Advanced anaerobic digestion of municipal sludge using a novel and energy-efficient radio frequency pretreatment system

Machine learning assisted modelling of anaerobic digestion of waste activated sludge coupled with hydrothermal pre-treatment

This study utilizes decision-tree-based models, including Random Forest, XGBoost, artificial neural networks (ANNs), support vector machine regressors, and K nearest neighbors algorithms, to predict sludge solubilization and methane yield in hydrothermal pretreatment (HTP) coupled with anaerobic digestion (AD) processes. Analyzing two decades of published research, we find that ANN models exhibit superior fitting accuracy for solubilization prediction, while decision-tree models excel in methane yield prediction. Pretreatment temperature is identified as pivotal among various variables, and heating time surprisingly emerges as equally significant as holding time for solubilization and surpasses it for methane yield. Contrary to prior expectations, the HTP method's impact on sludge solubilization and AD performance is minimal. This study underscores data-driven models' potential as resource-efficient tools for optimizing advanced AD processes with HTP. Notably, our research spans nearly two decades of lab, pilot, and full-scale studies, offering novel insights not previously explored.

Effective multipurpose sewage sludge and food waste reduction strategies: A focus on recent advances and future perspectives

Energy crisis and increasing rigorous management standards pose significant challenges for solid waste management worldwide. Several emerging diseases such as COVID-19 aggravated the already complex solid waste management crisis, especially sewage sludge and food waste streams, because of the increasingly large production year by year. As mature waste disposal technologies, landfills, incineration, composting, and some other methods are widespread for solid wastes management. This paper reviews recent advances in key sewage sludge disposal technologies. These include incineration, anaerobic digestion, and valuable products oriented-conversion. Food waste disposal technologies comprised of thermal treatment, fermentation, value-added product conversion, and composting have also been described. The hot topic and dominant research foci of each area are summarized, simultaneously compared with conventional technologies in terms of organic matter degradation or conversion performance, energy generation, and renewable resources production. Future perspectives of each technology that include issues not well understood and predicted challenges are discussed with a positive effect on the full-scale implementation of the discussed disposal methods.

Impact of solid content on hydrothermal pretreatment of municipal sludge prior to fermentation and anaerobic digestion

This study investigated the impact of the solid sludge content concentrations (SC) on hydrothermal pretreatment (HTP) before fermentation and anaerobic digestion. Five different SC of 3.5%, 7%, 10%, 12%, and 16% were investigated in two different scenarios. The first scenario entailed using only the pretreated samples as substrates, whereas in scenario two, the substrates included pretreated samples combined with the supernatant. Results revealed that the highest overall pCOD solubilization (considering HTP and fermentation) of 64% was achieved for the sample with 12% SC combined with supernatant. The maximum volatile fatty acids production of 2.8 g COD/L occurred with 10% SC without supernatant. The maximum methane yield of 291 mL CH₄/g VSS added was attained at 7% SC without supernatant. Furthermore, the results indicated that increasing the SC beyond 7% in scenario 1 and 10% in scenario two led to a decrease in methane yield. Additionally, optimizing for all desired endpoints may be difficult, and there are limits on the increase in SC concerning methane production.

Principles and potential of thermal hydrolysis of sewage sludge to enhance anaerobic digestion

Sewage sludge is rich source of carbon, nutrients, and trace elements and can be subjected to proper treatment before disposal to fulfill government legislation and protect receiving environments. Anaerobic digestion (AD) is a well-adopted technology for stabilizing sewage sludge and recovering energy-rich biogas and nutrient-rich digestate. However, a slow hydrolysis rate limits the biodegradability of sludge. In the present study we have attempted to explain the potential of thermal hydrolysis to enhance anaerobic digestion of sewage sludge. Thermal pretreatment improves biodegradability and recycling of the sludge as an excellent energy and nutrients recovery source at reasonable capital (CAPEX) and operational (OPEX) costs. Other pretreatments like conventional (below/above 100 °C), temperature-phased anaerobic digestion (TPAD), microwave and chemically mediated thermal pretreatment have also been accounted. This review provides a holistic overview of sludge's characterization and value-added properties, various techniques used for sludge pretreatment for resource recovery, emphasizing conventional and advanced thermal pretreatment, challenges in scale-up of these technologies, and successful commercialization of thermal pretreatment techniques.

