Anaerobic Digestion Digestate Research Articles

Treatment of Anaerobic Digester Liquids via Membrane Biofilm Reactors: Simultaneous Aerobic Methanotrophy and Nitrogen Removal.

Anaerobic digestion (AD) produces useful biogas and waste streams with high levels of dissolved methane (CH4) and ammonium (NH4+), among other nutrients. Membrane biofilm reactors (MBfRs), which support dissolved methane oxidation in the same reactor as simultaneous nitrification and denitrification (ME-SND), are a potential bubble-less treatment method. Here, we demonstrate ME-SND taking place in single-stage, AD digestate liquid-fed MBfRs, where oxygen (O2) and supplemental CH4 were delivered via pressurized membranes. The effects of two O2 pressures, leading to different O2 fluxes, on CH4 and N removal were examined. MBfRs achieved up to 98% and 67% CH4 and N removal efficiencies, respectively. The maximum N removal rates ranged from 57 to 94 mg N L-1 d-1, with higher overall rates observed in reactors with lower O2 pressures. The higher-O2-flux condition showed NO2- as a partial nitrification endpoint, with a lower total N removal rate due to low N2 gas production compared to lower-O2-pressure reactors, which favored complete nitrification and denitrification. Membrane biofilm 16S rRNA amplicon sequencing showed an abundance of aerobic methanotrophs (especially Methylobacter, Methylomonas, and Methylotenera) and enrichment of nitrifiers (especially Nitrosomonas and Nitrospira) and anammox bacteria (especially Ca. Annamoxoglobus and Ca. Brocadia) in high-O2 and low-O2 reactors, respectively. Supplementation of the influent with nitrite supported evidence that anammox bacteria in the low-O2 condition were nitrite-limited. This work highlights coupling of aerobic methanotrophy and nitrogen removal in AD digestate-fed reactors, demonstrating the potential application of ME-SND in MBfRs for the treatment of AD's residual liquids and wastewater. Sensor-based tuning of membrane O2 pressure holds promise for the optimization of bubble-less treatment of excess CH4 and NH4+ in wastewater.

Impact of lowering nitrogen content in pig manure through low crude protein diets on anaerobic digestion process stability, biogas yields, and digestate composition

Lowering crude protein in pig diets can reduce nitrogen (N) excretion and alter manure characteristics. Anaerobic digestion (AD) offers potential for converting pig manure into biogas and bio-based fertilizers (i.e., digestate). However, limited research exists on the effects of N content in pig manure on AD when pigs are fed diets with varying crude protein levels. This study investigated how lowering N content in pig manure through low crude protein diets may affect AD process stability, biogas generation, and digestate properties. Manures from different dietary treatments, named as control (CON), low N (LN), and very low N (VLN), with Total Kjeldahl Nitrogen concentrations of 5.87, 5.42, and 5.15 g/L, respectively, were investigated. Daily biogas production, composition (CH4, CO2, and H2S), and digestate properties were monitored over 13 fed-batch cycles (25 ± 4 days per cycle). The experiment was conducted at 20 ± 1 °C, a condition suited for milder climate regions, using six single-stage digesters operated in sequencing fed-batch mode. Data were analyzed by ANOVA using PROC MIXED with repeated measures. Results showed that the differences in N content in pig manure due to the three dietary treatments had a limited impact on biogas generation, with specific methane yields remaining similar over time. CH4 concentrations remained stable between 60 and 65 %, ensuring high-quality biogas despite dietary variations. Differences between treatments became more pronounced with increased organic loading rates (OLRs) due to variations in the amount of volatile solids fed. AD also remained stable (Total Volatile Fatty Acids/Total Alkalinity <0.25) even at an OLR of 2.15 g of chemical oxygen demand L−1 day−1, highlighting AD's robustness at lower temperatures. Digestate samples contained essential minerals beneficial for plant growth. More research is needed to explore varied manure compositions and feeding strategies to better understand the interactions of animal nutrition with AD.

