Utilising biohydrogen to increase methane production, energy yields and process efficiency via two stage anaerobic digestion of grass

A comprehensive review on anaerobic digestion with focus on potential feedstocks, limitations associated and recent advances for biogas production

With the escalating energy demand to accommodate the growing population and its needs along with the responsibility to mitigate climate change and its consequences, anaerobic digestion (AD) has become the potential approach to sustainably fulfil our demands and tackle environmental issues. Notably, a lot of attention has been drawn in recent years towards the production of biogas around the world in waste-to-energy perspective. Nevertheless, the progress of AD is hindered by several factors such as operating parameters, designing and the performance of AD reactors. Furthermore, the full potential of this approach is not fully realised yet due the dependence on people's acceptance and government policies. This article focuses on the different types of feedstocks and their biogas production potential. The feedstock selection is the basic and most important step for accessing the biogas yield. Furthermore, different stages of the AD process, design and the configuration of the biogas digester/reactors have been discussed to get better insight into process. The important aspect to talk about this process is its limitations associated which have been focused upon in detail. Biogas is considered to attain the sustainable development goals (SDG) proposed by United Nations. Therefore, the huge focus should be drawn towards its improvements to counter the limitation and makes it available to all the rural communities in developing countries and set-up the pilot scale AD plants in both developing and developed countries. In this regard, this article talks about the improvements and futures perspective related to the AD process and biogas enhancement.

Biomass conversion and radiocaesium (Rad-Cs) leaching behaviors of radioactive grass in anaerobic wet fermentation systems: Effects of pre-treatments

Persistent concerns regarding environmental hazards arise from the difficulty in disposing of radioactive plant-based wastes originating from the nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FNPP) in Japan in 2011. In this study, three anaerobic digestion (AD) strategies were proposed: Sole anaerobic wet fermentation, and wet fermentations with either alkaline-heat or ultrasonic pre-treatment, which were employed for long-term anaerobic treatment of a genuine radioactive grass stemming from the FNPP accident. The objectives of this work are to investigate the effects of pre-treatments on biomass conversion efficiency and to gain insight into the leaching behavior of radiocaesium (Rad-Cs) within AD processes. Experimental results indicate that by introducing alkaline-heat and ultrasonic pre-treatments to AD systems, the removal efficiencies of total solids (TS) from the raw grass increased by 60.8 % and 42.5 %, respectively, compared to sole wet fermentation. Pre-treatments have been shown to enhance the stability of AD systems, both in terms of enhancing methane production and mitigating pH fluctuations triggered by the accumulation of organic acids. Remarkably, even though the Rad-Cs leaching rate was highest when the AD system was fed with the alkaline-heat pre-treated grass, it remained unsatisfactory at only 5.77 %. We inadvertently isolated a soil-like component from the raw grass, and analyzed both its proportion in the raw grass and the radioactivity intensity. The results indicate that although the soil constituted only 9.51 % TS of the raw grass, it accounted for a significant 81.35 % of the total radioactivity. The soil, which has a pronounced affinity for ionic Cs, being mixed into the raw grass, was identified as the primary factor limiting the leaching efficiency of Rad-Cs throughout both the pre-treatment and wet fermentation phases.

Life Cycle Assessment of the Mesophilic, Thermophilic, and Temperature-Phased Anaerobic Digestion of Sewage Sludge

In this study the environmental impact of the anaerobic digestion (AD) of sewage sludge within an activated sludge wastewater treatment plant (WWTP) was investigated. Three alternative AD systems (mesophilic, thermophilic, and temperature-phased anaerobic digestion (TPAD)) were compared to determine which system may have the best environmental performance. Two life cycle assessments (LCA) were performed considering: (i) the whole WWTP (for a functional unit (FU) of 1 m3 of treated wastewater), and (ii) the sludge line (SL) alone (for FU of 1 m3 of produced methane). The data for the LCA were obtained from previous laboratory experimental work in combination with full-scale WWTP and literature. According to the results, the WWTP with TPAD outperforms those with mesophilic and thermophilic AD in most analyzed impact categories (i.e., Human toxicity, Ionizing radiation, Metal and Fossil depletion, Agricultural land occupation, Terrestrial acidification, Freshwater eutrophication, and Ozone depletion), except for Climate change where the WWTP with mesophilic AD performed better than with TPAD by 7%. In the case of the SL alone, the production of heat and electricity (here accounted for as avoided environmental impacts) led to credits in most of the analyzed impact categories except for Human toxicity where credits did not balance out the impacts caused by the wastewater treatment system. The best AD alternative was thermophilic concerning all environmental impact categories, besides Climate change and Human toxicity. Differences between both LCA results may be attributed to the FU.

