Anaerobic Digestion Of Sewage Sludge Research Articles

Integration of Reversible Solid Oxide Electrolysis (rSOC) to Wastewater Treatment Plants for Sustainable Green Gas Production and Balancing of Green Power

Biogas produced from anaerobic digestion (AD) of sewage sludge and organic wastes at wastewater treatment plants (WWTPs) can present an important energy contribution to a sustainable society. This raw biogas contains typically 50-65 molar% methane (CH4) and 50-35 molar% carbon dioxide (CO2). The raw AD biogas can be further upgraded to biomethane (with over 98 molar% CH4). This is a sustainably produced gas of biological origin which has same usability and energetic properties as fossil natural gas. Either by use as biofuel in transportation, local combined heat and power (CHP) or directly fed into the gas grid, biomethane has become a vital non-fossil energy source in Europe but also other parts of the world.During the CO2 separation step of biomethane upgrading, substantial amounts of CO2 are emitted to the atmosphere. A methanation unit can convert this CO2 into sustainable Synthetic Natural Gas (SNG), with a strong potential to be used as additional biomethane (SNG-Biomethane). The methanation unit can replace the conventional carbon dioxide (CO2) separation unit for biomethane upgrading, but needs addition of hydrogen from water electrolysis, e.g. high performance Solid Oxide Electrolysis (SOE). Such SOE can also be built as a novel high-performance reversible Solid Oxide Electrolysis (rSOC), as demonstrated in the European H2020 project BALANCE. Besides the SOE mode, such rSOC can operate also in the opposite fuel cell direction (SOFC): In times when power demands exceed sustainable supply, the SOFC mode of the rSOC can convert raw AD biogas directly to electricity. In this way, WWTPs can act as energy storage solutions to buffer temporal electrical energy surplus and produce domestic sustainable SNG to the transport sector or the gas grid. At the same time, the rSOC provides a concept of zero-emission power production. To highlight the zero-emission potential of the rSOC, this paper emphasizes author’s earlier results from the European H2020 project DEMOSOFC, which demonstrated zero-local emission SOFC using raw biogas from WWTP sewage sludge and organic wastes.In this paper, profitability estimations for optimal rSOC and methanation investment and operation strategies at a modern city level WWTP in Finland, Europe, are presented. Power used in rSOC electrolysis mode (SOEC) origins from intermittent green power sources such as wind power, while power produced in rSOC fuel cell mode (SOFC) uses raw AD biogas. For the assessment, we utilize a dedicated Power-to-X optimization model which combines optimally power market operations, dimensions of renewable power sources, plant dimensions and operational scheduling strategies. The assessment showed high profitability using European gas crisis power market data 2021-2022, resulting in sustainable biomethane net production costs of 30-70 €/MWh.

Bioenergy recovery and carbon emissions benefits of short-term bio-thermophilic pretreatment on low organic sewage sludge anaerobic digestion: A pilot-scale study

Sewage sludge in cities of Yangzi River Belt, China, generally exhibits a lower organic content and higher silt contentdue to leakage of drainage system, which caused low bioenergy recovery and carbon emission benefits in conventional anaerobic digestion (CAD). Therefore, this paper is on a pilot scale, a bio-thermophilic pretreatment anaerobic digestion (BTPAD) for low organic sludge (volatile solids (VS) of 4%) was operated with a long-term continuous flow of 200 days. The VS degradation rate and CH4 yield of BTPAD increased by 19.93% and 53.33%, respectively, compared to those of CAD. The analysis of organic compositions in sludge revealed that BTPAD mainly improved the hydrolysis of proteins in sludge. Further analysis of microbial community proportions by high-throughput sequencing revealed that the short-term bio-thermophilic pretreatment was enriched in Clostridiales, Coprothermobacter and Gelria, was capable of hydrolyzing acidified proteins, and provided more volatile fatty acid (VFA) for the subsequent reaction. Biome combined with fluorescence quantitative polymerase chain reaction (PCR) analysis showed that the number of bacteria with high methanogenic capacity in BTPAD was much higher than that in CAD during the medium temperature digestion stage, indicating that short-term bio-thermophilic pretreatment could provide better methanogenic conditions for BTPAD. Furthermore, the greenhouse gas emission footprint analysis showed that short-term bio-thermophilic pretreatment could reduce the carbon emission of sludge anaerobic digestion system by 19.18%.

