Bioenergy Recovery Research Articles

Design, fabrication, automation, and scaleup of anaerobic reactors for waste management and bioenergy recovery

AbstractDigitally controlled reactors can optimize biological reactions and process control through a neural network system. This study reports on the design, fabrication, and automation of a laboratory‐scale anaerobic reactor for the management of agrifood byproducts and bioenergy recovery. The process described here can digitally control the operational parameters, which is beneficial for stable methane production. The proposed process comprises the digital measurement of temperature, pH, humidity, biogas volume, and methane composition by integrating the data in a processor module. The proposed automated reactor can assist significantly in controlling and monitoring the anaerobic digestion process, providing decision making during waste management and bioenergy recovery. A case study is described with the application of automated reactors in a pilot‐scale plant, operated with the flow of 8 m3 slaughterhouse wastewater per day and a biogas production of 10 m3 h−1. The automated pilot‐scale process presents many advantages, including a continuous mode of operation and a faster adaptation of the microorganisms to the substrate, improving biogas production.

Dual-chamber up-flow fluidized hollow-fiber anaerobic membrane bioreactor (UF-HF-AnMBR) treating synthetic purified terephthalic acid wastewater: Granular sludge regranulation and methane production

Up-flow anaerobic sludge blanket is a representative way to treat high-strength industrial wastewater, but suffers from granule collapse and poor long-term performance. In this study, a dual-chamber up-flow fluidized hollow-fiber anaerobic membrane bioreactor (UF-HF-AnMBR) was constructed to enhance sludge regranulation and process stability for purified terephthalic acid (PTA) wastewater treatment. A stable and satisfactory aromatic compound removal (>99 %) was achieved at the organic loading rate of 7.15 ± 0.21 g-COD·Lreactor−1·d−1, with the methane yield of 0.345 ± 0.028 L·g-CODremoved−1. Although the granular sludge collapsed initially induced the pore blocking and membrane fouling, membrane separation promoted the agglomeration of small particles, sludge regranulation and biomass retention. Parallel factor analysis identified that the amino acid tryptophan in extracellular polymeric substances exerted a more critical effect on membrane fouling. The 16S rRNA and network analysis revealed the robust proliferation/enrichment of Methanosaeta, Pelotomaculum, etc., as well as the establishment of mutualistic symbiotic relationship among different species, ensured a stable microbial community and created a favorable circumstance for highly effective anaerobic treatment of PTA wastewater. This study proves that UF-HF-AnMBR can be an efficient and promising technique for sludge regranulation and microbe enrichment, advancing its application in PTA wastewater treatment and bioenergy recovery.

Impacts of micro- to nano-sized carbon supplements on mixed and archaea-free halophilic cultures when used for bioenergy recovery from saline wastewater

Due to the growing interest in enhancing dark fermentation and anaerobic digestion via carbon-based additives to exploit different lucrative synergies, process-based optimizations and mechanistic analyses are crucial. Herein, we studied the impacts of carbon supplements on halophilic microbial cultures as a process intensification way towards bioenergy recovery from saline wastewater. Three additives, i.e., powdered activated carbon, multiwall carbon nanotubes and graphene, were examined. Two sets of parallel batch experiments were conducted using archaea-free (for dark fermentation) and mixed (for anaerobic digestion) halophilic cultures at different supplement doses. Adding 30 mg/L of graphene and 100 mg/L of powdered activated carbon exhibited the maximum improvements in dark fermentation (H2-production) and anaerobic digestion (biogas production), i.e., ∼28% and ∼27% over the control, respectively. This underscored the dose-dependent impact of different-sized carbon amendments. Both the electron transport system activity and extracellular polymeric substances were improved with all amendments, indicating a stimulated electron transfer flux. However, distinct bacterial compositions were identified when using mixed and archaea-free cultures along with indicators for direct interspecies electron transfer establishment in the presence of archaea. Beyond this, such findings also supported the expanded role of conductive additives on H2-producing bacteria, including Clostridium. According to a techno-economic analysis, powdered activated carbon emerged as the best for optimizing bioenergy recovery (in terms of H2 or biogas) from saline wastewater, providing a solid basis for relevant applications.