A review on recent disposal of hazardous sewage sludge via anaerobic digestion and novel composting

The high investment cost required by modern treatment technologies of hazardous sewage sludge such as incineration and anaerobic digestion have discouraged their application by many developing countries. Hence, this review elucidates the status, performances and limitations of two low-cost methods for biological treatment of hazardous sewage sludge, employing vermicomposting and black soldier fly larvae (BSFL). Their performances in terms of carbon recovery, nitrogen recovery, mass reduction, pathogen destruction and heavy metal stabilization were assessed alongside with the mature anaerobic digestion method. It was revealed that vermicomposting and BSFL were on par with anaerobic digestion for carbon recovery, nitrogen recovery and mass reduction. Thermophilic anaerobic digestion was found superior in pathogen destruction because of its high operational temperature. Anaerobic digestion also had proven its ability to stabilize heavy metals, but no conclusive finding could confirm similar application from vermicomposting or BSFL treatment. However, the addition of co-substrates or biochar during vermicomposting or BSFL treatment may show synergistic effects in stabilizing heavy metals as demonstrated by anaerobic digestion. Moreover, vermicomposting and BSFL valorization had manifested their potentialities as the low-cost alternatives for treating hazardous sewage sludge, whilst producing value-added feedstock for biochemical industries.

The Effect of Detoxification of Lignocellulosic Biomass for Enhanced Methane Production

The aim of this research is to examine the effect of lignocellulosic biomass detoxification on the efficiency of the methane fermentation process. Both for corn straw and rye straw, the methane yield was expressed per volume of fermentation medium and per mass of volatile solids (VS) added. Lignocellulosic biomass was subjected of thermo-chemical and enzymatic sequential pretreatments. It was found that methane yield was higher by 22% when using the detoxification process. In these variants, CH4 yield was 18.86 L/L for corn straw and 17.69 L/L for rye straw; while methane yield expressed per mass of VS added was 0.31 m3/kg VS for corn straw and 0.29 m3/kg VS for rye straw. The inclusion of a detoxification step in pretreatments of biomass lignocellulosic increases the degree of organic substance decomposition and enhances methane yield. The results show that a two-step pretreatment, alkaline/enzymatic with a detoxification process, is necessary for the effective generation of high methane concentration biogas.

Enhancing methane production from dewatered waste activated sludge through alkaline and photocatalytic pretreatment

Waste activated sludge generated from wastewater treatment plants makes an abundant source of biomass. Its effective utilization through anaerobic digestion (AD) requires pretreatment to disintegrate the sludge matrix and increase organic matter availability. In this study, dewatered waste activated sludge (DWAS) was subjected to alkaline, photocatalytic, and alkaline-photocatalytic pretreatment for its disintegration and subsequent methane production using different concentrations of sodium hydroxide and titania nanoparticles. Individual pretreatment resulted in maximum disintegration degree (DD_sCOD) of 11.3 and 5.2% at 0.8% NaOH and 0.6 gTiO₂/L, respectively. Alkaline-photocatalytic pretreatment yielded 37% DD_sCOD at 0.8% NaOH-0.4 g/L TiO₂. As compared to control, AD at 0.4% NaOH and 0.5 g/L TiO₂ pretreatments yielded maximum methane, which was 50.4 and 32.6% higher. Similarly, alkaline-photocatalytic pretreatment at 0.4% NaOH-0.5 g/L TiO₂ yielded methane as 462 N mL/g VS, which was 71.1% higher. Modified Gompertz model fitted the methane yield data well.