Anaerobic digestion or composting? Small-scale plants design and holistic evaluations in a Sub-Saharan African context

In small developing settings, is it worth building anaerobic digestion (AD) or composting plants? This study explores the economic, management, and environmental dimensions of two small-scale alternatives for organic fraction municipal solid waste (OFMSW) treatment within the context of Lacor Hospital (Uganda): aerated static pile composting (S1) and AD with digestate composting (S2), both designed to manage approximately 347.5 tOFMSW annually. In the optimistic scenario, S1 achieves a cost savings of about −2.9 USD tOFMSW−1, while S2 incurs costs of 2.1 USD tOFMSW−1. In the pessimistic scenario, S1's costs rise to 3.9 USD tOFMSW−1, while S2 becomes more expensive at 9.5 USD tOFMSW−1. Management analysis underlines S2's complexity due to AD operations and digestate drying. Total normalized environmental impacts of S1 can be quantified with about 0.125 mPt tOFMSW−1, whereas S2 is equal to about −6.163 mPt tOFMSW−1. However, in an optimistic scenario, climate change endpoint category results are similar. On balance, the LCA analysis indicates that AD can be better than standalone composting. However, in developing settings serving approximately 3000 inhabitants, it is crucial to prioritize economic and management sustainability that can be obtained only by small-scale composting plants. These findings provide definite insights for small-scale waste management projects in low-income regions, offering valuable data and references for plant design and their replicability. The study sets the ultimate definition of the most feasible option to treat OFMSW in low-income settings: community composting. Unfortunately, economic barriers remain the main challenge: citizens should pay for the service and landfill management fees should be set by local governments.

Use of anaerobic digestate to substitute inorganic fertilisers for more sustainable nitrogen cycling

Fertiliser applications significantly contribute to agricultural greenhouse gas emissions. While crop yields and quality depend on the type and amount of chemical fertiliser supplied, there are limitations to fertiliser use, as continuous application of inorganic fertilisers can negatively affect soil fertility and result in mineral leaching. Organic fertiliser produced by Anaerobic Digestion (AD) of sewage waste (AD digestate) has been proposed as a substitute for chemical fertilisers. To investigate the potential for the use of AD digestate in pasture systems, grassland field trials were conducted on three independent farm enterprises in Ireland. AD digestate and chemical fertilisers were applied separately or in various combinations and the effects on soil mineral content, grass yield and quality compared. The analyses revealed significant improvement of soil nitrification activity along with improved fodder yield and quality. In particular, digestate application increased crude protein content of fodder by up to 22.8 % compared to fodder from plots without digestate application, that had up to 14.8% crude protein. The increase could be attributed to the white clover percentage increase from 2% (no AD digestate) to up to 31% on plots where AD digestate was applied. Strikingly, grass yields were 1.5-2-fold higher than the five-year national average (of 59.7 ± 4 kg DM/ha/day) on the plots where AD digestate was applied, either alone or in combination with chemical fertilisers. The results of this study highlight the potential of AD digestate application as a more sustainable alternative to inorganic fertilisers in grass-based pasture systems.

Metagenomic absolute quantification of antibiotic resistance genes and virulence factor genes-carrying bacterial genomes in anaerobic digesters

Sewage treatment works have been considered as hotspots for the dissemination of antibiotic resistance genes (ARGs). Anaerobic digestion (AD) has emerged as a promising approach for controlling the spread of ARGs while destroying biomass in sludge. Evaluating the impact of AD on ARG removal relies on the absolute quantification of ARGs. In this study, we quantified the ARG concentrations in both full-scale and lab-scale AD systems using a cellular spike-ins based absolute quantification approach. Results demonstrated that AD effectively removed 68 ± 18 %, 55 ± 12 %, and 57 ± 19 % of total ARGs in semi-continuous AD digesters, with solid retention times of 15, 20, and 25 days, respectively. The removal efficiency of total ARGs increased as the AD process progressed in the batch digesters over 40 days. A significant negative correlation was observed between digestion time and the concentrations of certain ARG types, such as beta-lactam, sulfonamide, and tetracycline. However, certain potential pathogenic antibiotic resistant bacteria (PARB) and multi-resistant high-risk ARGs-carrying populations robustly persisted throughout the AD process, regardless of the operating conditions. This study highlighted the influence of the AD process and its operating parameters on ARG removal, and revealed the broad spectrum and persistence of PARB in AD systems. These findings provided critical insights for the management of microbial hazards.

