Kinetic model implementation in raceway pond reactors with hydrodynamic and radiation fields

Vertical mixing plays a critical role in solar-driven processes using raceway pond reactors (RPRs). However, incorporating the time history of radiation for each fluid element is still an open issue. This work aims to develop an effective methodology using Computational Fluid Dynamics (CFD) tools that couples hydrodynamics and radiation fields into kinetic models of biomass growth or cell lysis enhanced from radiation. First, three methodologies to assess vertical mixing were investigated. It was found that, under typical RPR flow conditions, the velocity in the direction of solar light incidence can maintain particles in constant motion and a near-homogenous particle distribution. In addition, two RPR applications were studied regarding radiation influence analysis: the production of microalgae and an innovative approach for waste activated sludge (WAS) pre-treatment, fostering biogas production. Regarding microalgae production, coupling the biokinetic models with CFD data enables the development of a cost-effective computational methodology to describe the growth of microalgae cultures accounting for hydrodynamics and radiation fields. This work was successful in introducing hydrodynamics and radiation conditions in models to design and optimise RPRs. Reduced geometries based in 2D and Periodic Boundary Conditions were used for CFD simulations to make it feasible for RPRs design purposes.

Enhancing methane production in anaerobic digestion of waste activated sludge by combined thermal hydrolysis and photocatalysis pretreatment

Thermal hydrolysis (TH) is promising for sludge pretreatment, but the refractory substances generated at high temperatures inhibit anaerobic digestion. In this study, a novel combined TH and photocatalytic pretreatment method was proposed to improve the anaerobic digestion performance of waste activated sludge. The results showed that the combined pretreatment (170 °C, 0.5 g/L TiO2) increased methane yield by 66 % from 111 ± 5 m L/g VS to 185 ± 5 m L/g VS. After TH pretreatment, photocatalysis further promoted sludge solubilization by destroying extracellular polymeric substances, resulting in an increase in released soluble organic matter from 292 ± 16 mg/L to 4,091 ± 85 mg/L. In addition, photocatalysis improved the biodegradability of sludge by reducing the melanoidin and humic acid contents by 26 % and 20 %, respectively. The proposed novel pretreatment method effectively overcomes the bottleneck of TH technology and provides an alternative pretreatment technology for improving sludge resource recovery.

Exploring nanobubble technology for enhanced anaerobic digestion of thermal-hydrolysis pre-treated sewage sludge

Nanobubble technology was used to enhance anaerobic digestion (AD) of thermal-hydrolysis pre-treated sewage sludge for bioenergy recovery. The prepared air, CO2, and H2 nanobubble solutions, with concentrations of 9.88–10.2 × 107 bubbles/mL, remained stable for at least 7 days. After adding them into AD reactors, significantly higher CH4 production (37.1 %) was observed for the CO2 nanobubble treatment, followed by air (25.6 %) and H2 (14.5 %) nanobubble treatments, compared to the control group. CO2 nanobubble treatment performed the best in improving acidogenesis/acetogenesis, resulting in significantly higher volatile fatty acid generation during the initial 3–4 days. A comparison of reactors supersaturated and non-saturated with oxygen has demonstrated most of the biogas uplift observed to result from the nanobubbles rather than from initial oxygen soluble levels, demonstrating the crucial role of nanobubbles in upgrading AD. This study demonstrates, for the first time, that nanobubbles can provide additional benefits when combined with stablished sludge pre-treatment technologies.

Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS)