Optimization of thermal hydrolysis process for enhancing anaerobic digestion in a wastewater treatment plant with existing primary sludge fermentation

Many wastewater treatment plants (WWTPs) adopted primary sludge fermentation to produce sludge liquor for the biological denitrification process. The fermented primary sludge (FPS) is usually co-digested with thickened waste activated sludge (TWAS) in the anaerobic digestion (AD) process. To date, there has been limited information on how the sludge thermal hydrolysis process (THP) could be retrofitted for enhancing AD in WWTPs with the existing primary sludge fermentation process. This study assessed two THP retrofitting schemes, (FPS + TWAS and TWAS alone) combining different exposure times (15, 30, and 60 min) and temperatures (140, 160, and 180 °C). The results suggested that temperature had more impact on sludge solubilization than exposure times. Notably, 180 °C was the most effective for sludge solubilization under both schemes. However, a higher degree of solubilization did not necessarily lead to higher methane yields. The THP of FPS + TWAS attained considerably higher methane yield than the pretreatment of TWAS alone.

Pulsed electric field: An emerging pretreatment technology in a biogas production

This review focuses on current status of pulsed electric field (PEF) technology and its implementation in biogas production. First, basic principles of PEF and a schematic overview of typical PEF processing system were provided. Thereafter, lab- and pilot-scale PEF pretreatments of sludge with subsequent anaerobic digestion (AD) were provided. Furthermore, PEF technology, as an emerging technology for the lignocellulose (LC) pretreatment in biogas production which is still predominantly used at lab-scale, was outlined. Eventually, conclusion together with future perspectives and challenges were outlined.

Comparison of two advanced anaerobic digestions of sewage sludge with high-temperature thermal pretreatment and low-temperature thermal-alkaline pretreatment

Semi-continuous experiments were conducted to compare the performances and energy efficiencies of two advanced anaerobic digestions (AAD) of sewage sludge with high-temperature thermal pretreatment (HTTP, 160 ± 1 °C and 0.55 MPa for 30 min) and low-temperature thermal-alkaline pretreatment (LTTAP, 60 ± 1 °C and pH 12.0 ± 0.1 for 30 min), which had similar sludge disintegration degree (9.44-9.48%). At the steady period of a SRT 20 d, the two AAD had similar methane production (150.22 ± 9.55 ml/L/d and 151.02 ± 12.56 ml/L/d) and organic matter removals (22.54 ± 2.84% and 23.15 ± 2.46% for volatile solids-VS). The results of high-throughput sequencing showed that the methanogenic pathways of the two AAD were strictly hydrogenotrophic (AAD with HTTP) and hydrogenotrophic/acetoclastic methanogenesis (AAD with LTTAP), respectively. The energy balance analysis suggested that the AAD with LTTAP was superior to that with HTTP because the former had a higher energy efficiency (1.610) than the latter (1.358).

Magnetic biochar catalysts from anaerobic digested sludge: Production, application and environment impact

Regulated disposal or re-utilization of dewatered sludge is of economic benefits and can avoid secondary contamination to the environment; however, feasible and effective management strategies are still lacking. In this study, a peroxydisulfate/zero-valent iron (PDS-ZVI) system is proposed to destroy proteins in soluble extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) in anaerobic digested sludge (ADS) to improve the dewaterability. Moreover, ADS derived biochars supported via iron oxides (Fe-ADSBC) were generated by dewatering and thermal annealing. Intriguingly, the iron species was discovered to gradually transform from Fe₃O₄ to FeO with increased pyrolysis temperatures from 600 to 1000 °C. The manipulated iron species on the biochar can remarkably impact the catalytic activity in PDS activation and degradation of sulfamethazine (SMT). The in situ radical scavenging and capturing tests revealed that the principal reactive oxygen species (ROS) in Fe-ADSBC/PDS system experienced a variation from OH into SO₄^- at higher annealing temperature (1000 °C). In addition, the carbonaceous ADSBC can promote the catalytic activity of iron oxides by synergistically facilitating the adsorption of reactants and charge transfer through COFe bonds at the interfaces. This study enables the first insights into the properties and catalytic performance of Fe-ADSBC, meanwhile unveils the mechanism, reaction pathways, and environmental impacts of the ultimate transformation products (TPs) from SMT degradation in the Fe-ADSBC/PDS system. The study also contributes to developing value-added green biochar catalysts from bio-wastes towards environmental purification.