Overcoming nutrient loss during volatile fatty acid recovery from fermentation media by addition of electrodialysis to a polytetrafluoroethylene membrane stack

This research investigated the use of an innovative polytetrafluoroethylene (PTFE) membrane configuration coupled to electrodialysis for the in-situ removal of Volatile Fatty Acids (VFAs) from a mixed culture bioreactor. It was shown that by stacking the PTFE membranes to increase the active membrane surface area, shortened VFA recovery times was seen. The addition of electrodialysis to the PTFE membrane stack enabled the continuous extraction of VFAs from fermentation media whilst retaining essential nutrients and organic compounds in the diluate stream. Ammonium, phosphate and nitrate remained in the diluate chamber and did not cross the PTFE membrane stack. Up to 98% of total VFA recovery was achieved with the PTFE and electrodialysis system. The process was shown to extract from a reservoir of low VFA concentration to a reservoir with a VFA concentration 10 times higher. These results show that the addition of electrodialysis to PTFE provides a robust solution for the in-situ extraction of VFAs from fermentation media within bioreactors to support the demand for sustainable fuels and green chemical feedstocks.

Effects of Two-Stage Operation on Stability and Efficiency in Co-Digestion of Food Waste and Waste Activated Sludge

The two-stage anaerobic digestion (AD) technology attracts increasing attention due to its ability to collect both hydrogen and methane. A two-stage AD system feeding with food waste and waste activated sludge was investigated in order to achieve higher energy yield and organics removal. The two-stage process consists of a thermophilic H2-reactor and a mesophilic CH4-reactor, achieved the highest hydrogen and methane yields of 76.8 mL/g-VS and 147.6 mL/g-VS at hydraulic retention times (HRTs) of 0.8 d and 6 d, respectively. The co-digestion process in this study required much less external alkalinity to maintain the pH values than sole food waste digestion in the literature. Compared with the single-stage mesophilic methane AD process, the two-stage AD system had better performance on operation stability, biogas and energy yields, organics removal and chemical oxygen demand (COD) conversion at high organic loading rates (OLRs). According to the TA-cloning analysis, the dominant bacteria in H2-reactor was closely related to Clostridium sp. strain Z6 and species Thermoanaerobacterium thermosaccharolyticum. The dominant methanogens in two-stage and single-stage CH4-reactor were recognized as acetotrophic methanogens and hydrogenotrophic methanogens, respectively. The presence of the genus Nitrososphaera in the two CH4-reactors might contribute to the low NH4+-N concentration in digestate and low CO2 content in biogas.

Improvement of continuous hydrogen production using individual and binary enzymatic hydrolysates of agave bagasse in suspended-culture and biofilm reactors

Continuous hydrogen (H₂) production from individual (Stonezyme, IH) and binary (Celluclast-Viscozyme, BH) enzymatic hydrolysates of agave bagasse was evaluated in continuous stirred-tank reactors (CSTR) and trickling bed reactors (TBR). The volumetric H₂ production rates (VHPR) in CSTR were 13 and 2.25 L H₂/L-d with BH and IH, respectively. Meanwhile, VHPR of 5.76 and 2.0 L H₂/L-d were obtained in the TBR configuration using BH and IH, respectively. Differences on VHPR between reactors could be explained by substrate availability, which is intrinsic to the growth mode of each reactor configuration; while differences of VHPR between hydrolysates were possibly related to the composition of enzymatic hydrolysates. Furthermore, homoacetogenesis was strongly influenced by H₂ and substrate transfer conditions. Considering VHPR, H₂ yields, and costs of hydrolysis, hydrogen production from binary hydrolysates of agave bagasse was identified as the most promising alternative evaluated with scale-up potential for the production of energy biofuels.