Effects of Mixtures of Engineered Nanoparticles and Cocontaminants on Anaerobic Digestion.

The widespread application of nanotechnology inevitably leads to an increased release of engineered nanoparticles (ENPs) into the environment. Due to their specific physicochemical properties, ENPs may interact with other contaminants and exert combined effects on the microbial community and metabolism of anaerobic digestion (AD), an important process for organic waste reduction, stabilization, and bioenergy recovery. However, the complicated interactions between ENPs and other contaminants as well as their combined effects on AD are often overlooked. This review therefore focuses on the co-occurrence of ENPs and cocontaminants in the AD process. The key interactions between ENPs and cocontaminants and their combined influences on AD are summarized from the available literature, including the critical mechanisms and influencing factors. Some sulfides, coagulants, and chelating agents have a dramatic "detoxification" effect on the inhibition effect of ENPs on AD. However, some antibiotics and surfactants increase the inhibition of ENPs on AD. The reasons for these differences may be related to the interactive effects between ENPs and cocontaminants, changes of key enzyme activities, adenosine triphosphate (ATP) levels, reactive oxygen species (ROS) production, and microbial communities. New scientific opportunities for a better understanding of the coexistence in real world situations are converging on the scale of nanoparticles.

Energy Recovery Efficiency of Integrating Anaerobic Co-Digestion of Pig Slurry and Feedlot Cattle Manure and Hydrothermal Carbonization of Anaerobic Sludge Cake

Hydrothermal carbonization (HTC) is a technology designed to improve the efficiency of bioenergy recovery by subjecting biomass to high-temperature and high-pressure conditions. By integrating this technical feature with anaerobic digestion (AD), enhanced energy recovery efficiency is achieved in treating anaerobic digestate (AD-T). The study investigates enhancing bioenergy recovery efficiency through an integrated process, combining AD of livestock manure and HTC. The primary objective is to improve the energy conversion efficiency of biomass characterized by varying solid contents and chemical compositions. Shortening the hydraulic retention time (HRT) in AD of livestock manure resulted in decreased degradation rate efficiency within the AD-T. This led to increased solid material accumulation, which was crucial for the subsequent HTC reaction. The HTC reaction exhibited its maximum bioenergy recovery at 160 °C. The input energy of the livestock manure, obtained by mixing pig slurry and feedlot cattle manure in a 1:1 (w/w) ratio, was 171,167 MJ/day. Under different HRT conditions (40, 30, and 20 days), recoverable energy from AD of livestock manure ranged from 60,336 to 68,517 MJ/ton. Integration of HTC increased net bioenergy recovery to 106,493 to 130,491 MJ/day under corresponding HRT conditions, highlighting the potential of integrating HTC with AD from livestock manure for enhanced bioenergy recovery efficiency.

Integrated dark-fermentation-microbial electrosynthesis for efficient wastewater treatment and bioenergy recovery

High ammonia concentration in wastewater can hinder methane production rate in anaerobic digestion (AD)-microbial electrosynthesis systems (ADMES). To address this issue, a dual-chamber reactor was fabricated using an anion exchange membrane (AEM) to separate the dark-fermentation (DF) and ADMES process, preventing ammonia migration from the DF chamber to the ADMES chamber. As a result, the DF-ADMES achieved a high methane yield based on chemical oxygen demand (COD) of 0.35 L CH4/gCOD compared to control operation AD (0.23 L CH4/gCOD) and ADMES (0.30 L CH4/gCOD). Additionally, hydrogen could be recovered from the DF chamber which improved the energy efficiency of the DF-ADMES reactor (91.7 %) as compared to control AD (53.4 %) and ADMES (71.9 %). Thus, a dual-chamber DF-ADMES with an AEM separator could be a feasible design for scalable treatment of high nitrogen-containing wastewater and high bioenergy recovery.