Application of biochar to anaerobic digestion versus digestate: Effects on N emissions and C stability

Research attention is growing for biochar as amendment for anaerobic digestion (AD), as it may improve both the AD process and digestate properties. In this study, two biochars (from insect frass or the woody fraction of green waste, both pyrolyzed at 450 °C) were added (5 % w/w) during semi-continuous AD of organic kitchen waste and chicken manure. Biochar was mixed either during the AD process or to the digestate post-treatment. The impact of biochar addition on the AD process, NH3 and greenhouse gas emissions, as well as C and N mineralization following soil application, was examined. Biochar did not affect the biogas yield, but did lead to a reduction in NH3 levels in the biogas and NH4+-N sorption onto the biochars, reducing the risk of AD inhibition. N sorbed as NH4+-N onto the biochar was more available for mineralization than N sorbed as NH3. N sorption on biochar-amended digestates did not result in trade-offs when these digestates were applied to soil: the sorbed N was not released as NH3 or N2O after soil application and was not released as mineral N in either the short or medium term. The C-rich biochars increased the OC content of the digestate, which enhanced its value as soil improver. Additionally, mixing biochar with digestate decreased the soil CO2 emissions up to 33 % when it was added after AD; no priming effects were observed. These findings highlight the potential benefits of incorporating biochar into AD systems and the subsequent application of biochar-amended digestate in agricultural practices.

Treatment and Valorization of Agro-Industrial Anaerobic Digestate Using Activated Carbon Followed by Spirulina platensis Cultivation

The increased production of biogas through the anaerobic digestion (AD) process has raised several concerns regarding the management of liquid digestate, which can present some environmental risks if not properly handled. Among the different techniques to treat AD digestate, microalgae and cyanobacteria cultivation has emerged as a sustainable approach to valorizing digestate while producing valuable biomass for production of biofuels and high value bioproducts. However, the intrinsic parameters of the liquid digestate can strongly limit the microalgae or cyanobacteria growth as well as limit the uptake of residual nutrients. In this study, the detoxification potential of activated carbon (AC) was evaluated on agro-industrial liquid digestate prior to Spirulina platensis cultivation. Different doses of AC, ranging from 5 to 100 g/L, were tested during adsorption experiments in order to determine the adsorption capacity as well as the removal efficiency of several compounds. Experimental results showed the high reactivity of AC, especially towards phosphate (PO4-P), total phenol (TP) and chemical oxygen demand (COD). At a dosage of 50 g/L, the AC pretreatment successfully achieved 54.7%, 84.7% and 50.0% COD, TP and PO4-P removal, corresponding to adsorption capacity of 94.7 mgDCO/g, 17.9 mgTP/g and 8.7 mgPO4-P/g, respectively. Even if the AC pretreatment did not show significant effects on Spirulina platensis growth during toxicity assays, the AC adsorption step strongly participated in the digestate detoxification by removing hardly biodegradable molecules such as phenolic compounds.

Exploring the potential for biomethane production by the hybrid anaerobic digestion and hydrothermal gasification process: A review

For a cleaner and healthier planet, industries must decarbonize and move away from ‘take-make-waste’ linear economies to circular economies , in ways that maximize value and minimize negative environmental impact. Anaerobic digestion (AD) is a well-known technology used to process domestic and industrial wastes to produce biogas (mainly CH 4 ) but with large volumes of high moisture content digestate as by-product The digestate is both difficult and expensive to manage, often requiring more than half the operating cost of treatment plants. Hydrothermal gasification (HTG) can convert the recalcitrant digestate into renewable gases including CH 4 . So, for high carbon conversion a hybrid AD and HTG technology is an attractive solution. In the hybrid process , AD can be used to treat the wet biomass (an existing practice), and the liquid AD digestate can then be processed by HTG to optimize CH 4 yield. This paper reviews the pros and cons of the AD and HTG processes, examines the valorization of co-products, and assesses the potential of the hybrid process with additional information from other AD-thermochemical process hybrids that have widely reported economic feasibilities . • AD-HTG is an efficient process for sustainable energy production. • Hybrid AD-HTG results in higher power and CH 4 generation. • Benefits of recycling HTG products to higher energy density gases were discussed. • Valorization of by-product CO 2 to chemicals were discussed. • Current challenges and future directions of AD-HTG process were presented.