Heavy metals interact with the microbial community and affect biogas production in anaerobic digestion: A review

Heavy metals (HMs), which accumulate in digestion substrates, such as plant residues and livestock manure, can affect biogas yields during anaerobic digestion (AD). Low concentration of Cu²⁺ (0-100 mg/L), Fe²⁺ (50-4000 mg/L), Ni²⁺ (0.8-50 mg/L), Cd²⁺ (0.1-0.3 mg/L), and Zn²⁺ (0-5 mg/kg) promote biogas production, while high concentrations inhibit AD. Trace amounts of HMs are necessary for the activity of some enzymes. For example, Cu²⁺ and Cd²⁺ serve as cofactors in the catalytic center of cellulase and stimulate enzyme activity. High contents of Cd²⁺ and Cu²⁺ inhibit enzyme activity by disrupting protein structures. Trace amounts of HMs stimulate the growth and activity of methanogens, while high levels have toxic effects on methanogens. HMs affect the hydrolysis, acidification, and other biochemical reactions of organics in AD by changing the enzyme structure and they also impact methanogen growth. A better understanding of the impact of HMs on AD can provide valuable insights for improving the digestion of poultry manure and plant residues contaminated with HMs, as well as help mitigate HMs pollution. Although several studies have been conducted in this field, few comprehensive reviews have examined the effect of many common HMs on AD. This review summarizes the effects of HMs on the biogas production efficiency of AD and also discusses the effects of HMs on the activities of enzymes and microbial communities.

Ultrasound combined with dilute acid pretreatment of grass for improvement of fermentative hydrogen production

In this study, the dilute acid pretreatment combined with ultrasound was applied to improve fermentative hydrogen production from grass. The experimental results indicated that SCOD and soluble carbohydrate contents of grass was improved by 98.6% and 236.9% after the combined treatment, respectively. Surface morphology (SEM and AFM) and crystallinity analysis revealed that the combined pretreatment process could effectively destroyed the biomass structure and increased their surface area. Owing to the increased soluble organics proportion and better enzymatic accessibility of residual solids, the hydrogen yield reached 42.2 mL/g-dry grass after the combined treatment, which was 311.7%, 190.0% and 35.0% higher in comparison with the control, individual ultrasound and acid pretreated groups, respectively. Meanwhile, the combined treatment also increased the substrate utilization efficiency and induced a more efficient fermentation pathway. Bacterial community analysis revealed that more enrichment of Clostridium and less enrichment of Enterococcus contributed to the improved hydrogen production.

Enhancing Anaerobic Digestion: The Effect of Carbon Conductive Materials

Anaerobic digestion is a well-known technology which has been extensively studied to improve its performance and yield biogas from substrates. The application of different types of pre-treatments has led to an increase in biogas production but also in global energy demand. However, in recent years the use of carbon conductive materials as supplement for this process has been studied resulting in an interesting way for improving the performance of anaerobic digestion without greatly affecting its energy demand. This review offers an introduction to this interesting approach and covers the different experiences performed on the use of carbon conductive materials proposing it as a feasible alternative for the production of energy from biomass, considering also the integration of anaerobic digestion and thermal valorisation.