Postliminary treatment of food-waste digestate via combined hydrothermal carbonization and microbial fuel cell for bio-energy recovery: a comparative life cycle impact assessment

Multiple resource recovery from food-waste digestate by deploying hydrothermal carbonisation and microbial fuel cells. A green route for environmental sustainability and economic gains.

Comparative study of high-performance mesophilic and thermophilic anaerobic membrane bioreactors in the co-digestion of sewage sludge and food waste: Methanogenic performance and energy recovery potential

To support smart cities in terms of waste management and bioenergy recovery, the co-digestion of sewage sludge (SeS) and food waste (FW) was conducted by the anaerobic membrane bioreactor (AnMBR) under mesophilic and thermophilic conditions in this study. The biogas production rate of the thermophilic AnMBR (ThAnMBR) at the SeS to FW ratio of 0:100, 75:25, 50:50 and 100:0 was 2.84 ± 0.21, 2.51 ± 0.26, 1.54 ± 0.26 and 1.31 ± 0.08 L-biogas/L/d, inconspicuous compared with that of the mesophilic AnMBR (MeAnMBR) at 3.00 ± 0.25, 2.46 ± 0.30, 1.63 ± 0.23 and 1.30 ± 0.17 L-biogas/L/d, respectively. The higher hydrolysis ratio and the poorer rejection efficiencies of the membrane under thermophilic conditions, resulting that the permeate COD, carbohydrate and protein of the ThAnMBR was higher than that of the MeAnMBR. The lost COD that might be converted into biogas was discharged with the permeate in the ThAnMBR, which was partly responsible for the inconspicuous methanogenic performance. Furthermore, the results of energy recovery potential assessment showed that the energy return on investment (EROI) of the MeAnMBR was 4.54, 3.81, 2.69 and 2.22 at the four SeS ratios, which was higher than that of the ThAnMBR at 3.29, 2.97, 2.02 and 1.80, respectively, indicating the advantage of the MeAnMBR over the ThAnMBR in energy recovery potential. The outcomes of this study will help to choose a more favorable temperature to co-digest SeS and FW to support the construction of smart cities.

Pilot scale study of anaerobic treatment of food waste using ambient and solar heated digesters

ABSTRACT Food waste (FW) is high in nutrients and has gained global attention as an ideal substrate for bioenergy recovery through anaerobic digestion (AD). Ambient digesters have been widely used because of their ease of installation, low cost, and low energy input. However, to improve biogas production sustainably, there is a need to consider reactor heating using renewable energy such as solar. This study sought to apply psychrophilic and mesophilic biodigester temperatures for FW treatment. For ambient digestion, a complete-mix flexible biodigester, named STH-1000A, covered in a greenhouse structure was operated between 24 and 32 °C. A prototype complete-mix tank biodigester, named VUT-1000C, was designed and operated at mesophilic conditions of 37 °C through solar geyser heating. VUT-1000C produced 1200 L of biogas per day while STH-1000A 150 L/day. VUT-1000C and STH-1000A generated up to 1.8 and 0.4 kWh of electricity, respectively. The power balance showed that VUT-1000C used 68% of its power production and STH-1000A consumed 398%. Digester heating using solar geyser is a novel and promising technique for achieving mesophilic condition leading to improved biogas production.

Improving bioenergy recovery from municipal wastewater with a novel cloth-filter anaerobic membrane bioreactor