Comparison of liquid and dewatered digestate as inoculum for anaerobic digestion of organic solid wastes

In this study, the application of liquid and dewatered digestate was compared for the inoculation of the anaerobic digestion (AD) process. In addition to the specific methanogenic activity and biohydrogen potential tests, biochemical methane potential assay was also conducted using four different types of municipal and industrial waste streams (primary sludge, thickened waste activated sludge, source separated organics, and cattle manure). The specific methanogenic activity of the digestate cake (5.0 ± 0.5 mL-CH₄/g-VSS.d) was higher than that of the liquid digestate (3.4 ± 0.2 mL-CH₄/g-VSS.d) for the food to microorganism ratio of 0.5. The BMP results also revealed that regardless of the type of the substrate used, the application of the digestate cake as inoculum achieved statistically significantly higher methane production rate compared to the utilization of liquid digestate, most likely due to the lower concentration of dissolved contents (i.e., ammonia, soluble organic matter, heavy metals, etc.) in the diluted digestate cake. Despite the increased process rate, no statistically significant effect of the type of the inoculum was observed on the ultimate methane yield. The biohydrogen potential test revealed the similar performance of the digestate- and digestate cake-inoculated digesters in terms of biohydrogen and volatile fatty acids production. The findings of this study suggest that the digestate cake can be used as an effective alternative to the liquid digestate for the inoculation of full-scale anaerobic digesters, reducing the transportation volume by up to 90%.

Enzymatic pretreatment of lignocellulosic biomass for enhanced biomethane production-A review

The rapid depletion of natural resources and the environmental concerns associated with the use of fossil fuels as the main source of global energy is leading to an increased interest in alternative and renewable energy sources. Particular interest has been given to the lignocellulosic biomass as the most abundant source of organic matter with a potential of being utilized for energy recovery. Different approaches have been applied to convert the lignocellulosic biomass to energy products including anaerobic digestion (AD), fermentation, combustion, pyrolysis, and gasification. The AD process has been proven as an effective technology for converting organic material into energy in the form of methane-rich biogas. However, the complex structure of the lignocellulosic biomass comprised of cellulose, hemicelluloses, and lignin hinders the ability of microorganisms in an AD process to degrade and convert these compounds to biogas. Therefore, a pretreatment step is essential to improve the degradability of the lignocellulosic biomass to achieve higher biogas rate and yield. A system that uses pretreatment and AD is known as advanced AD. Several pretreatment methods have been studied over the past few years including physical, thermal, chemical and biological pretreatment. This paper reviews the enzymatic pretreatment as one of the biological pretreatment methods which has received less attention in the literature than the other pretreatment methods. This paper includes a review of lignocellulosic biomass composition, AD process, challenges in degrading lignocellulosic materials, the current status of research to improve the biogas rate and yield from the AD of lignocellulosic biomass via enzymatic pretreatment, and the future trend in research for the reduction of enzymatic pretreatment cost.

Hydrothermal pretreatment of source separated organics for enhanced solubilization and biomethane recovery

The objective of this research was to evaluate the effect of the hydrothermal pretreatment on the solubilization of source separated organics (SSO) as well as the biomethane recovery through the mesophilic batch anaerobic digestion process. For this purpose, the SSO was subjected to fifteen different pretreatment conditions within five different severity index (SI) values (3, 3.5, 4, 4.5, and 5). The pretreatment temperature, holding time, and pressure ranged from 150 to 240 °C, 5 to 30 min, and 476 to 3367 kPa, respectively. The highest solubilization improvement of ∼50% was achieved under the pretreatment condition of "220 °C-10 min-2323 kPa" corresponding to the SI value of 4.5. However, the maximum biomethane production yield of 280 mL/g TCOD_added and biomethane production rate of 30 mL/g TCOD_added were obtained under the less intense pretreatment conditions of "190 °C-20 min-1247 kPa" and "170 °C-30 min-786 kPa", respectively.