Improving mechanisms of biohydrogen production from grass using zero-valent iron nanoparticles

This paper investigated the improving mechanisms and microbial community dynamics of using zero-valent iron nanoparticles (Fe⁰ NPs) in hydrogen fermentation of grass. Results showed that Fe⁰ NPs supplement improved microbial activity and changed dominant microbial communities from Enterobacter sp. to Clostridium sp., which induced a more efficient metabolic pathway towards higher hydrogen production. Meanwhile, it is also proposed that Fe⁰ NPs could accelerate electron transfer between ferredoxin and hydrogenase, and promote the activity of key enzymes by the released Fe²⁺. The maximal hydrogen yield and hydrogen production rate were 64.7 mL/g-dry grass and 12.1 mL/h, respectively at Fe⁰ NPs dosage of 400 mg/L, which were 73.1% and 128.3% higher compared with the control group. Fe⁰ NPs also shorten the lag time and facilitated the hydrolysis and utilization of grass. This study demonstrated that Fe⁰ NPs could effectively improve hydrogen production and accelerate the fermentation process of grass.

Pretreatment of grass waste using combined ionizing radiation-acid treatment for enhancing fermentative hydrogen production

In this study, the combined ionizing radiation-acid pretreatment process was firstly applied to enhance hydrogen fermentation of grass waste. Results showed that the combined pretreatment synergistically enhanced hydrogen fermentation of grass waste. The SCOD and soluble polysaccharide contents of grass waste increased by 1.6 and 2.91 times after the combined pretreatment, respectively. SEM observation and crystallinity test showed the combined pretreatment effectively disrupted the grass structure. Owing to the more favorable substrate conditions, the hydrogen yield achieved 68 mL/g-dry grass_added after the combined pretreatment, which was 161.5%, 112.5% and 28.3% higher than those from raw, ionizing radiation pretreated and acid pretreated grass waste, respectively. The VS removal also increased from 13.9% to 25.6% by the combined pretreatment. Microbial community analysis showed that the abundance of dominant hydrogen producing genus Clostridium sensu stricto 1 increased from 37.9% to 69.4% after the combined pretreatment, which contributed to more efficient hydrogen fermentation.

Continuous hydrogen and methane production from Agave tequilana bagasse hydrolysate by sequential process to maximize energy recovery efficiency

Continuous H₂ and CH₄ production in a two-stage process to increase energy recovery from agave bagasse enzymatic-hydrolysate was studied. In the first stage, the effect of organic loading rate (OLR) and stirring speed on volumetric hydrogen production rate (VHPR) was evaluated in a continuous stirred tank reactor (CSTR); by controlling the homoacetogenesis with the agitation speed and maintaining an OLR of 44 g COD/L-d, it was possible to reach a VHPR of 6 L H₂/L-d, equivalent to 1.34 kJ/g bagasse. In the second stage, the effluent from CSTR was used as substrate to feed a UASB reactor for CH₄ production. Volumetric methane production rate (VMPR) of 6.4 L CH₄/L-d was achieved with a high OLR (20 g COD/L-d) and short hydraulic retention time (HRT, 14 h), producing 225 mL CH₄/g-bagasse equivalent to 7.88 kJ/g bagasse. The two-stage continuous process significantly increased energy conversion efficiency (56%) compared to one-stage hydrogen production (8.2%).

Increased biohydrogen yields, volatile fatty acid production and substrate utilisation rates via the electrodialysis of a continually fed sucrose fermenter

Electrodialysis (ED) removed volatile fatty acids (VFAs) from a continually-fed, hydrogen-producing fermenter. Simultaneously, electrochemical removal and adsorption removed gaseous H₂ and CO₂, respectively. Removing VFAs via ED in this novel process increased H₂ yields by a factor of 3.75 from 0.24molH₂mol^-1_hexose to 0.90molH₂mol^-1_hexose. VFA production and substrate utilisation rates were consistent with the hypothesis that end product inhibition arrests H₂ production. The methodology facilitated the recovery of 37g of VFAs, and 30L H₂ that was more than 99% pure, both of which are valuable, energy dense chemicals. Typically, short hydraulic and solid retention times, and depressed pH levels are used to suppress methanogenesis, but this limits H₂ production. To produce H₂ from real world, low grade biomass containing complex carbohydrates, longer hydraulic retention times (HRTs) are required. The proposed system increased H₂ yields via increased substrate utilisation over longer HRTs.