Conversion of Materials and Energy in Anaerobic Digestion of Sewage Sludge with High-Pressure Homogenization Pretreatment

High pressure homogenization (HPH) pretreatment can improve sludge anaerobic digestion; however, the relationship among the material, energy conversion, and gas production efficiency was unclear under different operating conditions in sludge anaerobic digestion by HPH pretreatment. In this study, the performance of HPH pretreatment before sludge anaerobic digestion was investigated, and the relationship among the material, energy conversion, and gas production efficiency was explored. HPH pretreatment induced organic solubilization, and a maximum soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) of about 30% was achieved. Results showed that HPH pretreatment significantly improved the biogas production of sludge anaerobic digestion; the maximum increase in CH4 yield was 57%; and the anaerobic digestion period was shortened by about 10 days. The ratio of CH4 yield increment to volatile dissolved solids (VDS) increment was 0.21 mL/mg. The CH4 yield increment of 1 L/g volatile solid (VS) required a specific energy of 0.10 MJ/kg total solid (TS) by increasing the pressure with one cycle and 0.72 MJ/kg TS by increasing the cycle at 60 MPa. The minimum additive energy consumption of HPH pretreatment was 125 J/mL CH4 yield increment at 20 MPa with one cycle. Considering CH4 yield improvement and energy conservation, HPH pretreatment should maintain a pressure of no more than 60 MPa in one cycle. This study provides a theoretical reference for the practical application of HPH pretreatment in anaerobic digestion. HPH holds promise as a potential strategy for sewage sludge pretreatment to produce CH4 in anaerobic digestion.

Anaerobic Digestion of Municipal Sewage Sludge Integrated with Brewery Wastewater Treatment: Importance of Temperature and Mixing Ratio

Brewery wastewater is characterized by a high organic matter content and low pH, which may cause serious ecological hazards if it is discharged without any treatment. In this study, brewery wastewater treatment was integrated with anaerobic digestion of municipal sewage sludge. Additionally, the effects of temperature and mixing ratio of brewery wastewater were investigated. The results showed that the brewery wastewater mixing ratio (v/v) of 20% could maximize the biogas production during anaerobic digestion at the temperature of 34 °C. Additionally, regulating the appropriate mixing ratio, increasing operating temperature and adjusting pH were effective ways to enhance anaerobic digestion efficiency. Furthermore, the distribution of microbial communities was confirmed to be significantly influenced by the mixing ratio of brewery wastewater using high-throughput DNA sequencing technology. With the increasing mixing ratio of brewery wastewater, Firmicutes gradually dominated instead of Chloroflexi. Meanwhile, Methanolinea and Methanosarcina became the dominant methanogens, while the proportion of Methanothrix was significantly reduced. The results of this study will provide data to support the practical process operation of anaerobic co-digestion of brewery wastewater and municipal sewage sludge.

Manufacture of artificial lightweight aggregates recycled from anaerobic digested sewage sludge and process optimization by machine learning modeling techniques

In this study, in an effort to manufacture lightweight aggregates recycled from anaerobic digested sewage sludge (ADSS), the firing process characteristics of artificial lightweight aggregates containing a large amount of ADSS were identified and the manufacturing process was optimized and modeled using machine learning techniques. Samples were prepared by rapid firing and according to the orthogonal arrangement experimental design, the particle density and water absorption rate were measured, and a pore analysis was conducted by 3D-CT and the mercury intrusion method. Random forest, SVR, and XGBoost techniques were used as machine learning techniques to analyze the data. As a result of the rapid firing test, samples with an ADSS content of 40 wt% or more were found to have a low density and were found not to show bloating. For aggregates containing more than 40% ADSS, a loss of organic matter played a greater role in the decreased density than bloating due to viscous behavior. Data expansion using an orthogonal arrangement table and modeling by the SVR method for the ADSS 50 mixture found an RMSE value of 0.013 with significant predicted values of the physical properties also obtained. Aggregates with a content of 50 wt% of anaerobically digested sludge were successfully prepared on a pilot scale, and this aggregate mix met EN-13055 standards.