Anaerobic membrane bioreactors (AnMBR) have been used for treating high-strength industrial wastewater at full-scale and the potential to use them for mainstream municipal wastewater treatment presents an important opportunity to turn energy-intensive plants into net-energy producers. However, several limitations of the AnMBR technology have prevented their adoption in the municipal wastewater industry, namely, high membrane cleaning energy demand and low membrane flux. This study demonstrated a novel AnMBR configuration that uses a commercially available cloth filter technology to address the key limitations of cleaning energy and membrane flux. The cloth filter anaerobic membrane bioreactor (CFAnMBR) is comprised of an anaerobic fixed-film bioreactor coupled with a cloth filter membrane with nominal pore size of 5 µm. The pilot CFAnMBR was operated for 150 days through the winter at a municipal wastewater plant in central Illinois (minimum/average influent temperature 5/13°C). The CFAnMBR increased membrane flux by more than 2 orders of magnitude (3,649 ± 1,246 L per meter squared per hour) and reduced cleaning energy demand by 78%—92% (0.0085 kWh/m3) relative to previously reported AnMBR configurations. With the CFAnMBR, average chemical oxygen demand and total suspended solids removal were 66% and 91%, respectively, and were shown to be increased up to 88% and 96% by in-line coagulant dosing with ferric chloride. Average headspace methane yield was 154 mL CH4/g CODremoved by the end of the study period with influent temperatures of 11°C± 4°C. The CFAnMBR resolves major limitations of AnMBR technology by employing a commercially-available technology already used for other municipal wastewater treatment applications.

Cation exchange resins enhance anaerobic digestion of sewage sludge: Roles in sequential recovery of hydrogen and methane

The recovery of renewable bioenergy from anaerobic digestion (AD) of sludge is a promising method to alleviate the energy problem. Although methane can be effectively recovered through sludge pretreatment by cation exchange resin (CER), the simultaneous enhancement of hydrogen and methane generation from AD using CER has not been extensively investigated. Herein, the effect of CER on the sequential recovery of hydrogen and methane and the corresponding mechanisms were investigated. When CER is introduced, the maximum increases for the hydrogen and methane production are 104.7 % and 35.3 %, respectively, confirming the sequential enhancement effects of CER on the hydrogen and methane production. Analyses of the variations in the main biochemical components with and without the effect of CER demonstrate that CER promotes sludge organic solubilisation, hydrolysis, and acidification in both hydrogen- and methane-production stages. Moreover, investigations of variations in the solid–liquid interfacial thermodynamics and removal rates of main multivalent metals of sludge reveal that the ion exchange reactions between the CER and sludge in the hydrogen-production stage provide the direct driving force of effective contact between bacteria and organic particulates. Additionally, the residual effect of the CER during methane production reduces the energy barrier for mass transfer and provides a driving force for this transfer. Further analyses of the microbial community structure and metagenomics indicate that CER directly drives the enrichment of hydrogen-producing bacteria (+ 15.1 %) and key genes encoding enzymes in the hydrogen-production stage. Moreover, CER indirectly induces the enrichment of methane-producing anaerobes (e.g. Methanosaeta: + 16.7 %, Methanosarcina: + 316.5 %); enhances the bioconversion of different substrates into methyl-coenzyme M; and promotes the metabolism pathway of acetoclastic process and CO2 reduction in the methane-production stage. This study can provide valuable insights for simultaneously enhancing the production of hydrogen and methane from AD through sequential recovery.

Simultaneous production of polyhydroxybutyrate and biogas from paper mill sludge through sodium citrate-mediated disperser-induced phase separated pretreatment

Paper mill waste activated sludge is an excess sludge produced from the paper industry wastewater treatment plant. Because of the bulk nature, its disposal concerns both economic and environmental perspectives. However, it widely consists of organic components and is considered a major attraction for its valorization to produce bioenergy and valued products. During the bioenergy and valued product retrieval, the enzymatic hydrolysis got hampered due to the presence of exopolymeric substances and thus require proper pretreatment prior to the product recovery. The application of two different phases of pretreatment by removing the flocs in the first step and then followed by the cell disintegration by mechanical means helps in enhancing the organics efficiently in soluble phase. In this study, the upshot of sodium citrate mediated disperser disintegration on sludge from a paper mill was explored enhance enzymatic hydrolysis. Firstly, the floc deaggregation results at 0.05 g sodium citrate per g suspended solid with the maximum floc deaggregation degree of 89 %. Further, the consequence of disperser disintegration on the floc deaggregated sludge sample shows solid reduction and solubilization of 12.5 % and 17.5 %, respectively, higher than the disperser alone disintegration (9.57 % and 13.4 %). The anaerobic biodegradability assay shows the maximum biogas production of 174.3 mL/g of chemical oxygen demand while utilizing pretreated settled sludge as substrate. There was no accumulation of polyhydroxybutyrate using pretreated sludge supernatant. Thus, the phase-separated pretreatment by sodium citrate-induced disperser pretreatment is a suitable approach for enhancing hydrolysis during bioenergy recovery.