Maximising biohydrogen yields via continuous electrochemical hydrogen removal and carbon dioxide scrubbing

The use of electrochemical hydrogen removal (EHR) together with carbon dioxide removal (CDR) was demonstrated for the first time using a continuous hydrogen producing fermenter. CDR alone was found to increase hydrogen yields from 0.07molH2molhexose to 0.72molH2molhexose. When CDR was combined with EHR, hydrogen yields increased further to 1.79molH2molhexose. The pattern of carbohydrate utilisation and volatile fatty acid (VFA) production are consistent with the hypothesis that increased yields are the result of relieving end product inhibition and inhibition of microbial hydrogen consumption. In situ removal of hydrogen and carbon dioxide as demonstrated here not only increase hydrogen yield but also produces a relatively pure product gas and unlike other approaches can be used to enhance conventional, mesophilic, CSTR type fermentation of low grade/high solids biomass.

Process contribution evaluation for COD removal and energy production from molasses wastewater in a BioH2-BioCH4-MFC-integrated system

In this study, a three-stage-integrated process using the hydrogenic process (BioH₂), methanogenic process (BioCH₄), and a microbial fuel cell (MFC) was operated using molasses wastewater. The contribution of individual processes to chemical oxygen demand (COD) removal and energy production was evaluated. The three-stage integration system was operated at molasses of 20 g-COD L^-1, and each process achieved hydrogen production rate of 1.1 ± 0.24 L-H₂ L^-1 day^-1, methane production rate of 311 ± 18.94 mL-CH₄ L^-1 day^-1, and production rate per electrode surface area of 10.8 ± 1.4 g m^-2 day^-1. The three-stage integration system generated energy production of 32.32 kJ g-COD^-1 and achieved COD removal of 98 %. The contribution of BioH₂, BioCH₄, and the MFC reactor was 20.8, 72.2, and, 7.0 % of the total COD removal, and 18.7, 81.2, and 0.16 % of the total energy production, respectively. The continuous stirred-tank reactor BioH₂ at HRT of 1 day, up-flow anaerobic sludge blanket BioCH₄ at HRT of 2 days, and MFC reactor at HRT of 3 days were decided in 1:2:3 ratios of working volume under hydraulic retention time consideration. This integration system can be applied to various configurations depending on target wastewater inputs, and it is expected to enhance energy recovery and reduce environmental impact of the final effluent.

Influence of Hydraulic Retention Time and Reactor Configuration During Fermentation of Diluted Chicken Manure

In this study, single-stage and two-phase semi-continuous thermophilic anaerobic reactors fed with diluted (3 % total solids (TS) and 1.8 % volatile solids (VS)) chicken manure at three different hydraulic retention times (HRTs) were compared interms of biogas production rate, methane content of the produced biogas, and VS and TS removal. Along the study, HRTs of 16, 12, and 8 days were implemented to the single-stage and the two-phase systems. It was observed that the single-stage anaerobic system was superior to the two-phase anaerobic system according to their biogas production rates (517 vs. 356, 551 vs. 359, 459 vs. 386 (mL/g VS_feed)) at all HRTs. On the other hand, methane content of the biogas produced was higher in the two-phase system compared to the single-stage system.

Assessment of increasing loading rate on two-stage digestion of food waste

A two-stage food waste digestion system involved a first stage hydrolysis reactor followed by a second stage methanogenic reactor. Organic loading rates (OLR) were increased from 6 to 15 g VS L(-1) d(-1) in the hydrolysis reactor and from 2 to 5 g VS L(-1) d(-1) in the methanogenic reactor. The retention time was fixed at 4 days (hydrolysis reactor) and 12 days (methane reactor). A single-stage digester was subjected to similar loading rates as the methanogenic reactor at 16 days retention. Increased OLR resulted in higher quantities of liquid fermentation products from the first stage hydrolysis reactor. Solubilisation of chemical oxygen demand peaked at 47% at the maximum loading. However, enhanced hydrolysis yields had no significant impact on the specific methane yields. The two-stage system increased methane yields up to 23% and enriched methane content by an average of 14% to levels of 71%.