Derivation of Optimal Operation Factors of Anaerobic Digesters through Artificial Neural Network Technology

The anaerobic digestion of sewage sludge in South Korean wastewater treatment plants is affected by seasonal factors and other influences, resulting in lower digestion efficiency and gas production, which cannot reach optimal yields. The aim of this study was to improve the digestion efficiency and gas production of sludge anaerobic digestion in a wastewater treatment plant (WWTP) by using data mining techniques to adjust operational parameters. Through experimental data obtained from the WWTP in Daegu City, South Korea, an artificial neural network (ANN) technology was used to adjust the range of the organic loading rate (OLR) and hydraulic retention rate (HRT) to improve the efficiency and methane gas production from anaerobic sludge digestion. Data sources were normalized, and data analysis including Pearson correlation analysis, multiple regression analysis and an artificial neural network for optimal results. The results of the study showed a predicted 0.5% increase in digestion efficiency and a 1.3% increase in gas production at organic loads of 1.26–1.46 kg/m3 day and an HRT of 26–30 days. This shows that the ANN model that we established is feasible and can be used to improve the efficiency and gas production of sludge anaerobic digestion.

Microbial electrolysis cell simultaneously enhancing methanization and reducing hydrogen sulfide production in anaerobic digestion of sewage sludge

The effects of microbial electrolysis cells (MECs) at three applied voltages (0.8, 1.3, and 1.6 V) on simultaneously enhancing methanization and reducing hydrogen sulfide (H2S) production in the anaerobic digestion (AD) of sewage sludge were studied. The results showed that the MECs at 1.3 V and 1.6 V simultaneously enhanced the methane production by 57.02 and 12.70% and organic matter removal by 38.77 and 11.13%, and reduced H2S production by 94.8 and 98.2%, respectively. MECs at 1.3 V and 1.6 V created a micro-aerobic conditions for the digesters with oxidation-reduction potential as −178∼-232 mv, which enhanced methanization and reduced H2S production. Sulfur reduction, H2S and elemental sulfur oxidation occurred simultaneously in the ADs at 1.3 V and 1.6 V. The relative abundances of sulfur-oxidizing bacteria increased from 0.11% to 0.42% and those of sulfur-reducing bacteria decreased from 1.24% to 0.33% when the applied voltage of MEC increased from 0 V to 1.6 V. Hydrogen produced by electrolysis enhanced the abundance of Methanobacterium and changed the methanogenesis pathway.

Implementation at full scale of demand-driven biogas production from anaerobic digestion of sewage sludge

Abstract In a net-zero emissions scenario, a secure supply of electricity involves renewable generators that can flexibly increase their production when needed. Currently, electricity generation from biogas in the water industry is most commonly at a steady level, given Anaerobic Digestion (AD) is traditionally operated in steady state. This research demonstrated at different scales that demand-driven biogas production from AD of sewage sludge is feasible. Performance parameters are not negatively affected by a flexible feeding schedule and stability parameters show transitional imbalances that do not threaten the overall process. This paper presents the trial implementation in digesters of volume 3800 m3, which became permanent. Economic and environmental benefits exist; however, in order to unlock the full potential of flexible electricity generation from sewage sludge, synergies between technical, operational and political factors in the water and energy sectors need to be developed.