Zero-waste: Carbon and SiO2 composite materials from the solid residue of the hydrothermal liquefaction of anaerobic digestion digestate for Li-ion batteries

Hydrothermal liquefaction (HTL) of wet biomass waste (e.g., from anaerobic digestion (AD) digestate) is an efficient technology to produce bio-crude, a hydrocarbon based renewable fuel. However, the solid waste by-product has limited use and could be an environmental nuisance if not managed appropriately. Herein, we demonstrate a sustainable way of transforming the solid residue generated through HTL of AD of sugarcane bagasse digestate into potential electrodes for lithium-ion battery (LIB). Briefly, the solid residue from the HTL process was converted into hard carbon (HC@WA) or converted through KOH activation to a carbon with a higher specific surface area carbon material (HC@AA). The structural and morphological characterizations confirm the encapsulation of SiO2 nanoparticles in the carbon matrix to form composite materials. When employed as anode materials in LIB, HC@WA and HC@AA achieve specific capacities of 452 mAh/g and 319 mAh/g respectively, and with long term cycling stability. The practical feasibility of the electrode materials was assessed by fabricating full cells with commercial LiFePO4 (LFP) cathode material. The full cells delivered good specific capacities and cycling stabilities, confirming the potential of the composite materials as electrodes for LiB. Thus, the solid residue after HTL of AD digestate could be one of the sustainable sources to develop materials for energy storage systems, thereby paving the way towards zero-waste and clean energy for a circular economy.

Coupling hydrothermal carbonization with anaerobic digestion: an evaluation based on energy recovery and hydrochar utilization

This work evaluates the effect of hydrothermal carbonization (HTC) as a pretreatment and post-treatment technique to anaerobic digestion (AD) of dairy sludge. HTC's effect on AD was evaluated based on energy recovery, nutrient transformation, and hydrochar utilization. The first approach was executed by performing HTC under a range of temperatures before mesophilic AD. HTC optimal pretreatment temperature was 210 °C for 30 min residence time. HTC pretreatment significantly increased the methane yield potential by 192%, the chemical oxygen demand removal by 18%, and the sludge biodegradability during AD by 30%. On the other hand, the application of HTC after AD (post-treatment) increased the total energy production, i.e., in addition to methane, a hydrochar with a caloric value of 10.2 MJ/kg was also obtained. Moreover, HTC post-treatment improved the steam gasification performance of the AD digestate. From the fertilizer quality point of view, HTC implementation generally boosted the concentrations of macro, micro, and secondary nutrients, suggesting its suitability for use as a liquid fertilizer. Overall, the findings of the present study indicate that if bioenergy production were the main target, HTC post-treatment following AD would lead to the most promising outcomes.

Variation of Metagenome From Feedstock to Digestate in Full-Scale Biogas Plants.

Anaerobic digestion (AD) has been widely used to resolve the problem of organic wastes worldwide. Previous studies showed that the types of feedstock have a great influence on the AD microbiome, and a huge number of AD populations are migrated from upstream feedstocks. However, the changes of microbial compositions from feedstock to AD digestate are still less understood. We collected feedstock samples from 56 full-scale biogas plants, generated 1,716 Gb feedstock metagenomic data in total, and constructed the first comprehensive microbial gene catalog of feedstock containing 25.2 million genes. Our result indicated that the predominant phyla in feedstock are Firmicutes, Bacteroidetes, and Proteobacteria, which is similar to that in AD digestate, and the microbial diversity of feedstock samples is higher than that of AD digestate samples. In addition, the relative abundance of most genes involved in methanogenesis increase from feedstock to AD digestate. Besides, the amount of antibiotic resistance genes (ARGs) and pathogenic bacteria in AD are effectively reduced compared to feedstocks. This study provides a comprehensive microbial gene catalog of feedstock, and deepens the understanding of variation of microbial communities from feedstock to AD digestate of full-scale AD. The results also suggest the potential of AD to reduce the level of ARGs and pathogens in animal manure.