The anaerobic digestion of wastewater treatment sludge (WAS) produces a “green” biogas while reducing the amount of residual sludge. To increase the yield of biogas, several individual or combined pre-treatment methods of WAS can be used. These pre-treatment methods substantially reduce the amount of volatile suspended solids (VSSs) and their associated total chemical oxygen demand (TCOD). Pre-treating the sludge will increase the methane yield by 15 to 30%. Although the individual methods have been dealt with in research and large-scale operations, the combined (hybrid) methods have not previously been reviewed. Here, different hybrid treatment methods are reviewed, including (1) thermochemical hydrolysis pre-treatment, using an alkaline or acid addition to enhance solubilization of the sludge cells and increase biogas production; (2) alkaline and high-pressure homogenizer pre-treatment, combining a chemical and mechanical treatment; (3) alkaline and ultrasound pre-treatment, capable of solubilizing organic sludge compounds by different mechanisms, such as the fast and effective ultrasound disruption of cells and the increasing effect of the alkaline (NaOH) treatment; (4) combined alkaline and microwave pre-treatment, which enhances sludge solubilization by at least 20% in comparison with the performance of each separate process; (5) microwave (MW) and peroxidation pre-treatment of WAS suspended solids (SSs), which are quickly (<5 min) disintegrated by MW irradiation at 80 °C; (6) ultrasound and peroxidation pre-treatment, with ozone and peroxides as powerful oxidizing agents; and (7) pulsed electric field (PEF) pretreatment. All literature findings are assessed, discussing relevant operation conditions and the results achieved.

Ultrasonic Disintegration of Municipal Sludge: Fundamental Mechanisms, Process Intensification and Industrial Sono-Reactors.

Sludge disintegration is an environmental and industrial challenge that requires intensive research and technological development. Sludge has a complex structure with a high yield of various chemical and biological compounds. Anaerobic digestion is the most commonly used process for sludge disintegration to produce biogas, detoxify sludge, and generate biosolids that can be used in agriculture . Biological cell lysis is the rate-limiting cell lysis. This review discusses the application of sonolysis as a sludge pretreatment for enhanced anaerobic digestion via three combined processes: thermal destruction, hydrochemical shear forces, and radical oxidation. The mechanistic pathways of sono-pretreatment to enhance biogas, sludge-enhanced dewatering, activation of filamentous bacteria, oxidation of organic pollutants, release of heavy metals, reduction of bulking and foaming sludge, and boosting ammonia-oxidizing bacterial activity are discussed in this review. This article also discusses the use of ultrasound in sludge disintegration, highlighting its potential in conjunction with Fenton and cation-binding agents, and reviews common large-scale sonoreactors available on the market..

Optimization of microwave intensified slaughterhouse sludge pretreatment method to enhance anaerobic digestion process by response surface methodology

Optimization of microwave intensified slaughterhouse sludge pretreatment method to enhance anaerobic digestion process by response surface methodology

Synergic effect of thermo-chemical pretreatment of waste-activated sludge on bio-methane enhancement

Sustainable and environmentally friendly energy production is feasible via anaerobic digestion (AD) of organic wastes, such as waste-activated sludge (WAS). However, due to its limited degradation, a pretreatment strategy is applied to WAS to enhance its bio-degradation and, thus, biogas yield. Alkaline (0.5%–9% g NaOH/gTS, 30 min), microwave (MW) (90°C–175°C), and hybrid (0.5% g NaOH/gTS +125°C) pretreatments were applied to WAS. The characterization of untreated and pretreated WAS revealed that with higher alkaline and MW pretreatment, the soluble chemical oxygen demand (sCOD), carbohydrate, and protein increased; however, the readily biodegradable COD (rbCOD) rate was unlike the sCOD. The sCOD was 7%–18%, 8%–23%, and 37% for alkaline, MW, and hybrid pretreatments, respectively. Stronger alkaline and MW pretreatment induced higher turbidity, capillary suction time, and lower average particle size. AD of alkaline-, MW-, and hybrid-pretreated WAS produced 94% (0.5% NaOH), 125% (MW at 125°C), and 199% (0.5% NaOH and MW at 125°C) increased biogas, respectively, compared to the AD of untreated sludge. The AD data on the alkaline-, MW-, and hybrid-pretreated BMP assays fitted well with the modified Gompertz model with a coefficient of determination above 0.95. The PCA analysis showed that biogas production is closely correlated with pretreatment temperature, VFA production, rbCOD, sCOD, and soluble carbohydrates and protein. Microbial genome sequencing analysis showed an improvement in microbial abundance and diversity. Acetoclastic methanogen (Methanothrix) growth was improved by 37% (MW pretreatment). Abundances of Methanosarcina, using all three metabolic pathways for methanogenesis, were 17, 21, 11, and 48% in the control, alkaline-, MW-, and hybrid-pretreated digestate, respectively, corresponding to 186% improvement in hybrid pretreatment when compared to the control.