Anaerobic digestion of sewage sludge using anaerobic dynamic membrane bioreactor under various sludge composition and organic loading rates

This study investigates the effects of sludge compositions and organic loading rates (OLRs) on stable biogas production during sludge digestion. Batch digestion experiments evaluate the effects of alkaline-thermal pretreatment and waste activated sludge (WAS) fractions on the biochemical methane potential (BMP) of sludge. A lab-scale anaerobic dynamic membrane bioreactor (AnDMBR) is fed with a mixture of primary sludge and pretreated WAS. Monitoring of volatile fatty acid to total alkalinity (FOS/TAC) helps maintain operational stability. The highest average methane production rate of 0.7 L/L·d is achieved when the OLR, hydraulic retention time, WAS volume fraction, and FOS/TAC ratio are 5.0 g COD/L·d, 12 days, 0.75, and 0.32, respectively. This study finds functional redundancy in two pathways: hydrogenotrophic and acetolactic. An increase in OLR promotes bacterial and archaeal abundance and specific methanogenic activity. These results can be applied to the design and operation of sludge digestion for stable, high-rate biogas recovery.

Integration of Reversible Solid Oxide Electrolysis (rSOC) to Wastewater Treatment Plants for Sustainable Green Gas Production and Balancing of Green Power

Biogas produced from anaerobic digestion (AD) of sewage sludge and organic wastes at wastewater treatment plants (WWTPs) can present an important energy contribution to a sustainable society. Raw AD biogas contains 50-65 molar% methane (CH4) and 50-35 molar% carbon dioxide (CO2) and can be upgraded to biomethane (with over 98 molar% CH4). During the CO2 separation step of biomethane upgrading, substantial amounts of CO2 are emitted to the atmosphere. A methanation unit can convert this CO2 into sustainable Synthetic Natural Gas (SNG) and replace the conventional CO2 separation unit for biomethane upgrading, but needs addition of hydrogen from water electrolysis. The water electrolysis can be built as a novel high-performance reversible Solid Oxide Electrolysis (rSOC), as demonstrated in the European H2020 project BALANCE. To highlight the zero-emission potential of the rSOC, this paper emphasizes author’s earlier results from the European H2020 project DEMOSOFC, demonstrating zero-local emission SOFC using raw biogas from WWTP sewage sludge and organic wastes. In this paper, profitability estimations for optimal rSOC and methanation investment and operation strategies at a modern city level WWTP in Finland, Europe, are presented. Power used in rSOC electrolysis mode (SOEC) origins from intermittent wind power, while power produced in rSOC fuel cell mode (SOFC) uses raw AD biogas. A dedicated Power-to-X optimization model is used, optimizing power market operations, dimensions of renewable power sources, plant dimensions and operational scheduling. High profitability is shown using European energy crisis market data 2021-2022, resulting in sustainable biomethane net production costs of 30-70 €/MWh.

Clean production of power and heat for waste water treatment plant by integrating sewage sludge anaerobic digester and solid oxide fuel cell

A Sewage Sludge Anaerobic Digester (SSAD) system integrated with a Solid Oxide Fuel Cell (SOFC) was modeled and optimized to reach an efficient/cost effective system for producing clean power and heat in Parand wastewater treatment plant (WWTP). The system modeling was performed by the use of 4E analyses (energy, exergy, economic, environmental). Based on 4E modeling and by the use of two objective functions (payback period and exergy efficiency) and seven design variables, SSAD-SOFC system was optimized for shorter payback period and higher exergy efficiency. This multi-objective optimization provided the optimum system operating specifications. From this type of investigation which was not observed in literature for SSAD-SOFC system, the results showed 58747 m3/day of biogas production as well as 7.5 MWel and 5.3 MWth electricity and heating generation, respectively at the optimum point. Furthermore, at the optimum point the integrated system overall energy and exergy efficiency were also 28% and 21%, with payback period of 1.7 years. Moreover, in comparison with a traditional system in which WWTP had to buy electricity and natural gas from the grid and burns fuel for heating requirements, the relative annual benefit of the SSAD-SOFC system increased by 17.9%. Finally, results from multi-objective optimization showed 6.9% improvement in overall energy efficiency and 7.5% enhancement in exergy efficiency in comparison with those for single-objective optimization with only payback period as the single objective function. The development of a sustainable and cost-effective procedure for optimum design of WWTP for electricity and heat generation as well as implementing of the above analysis in other integrated systems with similar equipment are implications of this research.