Performance and Energy Utilization Efficiency of an Expanded Granular Sludge Bed Reactor in the Treatment of Cassava Alcohol Wastewater

In recent years, expanded granular sludge blanket (EGSB) reactor has been widely used in the treatment of high-concentration organic wastewater, but its research mainly focused on treatment efficiency and microbial community composition. There were few studies on the relationship of operation conditions and energy utilization efficiency. Therefore, the methanogenic characteristics and energy utilization efficiency of EGSB reactor were studied by using cassava alcohol wastewater (CAW) as a raw material at (36 ± 1) °C. The results show that the degradation of volatile fatty acids (VFAs) is an important step affecting methane generation compared to the hydrolysis stage. When organic load rate (OLR) was 12.73 gCOD/L·d, the chemical oxygen demand (COD) removal rate was above 95%, the methane production efficiency of raw material was 202.73 mLCH4/ gCOD·d, the four-stage conversion efficiency was the highest, and the energy utilization efficiency was 62.26%, which was the optimal stage for EGSB reactor to treat CAW. These findings support high-efficiency bioenergy recovery from CAW in practice and highlight the potential wide application of high-performance anaerobic reactors for CAW.

Valorization and co-treatment of hazardous petroleum refinery oily sludge and sewage associated with bioenergy recovery in tubular microbial fuel cell.

Petroleum refineries generate large amounts of oily sludge which is normally loaded with different residual hazardous petroleum derivatives. Also, the residential complexes affiliated to the petroleum refineries generate considerable volumes of sewage. This study was devoted to investigate the potential of energy recovery from co-bioelectrochemical treatment of petroleum refinery oily sludge (PROS) and sewage using a tubular dual-chambers microbial fuel cell (MFC). Initially, the MFC was operated in a fill and draw mode of 4 cycles, each cycle at a different organic load (OL). The results revealed that maximum removal efficiencies of the organic content as COD were 93.67%, 98.57%, 99.64%, and 99.74%, whereby maximum power outputs were 225 ± 10, 324 ± 7, 1230 ± 18, and 1156 ± 14 mW/m3 for cycle1of OL1 (1138 ± 60 mg/L), cycle2 of OL2 (7000 ± 75 mg/L), cycle3 of OL3 (13,890 ± 50 mg/L), and cycle4 of OL4 (17,100 ± 150 mg/L), respectively. Based on those promising results, the MFC was operated continually for 60 days by feeding the MFC with PROS and sewage at organic loading of 13,000 ± 1000 mg/L. Significant results concerning COD and TPH elimination efficiency > 99.85% and 94.12%, respectively were obtained associated with power output of 1225 ± 25 mW/m3.

Corn ethanol production with thin stillage anaerobic digestion for bioenergy recovery: A technical and economic evaluation