Comparison of Different Electricity-Based Thermal Pretreatment Methods for Enhanced Bioenergy Production from Municipal Sludge

This paper presents results for a comprehensive study that compares the performance of three electricity-based thermal pretreatment methods for improving the effectiveness of anaerobic digestion (AD) to process municipal wastewater sludge. The study compares thermal pretreatment using conventional heating (CH), microwave (MW), and radio frequency (RF) heating techniques. The effectiveness of the pretreatment methods was assessed in terms of chemical oxygen demand (COD) and biopolymers solubilization, AD bioenergy production, input electrical energy, and overall net energy production of the sequential pretreatment/AD process. The heating applicators for the bench-scale testing consisted of a custom-built pressure-sealed heating vessel for CH experiments, an off-the-shelf programmable MW oven operating at a frequency of 2.45 GHz for MW heating experiments, and a newly developed 1 kW RF heating system operating at a frequency of 13.56 MHz for RF heating experiments. Under identical thermal profiles, all three thermal pretreatment methods achieved similar sludge disintegration in terms of COD and biopolymer solubilization as well as AD bioenergy production (p-value > 0.05). According to the energy assessment results, the application of CH and MW pretreatments resulted in overall negative energy production, while positive net energy production was obtained through the sequential pretreatment/AD process utilizing RF pretreatment.

A comprehensive insight into the effects of microwave-H2O2 pretreatment on concentrated sewage sludge anaerobic digestion based on semi-continuous operation

This study gave a comprehensive insight into the effects of microwave-H₂O₂ pretreatment on concentrated sludge anaerobic digestion (AD). Results showed that mixture of activated sludge and pretreated sludge at mass ratio of 1:1 was efficient for enhancing AD, methane production was increased from 215.51 mL/g VS_added to 258.38 mL/g VS_added. Pretreatment resulted in the change of sludge properties, such as solubilization of organic matters, decrease of viscosity. For concentrated sludge AD, the "pH buffer system" ensured the "inhibitory steady state" during start-up period, and significant archaeal community shift from Methanosaeta to Methanosarcina ensured the true steady-state operation. Pretreatment further increased the relative abundance of functional microbia such as Bacteroidales, Clostridiales and Methanosarcina, and resulted in the decrease of viscosity of digested sludge in reactors. Based on the CFD simulation, the flow velocity became greater than the control. These comprehensive effects might contribute to enhanced efficiency of concentrated sludge AD.

Enhanced Methane Production from Food Waste Using Cysteine To Increase Biotransformation of l-Monosaccharide, Volatile Fatty Acids, and Biohydrogen

The enhancement of two-stage anaerobic digestion of polysaccharide-enriched food waste by the addition of cysteine-an oxygen scavenger, electron mediator, and nitrogen source-to the acidification stage was reported. It was found that in the acidification stage the accumulation of volatile fatty acids (VFA), which mainly consisted of acetate, butyrate, and propionate, was increased by 49.3% at a cysteine dosage of 50 mg/L. Although some cysteine was biodegraded in the acidification stage, the VFA derived from cysteine was negligible. In the methanogenesis stage, the biotransformations of both VFA and biohydrogen to methane were enhanced, and the methane yield was improved by 43.9%. The mechanisms study showed that both d-glucose and l-glucose (the model monosaccharides) were detectable in the hydrolysis product, and the addition of cysteine remarkably increased the acidification of l-glucose, especially acetic acid and hydrogen generation, due to key enzymes involved in l-glucose metabolism being enhanced. Cysteine also improved the activity of homoacetogens by 34.8% and hydrogenotrophic methanogens by 54%, which might be due to the electron transfer process being accelerated. This study provided an alternative method to improve anaerobic digestion performance and energy recovery from food waste.