Biogas and Syngas Production from Sewage Sludge: A Sustainable Source of Energy Generation

Sewage sludge to energy conversion is a sustainable waste management technique and a means of militating against the environmental concerns associated with its disposal. Amongst the various conversion technologies, anaerobic digestion and gasification have been identified as the two most promising. Therefore, this study is focused on a detailed evaluation of the anaerobic digestion and gasification of sewage sludge for energy production. Moreover, the key challenges hindering both technologies are discussed, as well as the practical measures for addressing them. The applicable pretreatment measures for efficient transformation into valuable energy vectors were further evaluated. Specifically, the study evaluated various properties of sewage sludge in relation to gasification and anaerobic digestion. The findings showed that a high ash content in sewage sludge results in sintering and agglomeration, while a high moisture content promotes tar formation, which has been identified as one of the key limitations of sewage sludge gasification. More importantly, the application of pretreatment has been shown to have some beneficial features in promoting organic matter decomposition/degradation, thereby enhancing biogas as well as syngas production. However, this has additional energy requirements and operational costs, particularly for thermal and mechanical methods.

Process Improvement of Biogas Production from Sewage Sludge Applying Iron Oxides-Based Additives

Iron additives are effective in the anaerobic sewage sludge digestion process, but the composition and dosage of these additives are not precisely defined. This research investigates the effects of three iron oxides-based additives on the destruction of volatile solids, the production and quality of biogas, as well as the quality of the supernatant. Additive No 1 contained >41.5% of FeO and >41.5% of Fe2O3, additive No 2 contained ≥86% of Fe3O4, and additive No 3 contained ≥98% of Fe3O4. The best results were obtained by applying an iron oxides-based additive with a higher content of divalent iron oxide. The increase in efficiency of the VSs destruction was not significant and on average 2.2%. The increase in biogas production was on average 20% while the average increase in the content of methane in the biogas was 6.3%. Applying the additive, the reduction in the concentration of ammonium nitrogen in the supernatant was up to 28%, as well as a reduction in the concentration of phosphate phosphorus in the supernatant by up to 3.1 times could be expected compared to the case when the additive was not applied. The dose of additive No 1 was between 7.5 g/kg of dry solids and 15 g/kg of dry solids in the lab-scale test. The dose was specified in the full-scale test, and the recommended dose of the additive was 10 g/kg of dry solids to improve biogas production.

Key properties identification of biochar material in anaerobic digestion of sewage sludge for enhancement of methane production

Owing to the intricate physicochemical characteristics of biochar material, a comprehensive understanding of their effects and mechanisms involved in anaerobic digestion still needs to be explored. In this study, the methane yield of sludge hydrolysate was inhibited by 13.3% after the addition of granular activated carbon (GAC), which was due to the predominance of non-selective adsorption by GAC. Whereas the shortened lag phase time (38.2%), the improved methane production rate (16.6%), and the increased methane yield (7.6%) were achieved after the algal biochar (ABC) addition. The methanogenic improvements by ABC were attributed to its redox capacity other than pH buffering capacity. Quantitative results showed that the content of phenolic and lactonic groups were 4.8 and 2.3 times higher than that of GAC, respectively, which endowed ABC with a stronger redox capacity, especially electron-donating capacity. Surface oxygen-containing functional groups like phenolic and lactonic groups of biochar material should be valued for the improvement of anaerobic digestion. The findings of this study provided a comprehensive insight into the connection between mechanisms and related biochar properties during the anaerobic digestion of sewage sludge, which also provided an important clue to selecting, even harnessing desirable biochar material for a given biotechnological process.