The effects of saline toxicity and food-based AD digestate on the earthworm Allolobophora chlorotica

Anaerobic digestion (AD) is used to produce biogas and can offer a solution in waste management. Digestate, the AD by-product, can be applied to soil to improve fertility. However, the response of soil biological communities is not fully understood. There are mixed reports on its impact on earthworm survival. This study aimed to investigate digestate effects on earthworm mortality, and to elucidate potential mechanisms underlying it, if observed, after digestate application to soil. Juvenile and adult Allolobophora chlorotica were used as model organisms and added to microcosms prepared in a glasshouse trial. Five replicated treatments were: liquid Digestate; Osmotic-Stress (i.e. same salt concentration as digestate); Labile-C (i.e. same Biological Oxygen Demand as Digestate); Synthetic-Digestate a mixture of Osmotic-Stress and Labile-C (i.e. same salt concentration and BOD as digestate); and Water as the control. Treatments were applied at two different standardised rates equivalent to the digestate’s N content (i.e. 150 kg N ha−1 eq. or 300 kg N ha−1 eq.). The two development stages of A. chlorotica had different responses to treatments. Adult biomass was significantly greater in the Water control R150 treatment than in Digestate. Significantly lower juvenile biomass was observed in the Digestate R300 treatment than in the Labile-C and Water control treatments. The biomass of adults in the Labile-C R300 treatment was significantly greater than in the Digestate, Osmotic-Stress, Synthetic-Digestate and Water control treatments. Both life-stages exhibited a decline in biomass across all treatments, but the adults had higher mortality rates. The biomass of adults and juveniles declined, respectively, by 90% and 62% for Digestate applied at the lower rate, and by 96% and 90% at the higher rate. Whereas the abundance of adults and juveniles exhibited 80% and 24% drop at the lower rate, and a 90% and 84% drop at the higher rate. This study demonstrates that digestate can have negative impact on earthworm morbidity and mortality when applied to soil at 60% water filed pore space, with most of the total weight loss per pot due to reduced earthworm abundance. A likely hypothesis could be the osmotic stress induced by salts present in the digestate. However, there are other factors that interact with this effect, including possibly anaerobic impacts caused by high water content soils, as well as other mechanisms that have not been fully elucidated through this experimental design. Nevertheless, this work provides the basis for further ecotoxicology studies on the impact of digestate applied to soil. Further, while this works has shown that digestate can negatively impact A. chlorotica survival, whether the same is true for other earthworm species, ecotypes and life-cycle stages warrants further investigation. Considering the important role that worms play in soil health, field scale studies are also required to monitor the impacts of repeated digestate application on earthworm communities.

The inhibitory impact of ammonia on thermally hydrolyzed sludge fed anaerobic digestion.

This study evaluated the impact of ammonia on mesophilic anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) treating a mixture of primary sludge and waste activated sludge and operated under constant organic loading rate of 9kgCOD/m3 /d. Free ammonia concentrations in the digesters were varied between 37 and 966mgNH3 -N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54±5% (at <554mgNH3 -N/L) to 35±6% at the maximum free ammonia concentration of 966mgNH3 -N/L was observed at steady-state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia thus mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847±222mgNH3 -N/L for methanogens was estimated based on the digester steady-state methane rates dynamics. This study showed that current THP AD digesters (typically 110-260mgNH3 -N/L) operate under 12%-18% ammonia inhibition for methanogenesis. Operation under SRT of 15days, about 2 times more than needed to retain methanogens, can compensate for lower methanogens rates and avoid performance impacts. The later shows a good potential to operate under higher free and total ammonia concentration without jeopardizing performance. PRACTITIONER POINTS: Only from a free ammonia concentration above 554mgNH3 -N/L, decreased volatile solids reduction and biogas yield were observed. A volatile solids reduction of 35±6% at maximum free ammonia concentration of 966mgNH3 -N/L was still achieved. A Monod inhibition constant for methanogens of 847±222mgNH3 -N/L was estimated. It was estimated that current THP AD systems (110-260mgNH3 -N/L) operate under 12%-18% NH3 inhibition for methanogenesis.

One-pot synthesis of digestate-derived biochar for carbon dioxide capture

Digestate from Anaerobic Digestion (AD) is a very common waste produced by European biogas plants and its current method of reuse is as a fertilizer or as soil amendment. This in turn however gives rise to some other environmental concerns which include greenhouse gas emissions and volatilization of ammonia causing ammonia pollution. Biochar is an environmentally friendly and low-cost product for CO2 capture and sequestration. Thus, we have provided one simple method to convert AD digestate into biochar-based sorbents of CO2. In addition, the nitrogen-functionalised porous carbon has been reported to be effective in improving the uptake of CO2. However, most of them were prepared using multiple steps. In this work, N-doped biochar was prepared by a one-step method of modifying AD digestate by urea. The results show that the addition of N can increase CO2 uptake significantly. Thus, the adsorbent modified by urea with a ratio of 1:1 results in the highest CO2 uptake capacity (1.22 mmol g−1), indicating digestate’s potential application for carbon capture and storage.