The generation of bioenergy from thin stillage in a corn ethanol mill provides a new perspective for the use of by-products in a circular economy concept. Alternative technological routes for the corn ethanol by-products energy and nutrients recovery must be investigated to assess present and future decision-making toward economic and environmental indicators. The theoretical biogas production from an up-flow anaerobic sludge blanket (UASB) reactor in a corn-ethanol mill and the potential thermal and electrical power generation along with the avoided greenhouse gas (GHG) emissions were estimated for a large-scale industrial facility. This work aimed to assess the anaerobic digestion (AD) contribution to techno-economic indicators of a corn ethanol mill in three scenarios: 1) Standard corn-to-ethanol mill; 2) AD-CHP - thin stillage AD to produce biogas and its burning in a combined heat and power (CHP) system; 3) AD-BU - biogas purification to obtain and commercialize biomethane. The existent Brazilian market and fuels and biofuel policies place the system without AD in a slightly better position when analyzing financial indicators related to investment return. In contrast, the global energy and mass balances considering thin stillage AD showed that in the AD-CHP scenario, processing 1 ton of thin stillage would produce 10.37 m³ of biogas, which could generate 119.9 MJ of heat and 24.7 kWh of electricity in a CHP system, favoring a greener energy scenario. The energy recovered from biogas returns to the industry, supplying 7.15% of the heat required for steam production and 52.74% of the electricity requirement for industrial processing. As a renewable energy source, biogas would replace fossil sources for energy generation avoiding GHG emissions. Extrapolating the results of the present work to the Brazilian national scale, the biogas yield from the corn-to-ethanol industry would be 179.68 million m³ , the thermal energy generation from biogas could mitigate 125,574.42 ton CO2eq, and the electricity replacement could mitigate 58,193.02 ton CO2eq. Besides the mentioned energy generation and environmental benefits, the financial indicators for the examined scenarios showed that the Brazilian market and public policies related to biogas/biomethane are constraints to overcoming to larger adoption of AD by the corn-to-ethanol industry.

Environmental impacts of a novel biorefinery platform integrated with power-to-protein technology to decrease dependencies on soybean imports

The consistent population growth is directly tied to the annual rise in livestock production, placing a substantial burden on the crop sector that supplies animal feed. The Danish government has been relying on importing soybeans and soybean meal to be used as animal feed. However, this sparked environmental concerns that require more environmentally friendly solutions, such as self-sufficiency in animal feed production. The rise of green biorefineries allows new avenues of animal proteinaceous feed production using green biomass to produce leaf protein concentrate (LPC) and utilize side-stream products, such as brown juice and press cake, for feed-quality products. This study evaluated the combination of grass-clover biorefinery and the power-to-X concept, including power-to-protein technology, for its environmental sustainability through a consequential life cycle assessment (CLCA). The production of protein concentrate from organic grass clover exhibits optimal environmental performance when press cake and brown juice are used for bioenergy recovery. The findings indicate that combining a green biorefinery with power-to-protein to fully valorize the carbon and nitrogen content of brown juice and press cake into feed-grade protein can increase the environmental benefits. Such an integration resulted in an avoided impact of −995.9 kg CO2-eq/tonne of protein concentrate. The avoided impacts of climate change could be higher within the first 20 years due to a higher carbon sequestration rate. However, even after 20 years when a new carbon balance in the soil is reached, the environmental gain could be big enough to encourage the production and use of organic grass-clover protein concentrate.

Analysing the mechanism of food waste anaerobic digestion enhanced by iron oxide in a continuous two-stage process

The food waste (FW) digestion performance can be enhanced by introducing iron oxide (IO) into digesters. However, the role of IO in continuous two-stage digesters in enhancing the FW anaerobic digestion remains unclear. In this study, the effect of IO on the bioenergy recovery from a two-stage digestion process was investigated. The bioenergy recovery was significantly increased by up to 208.43 % with IO addition. The activities of dehydrogenase, α-amylase, and protease increase by 0.82–1.44, 7.24–14.56 and 7.97–20.45 times, respectively, as compared with that of the blank. With IO addition, the metabolic pathway in hydrolytic–acidogenic (HA) reactor shifted from lactic acid fermentation to butyric fermentation, which promoted stable methane production in methanogenic (MG) reactor. The activity of coenzyme F420 increased by 19.19–39.01 times, indicating that IO facilitated FW digestion by promoting hydrogenotrophic methanogenesis. The enhancement in the enzyme activity was attributable to the Fe2+ generated by dissimilatory iron reduction. According to the microbial analysis, IO enhanced interspecies hydrogen transfer between Methanobacterium and Syntrophomonas. Furthermore, IO improved direct interspecies electron transfer between Geobacter sulfurreducens and Methanosarcina. The effluent recirculation strategy greatly facilitated the hydrolysis and acidification of FW, which was critical for improving the two-stage process performance.