Enhancing Mesophilic Anaerobic Digestion of Waste-Activated Sludge through Heat Pretreatment and Kinetic Modeling

Sewage sludge is a useful raw material for the production of renewable energy due to its stable annual output. In this study, the enhancement of mesophilic anaerobic digestion of sewage sludge through heat pretreatment at 95 °C for 30 min was tested in an anaerobic moving bed biofilm reactor (hAMBBR). The sludge retention time was set at 20, 15, 10, and 5 days during 300 days of operation and compared to a traditional anaerobic continuous stirred tank reactor (AnCSTR) without pretreatment. Results of this research indicate that the digestion ratio of volatile soluble solids in the hAMBBR process could be improved by 50%, and the average conversion ratio of methane could be increased by 45%. When the sludge retention time (SRT) was shortened to 5 days, the methane production approached twice that of the contrast reactor. The expanded anaerobic digestion model, including activated sludge models, was utilized for operation simulation. The effect of sludge retention time (SRT) shortening on volatile suspended solids (VSS) digestibility and methane production was well reproduced with simulations. The research conclusion reveals the impact of pretreatment and reactor types on anaerobic digestion and provides the scientific basis for improving methane production and process efficiency in anaerobic digestion.

Biological Methanation in an Anaerobic Biofilm Reactor—Trace Element and Mineral Requirements for Stable Operation

Biological methanation of carbon dioxide using hydrogen makes it possible to improve the methane and energy content of biogas produced from sewage sludge and organic residuals and to reach the requirements for injection into the natural gas network. Biofilm reactors, so-called trickling bed reactors, offer a relatively simple, energy-efficient, and reliable technique for upgrading biogas via ex-situ methanation. A mesophilic lab-scale biofilm reactor was operated continuously for nine months to upgrade biogas from anaerobic sewage sludge digestion to a methane content >98%. To supply essential trace elements to the biomass, a stock solution was fed to the trickling liquid. Besides standard parameters and gas quality, concentrations of Na, K, Ca, Mg, Ni, and Fe were measured in the liquid and the biofilm using ICP-OES (inductively coupled plasma optical emission spectrometry) to examine the biofilms load-dependent uptake rate and to calculate quantities required for a stable operation. Additionally, microbial community dynamics were monitored by amplicon sequencing (16S rRNA gene). It was found that all investigated (trace) elements are taken up by the biomass. Some are absorbed depending on the load, others independently of it. For example, a biomass-specific uptake of 0.13 mg·g−1·d−1 for Ni and up to 50 mg·g−1·d−1 for Mg were measured.

Comprehensive evaluation of sewage sludge anaerobic digestion process with different digestate treatments.

Anaerobic digestion is one of the most promising methods for reducing sewage sludge and recovering energy. In the present study, a comparative life cycle assessment (LCA) of sewage sludge anaerobic digestion processes with different digestate treatments, including mesophilic anaerobic digestion with digestate landfilling (CAD-1) and digestate incineration (CAD-2), thermophilic anaerobic digestion combined with thermal hydrolysis pre-treatment with digestate land use (THPAD-1), and digestate incineration (THPAD-2), was performed to evaluate their environmental, resource, economic, and comprehensive performances using the SimaPro software. Environmental impact analysis revealed marine ecotoxicity, freshwater ecotoxicity, and human carcinogenic toxicity as the most obvious impacts, resulting in the most significant damage to human health. Resource analysis indicated that anaerobic digestion combined with cogeneration and digestate incineration is advantageous to high energy recovery, but digestate incineration is disadvantageous to economic performance because of increased investment costs. Comparison of the four processes revealed that THPAD-2 results in the largest environmental damage, whereas CAD-1 has the smallest load. Meanwhile, THPAD-2 and THPAD-1 exhibit the best resource performance and net economic benefit, respectively. The comprehensive evaluation indices revealed that THPAD-1 and CAD-2 show better comprehensive performance. And the deep drying incineration process exhibited better comprehensive performance than sewage sludge anaerobic digestion processes.