Influence of Anaerobic Mesophilic and Thermophilic Digestion on Cytotoxicity of Swine Wastewaters.

Recycling wastewater from animal production for fertilizers using anaerobic digestion (AD) is a common method to recover the nutrients in the digestate. However, the digestate toxicity is not well understood because AD is mainly designed for chemical oxygen demand reduction. This study determined the toxicity during AD and the controlling factors with the goal to improve digestate safety during farmer handling to reuse the nutrients. Thermophilic and mesophilic AD of two swine wastewater sources were studied. Mammalian cell cytotoxicity revealed that the effluent after thermophilic digestion was at least 69% more toxic than the mesophilic effluent, owing to higher ammonia and total organic carbon in the former. Ammonia accounted for >55% total cytotoxicity, and the organics of the thermophilic digestate were twice more toxic than those in the mesophilic digestate. Despite less toxicity contribution than the ammonia, the organics did demonstrate significant adverse effects on the thiol-mediated cellular protection mechanism. For swine wastewater nutrient recovery, converting ammonia to less toxic nitrogen forms could lower the toxic hazard of the AD digestate. With much less ammonia, the organics would be the remaining decisive factor for toxicity, which is favorably reduced using thermophilic AD over mesophilic. If the ammonia is not reduced, mesophilic AD would generate a less toxic digestate.

Ranking hazards pertaining to human health concerns from land application of anaerobic digestate

Anaerobic digestion (AD) has been identified as one of the cleanest producers of green energy. AD typically uses organic materials as feedstock and, through a series of biological processes, produces methane. Farmyard manure and slurry (FYM&S) are important AD feedstock and are typically mixed with agricultural waste, grass and/or food wastes. The feedstock may contain many different pathogens which can survive the AD process and hence also possibly be present in the final digestate. In this study, a semi-quantitative screening tool was developed to rank pathogens of potential health concern emerging from AD digestate. A scoring system was used to categorise likely inactivation during AD, hazard pathways and finally, severity as determined from reported human mortality rates, number of global human-deaths and infections per 100,000 populations. Five different conditions including mesophilic and thermophilic AD and three different pasteurisation conditions were assessed in terms of specific pathogen inactivation. In addition, a number of scenarios were assessed to consider foodborne incidence data from Ireland and Europe and to investigate the impact of raw FYM&S application (without AD and pasteurisation). A sensitivity analysis revealed that the score for the mortality rate (S3) was the most sensitive parameter (rank coefficient 0.49) to influence the final score S; followed by thermal inactivation score (S1, 0.25) and potential contamination pathways (S2, 0.16). Across all the scenarios considered, the screening tool prioritised Cryptosporidium parvum, Salmonella spp., norovirus, Streptococcus pyogenes, enteropathogenic E. coli (EPEC), Mycobacterium spp., Salmonella typhi (followed by S. paratyphi), Clostridium spp., Listeria monocytogenes and Campylobacter coli as the highest-ranking pathogens of human health concern resulting from AD digestate in Ireland. This tool prioritises potentially harmful pathogens which can emerge from AD digestate and highlights where regulation and intervention may be required.

Thermophilic anaerobic digestion of cattail and hydrothermal carbonization of the digestate for co-production of biomethane and hydrochar

Thermophilic anaerobic digestion (AD) of cattail followed by hydrothermal carbonization (HTC) was studied. The intent of the research was to develop agricultural waste-based biorefining technologies for bioenergy production along with value-added products. Cattail was anaerobically digested at 55 °C for 14 days and protein and cellulose components were partially degraded. The average methane yield was 230–280 mL/g volatile solids and the total solids decreased by 33–55%. When the particle size of cattail was reduced from 1 in. to 1 mm, the lag phase was shortened from 1.48 to 0 d. Following the AD process of cattail, the AD digestate was hydrothermally carbonized at 250 °C for 4 h, yielding approximately 6.7–7.5 wt % gaseous products, 64 wt % liquid products and 28 wt % hydrochar. The gaseous products contained >5000 ppm H2S and liquid products possessed fewer chemicals and higher ratio of phenolic compounds compared to the liquid products from HTC of original cattail. The hydrochar had a higher carbon content (76.8–79.8%) and a higher specific surface area (∼10 m2/g) than those of the feedstock. Hydrochar was further activated by using Na2CO3, NaHCO3 and NaCl. The activation process increased the carbon content and specific surface area to 84–93% and 250–630 m2/g, respectively.