Integrated urban water management by coupling iron salt production and application with biogas upgrading

Integrated urban water management is a well-accepted concept for managing urban water. It requires efficient and integrated technological solutions that enable system-wide gains via a whole-of-system approach. Here, we create a solid link between the manufacturing of an iron salt, its application in an urban water system, and high-quality bioenergy recovery from wastewater. An iron-oxidising electrochemical cell is used to remove CO2 (also H2S and NH3) from biogas, thus achieving biogas upgrading, and simultaneously producing FeCO3. The subsequent dose of the electrochemically produced FeCO3 to wastewater and sludge removes sulfide and phosphate, and enhances sludge settleability and dewaterability, with comparable or superior performance compared to the imported and hazardous iron salts it substitutes (FeCl2, and FeCl3). The process enables water utilities to establish a self-reliant and more secure supply chain to meet its demand for iron salts, at lower economic and environmental costs, and simultaneously achieve recovery of high-quality bioenergy.

Carbon-neutral potential in treatment of N, N-dimethylformamide-containing industrial wastewater: Comparative investigation of energy conservation, carbon reduction and expenditure between UASB and Fenton process

The methanogenic degradation of N, N-dimethylformamide (DMF) is a sustainable and low-carbon technology for the treatment of high-strength industrial wastewater. Compared with the state-of-the-art chemical degradation method like Fenton, the UASB process exhibited its promising superiority in bio-energy recovery, economic profit gaining and carbon emission reduction. The lab-scale UASB obtained high DMF removal efficiency of over 98 % with methane recovered as bio-energy at a limited organic loading rate (OLR) of 4.2 COD kg/m3/d. Compared with the Fenton process, the UASB process is demonstrated more suitable to the carbon-neutral treatment of high-strength DMF-containing wastewater: The Fenton process is suitable to the relative low COD concentration below 1500 mg/L; The UASB was capable of covering all electricity consumption when COD > 3586 mg/L; The UASB also achieved zero CO2 emission when COD > 5720 mg/L; The UASB process obtains net profit when COD > 3616 mg/L. This study provided clear evidence of the promising potential and remarkable feasibility of bio-energy production in carbon-neutral treatment of DMF-containing wastewater.

Impact of Mechanical Stirring and Percolate Recirculation on the Performances of Dry Anaerobic Digestion

Dry anaerobic digestion (DAD) is an attractive method for simultaneous organic waste disposal and bioenergy recovery. DAD has the problems of low methane yields, low reaction rates, and easy inhibition due to its limited mass transfer and heat transfer. In this work, two methods of mechanical stirring and percolate recirculation were compared regarding their capacities of improving the mass transfer and enhancing the performances of DAD in batch experiments with sorghum stalks as a substrate. The cumulative biogas yield and system stability were investigated when the stirring linear velocity was 0 cm/s, 22 cm/s, 44 cm/s, 66 cm/s, and 88 cm/s. When the stirring linear velocity was 88 cm/s, the cumulative biogas yield and methane content were highest. The computational fluid dynamics (CFD) simulation indicated that the shearing force near the stirring shaft was largest. When the linear velocity of the stirring paddle was 88 cm/s, the shearing force at a radial distance close to center was about −140 N/m2. When the ratio of the material stacking height to the reactor diameter (H/D) was 3:2, the AD showed the best performance. A higher material stacking height promoted the contact between the microorganisms and the substrate and enhanced the biogas production. By combining percolate recirculation and mechanical stirring, the cumulative biogas yield increased by 28% compared with the static DAD process because of the promotion of mass transfer in the DAD.