Ammonia tolerance and microbial community in thermophilic co-digestion of sewage sludge initiated with lignocellulosic biomass

Rice straw is a useful lignocellulosic biomass for controlling ammonia inhibition in the thermophilic anaerobic digestion of sewage sludge. However, it is challenging to procure rice straw throughout the year because of its seasonal production. This study investigated methane production in a laboratory-scale digester by gradually decreasing rice straw addition to solid thermophilic sewage sludge digestion. The decrease in rice straw did not accumulate volatile fatty acids and stabilized methane production. Even with increased sludge concentration without rice straw, methane production continued under high ammonia conditions. Ammonia tolerance of the digested sludge of the experimental digester was higher than that of conventionally digested sludge. The cellulose-degrading bacteria Clostridia and high ammonia-resistant archaea Methanosarcina were dominant in the experimentally digested sludge. The community was maintained for over 200 days after discontinuing the rice straw supply. These findings suggest that anaerobic digestion initiation with rice straw is appropriate to facilitate ammonia-tolerant communities.

Application of Magnetite-Nanoparticles and Microbial Fuel Cell on Anaerobic Digestion: Influence of External Resistance.

In this paper, the application of magnetite-nanoparticles and a microbial fuel cell (MFC) was studied on the anaerobic digestion (AD) of sewage sludge. The experimental set-up included six 1 L biochemical methane potential (BMP) tests with different external resistors: (a) 100 Ω, (b) 300 Ω, (c) 500 Ω, (d) 800 Ω, (e) 1000 Ω, and (f) a control with no external resistor. The BMP tests were carried out using digesters with a working volume of 0.8 L fed with 0.5 L substrate, 0.3 L inoculum, and 0.53 g magnetite-nanoparticles. The results suggested that the ultimate biogas generation reached 692.7 mL/g VSfed in the 500 Ω digester, which was substantially greater than the 102.6 mL/g VSfed of the control. The electrochemical efficiency analysis also demonstrated higher coulombic efficiency (81.2%) and maximum power density (30.17 mW/ m2) for the 500 Ω digester. The digester also revealed a higher maximum voltage generation of 0.431 V, which was approximately 12.7 times the 0.034 V of the lowest-performing MFC (100 Ω digester). In terms of contaminants removed, the best-performing digester was the digester with 500 Ω, which reduced contaminants by more than 89% on COD, TS, VS, TSS and color. In terms of cost-benefit analysis, this digester produced the highest annual energy profit (48.22 ZAR/kWh or 3.45 USD/kWh). This infers the application of magnetite-nanoparticles and MFC on the AD of sewage sludge is very promising for biogas production. The digester with an external resistor of 500 Ω showed a high potential for use in bioelectrochemical biogas generation and contaminant removal for sewage sludge.

Effects of Magnetic Biochar Addition on Mesophilic Anaerobic Digestion of Sewage Sludge.

As a low-cost additive to anaerobic digestion (AD), magnetic biochar (MBC) can act as an electron conductor to promote electron transfer to enhance biogas production performance in the AD process of sewage sludge and has thus attracted much attention in research and industrial applications. In the present work, Camellia oleifera shell (COS) was used to produce MBC as an additive for mesophilic AD of sewage sludge, in order to explore the effect of MBC on the mesophilic AD process and its enhancement mechanism. Analysis by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectrometry (FTIR), and X-ray diffraction (XRD) further confirmed that biochar was successfully magnetized. The yield of biogas from sewage sludge was enhanced by 14.68-39.24% with the addition of MBC, and the removal efficiency of total solid (TS), volatile solids (VS), and soluble chemical oxygen demand (sCOD) were 28.99-46.13%, 32.22-48.62%, and 84.18-86.71%, respectively. According to the Modified Gompertz Model and Cone Model, the optimum dosage of MBC was 20 mg/g TS. The maximum methane production rate (Rm) was 15.58% higher than that of the control reactor, while the lag-phase (λ) was 43.78% shorter than the control group. The concentration of soluble Fe2+ and Fe3+ were also detected in this study to analyze the function of MBC for improving biogas production performance from sewage sludge. The biogas production was increased when soluble Fe3+ was reduced to soluble Fe2+. Overall, the MBC was beneficial to the resource utilization of COS and showed a good prospect for improving mesophilic AD performance.