Solid-State KOH Pretreatment of Corn Straw for Anaerobic Digestion: Methane Yield Enhancement, Potassium Flow Analysis, and Preliminary Economic Assessment

Pretreatment has been widely used to improve the anaerobic digestion (AD) efficiency of cellulosic substrate. However, intrinsic shortcomings of traditional pretreatment techniques limit their industrial application. In this study, a solid-state alkaline pretreatment with KOH to improve the AD efficiency of corn straw was developed, which could largely decrease the chemical needed during pretreatment. In addition, it could be done during the feedstock storage without the need of liquid/solid separation before AD. Furthermore, the potassium added during pretreatment could be preserved in AD digestate, which could benefit its utilization as fertilizer. Results showed the solid-state KOH pretreatment significantly improved the methane yield and AD efficiency that depended on KOH loading. Under 8% KOH load, the maximum methane yield was obtained, which was 31.9% higher than the original corn straw. Structural analysis showed the intact structures (especially lignin) of corn straw were partly destroyed during ...

Submersible probe type microbial electrochemical sensor for volatile fatty acids monitoring in the anaerobic digestion process

Abstract: Microbial electrochemical sensor with a more simplified architecture is required in the anaerobic digestion (AD) process for in situ on-line volatile fatty acids (VFAs) monitoring. In this study, a submersible probe type microbial electrochemical sensor was constructed, and the current signal showed a linear relationship with the VFAs concentration (0–200 mM) at the specific reaction time from 10 to 120 min (R2 > 0.96), with the slope increased from 0.87 × 10−3 to 1.70 × 10−3 mA mM −1. The sensor showed a good selectivity when VFAs were co-existed with 2 g/L of additional organic matters (cellulose, yeast extract, peptone, and potassium oleate), and 0.1% formaldehyde. Particular attention should be paid when additional electroactive matter such as Cu(II) as well as high concentration of TAN (above 200 mg/L) was co-existed with VFAs in the digestate. When amending the artificial AD digestate with extra 50 mM NaCl to increase the ionic strength, the current generation increased from 0.064 ± 0.002 to 0.185 ± 0.001 mA (50 mM VFAs at 60 min). The sensor was successfully applied for actual digestate samples detection and the results did not show significance differences with the total VFAs measured by GC. The submersible probe type sensor depicted a good repeatability during one-month operation. This study provides a direct approach based on microbial electrochemical technology for VFAs monitoring in the AD process.

A Novel Mathematical Model for Predicting Performance of the Sequential Batch Anaerobic Digestion Process: Part I

Anaerobic digestion (AD) is a promising technology to produce renewable energy from organic wastes and achieve desirable solids reduction. One critical step for successful and continuous operation of an AD system is the startup phase, when fresh feedstock is added periodically to the digester after a portion of finished material is discharged, until reaching a pseudo-steady state. This process, identified as a sequential batch AD process, describes how many solid-state and liquid AD systems are managed. Currently, the startup and optimization of such a digester is largely determined by trial and error based on several months of continuous monitoring and adjustment by experienced operators. This research proposes a novel mathematical model to predict outputs per unit volume of a sequential batch AD process during the startup phase as well as at steady state. Important model inputs include the feedstock‘s total solids (TS) content, degradable solids content (α), digestion rate (k), batch time (∆t), digestate recycle rate (r), and water supplementation (X2). The model predicts the total methane yield (U), final end product TS and α, and mass quantity (X1, Qin, QCH4) at steady state. The model provides useful information for the design, operation, and optimization of large-scale AD digesters. Equations are used to identify r values for stable reactor performance given k and ∆t, assuming ratios of feedstock to digestate between 1 and 6.