Psychrophilic anaerobic digestion of lignocellulosic biomass: a characterization study

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.

Insights into enzyme activity and phosphorus conversion during kitchen waste composting utilizing phosphorus-solubilizing bacterial inoculation

The main objective of this research was to investigate the effects of Phosphorus-Solubilizing Bacterial (PSB) inoculant on the bacterial structure and phosphorus transformation during kitchen waste composting. High throughput sequencing, topological roles, and multiple analysis methods were conducted to explain the links between phosphorus fractions, enzyme contents, and microbial community structure and function. The findings indicated that bacterial inoculant improved environmental parameters and increased the concentration of total phosphorus, Olsen phosphorus, citric acid phosphorus, OM decomposition, and bacterial diversity. Network analysis concluded that the inoculation treatment was more complex (nodes and edges) and contained more positive links than the control, implying the inoculation effect. The structural equation model also displayed that pH and enzyme activity directly enhanced the phosphorus conversion and bacterial structure. Overall, these results suggest that bacterial inoculation may considerably increase enzyme activity, thus improving biological phosphorus transformation and nutrient content in composting products.

Bioaugmentation improves batch psychrophilic anaerobic co-digestion of cattle manure and corn straw

Low temperatures result in poor anaerobic digestion (AD). To investigate whether bioaugmentation can improve anaerobic co-digestion of cattle manure and corn straw at 20 °C, five different doses of methanogenic propionate-degrading culture (4%, 8%, 12%, 14%, and 16%) were added to batch AD systems to compare bioaugmentation performance. The results showed that the methane production of all the bioaugmented digesters was enhanced compared to the control, increasing 2.80-4.20-fold with digestion times (T₈₀) shorter by 11-22 d. The recommended dose for biogas production was 14%, and the recommended dose for the highest bioaugmentation efficiency of microbes was 4%. These improvements were due to the addition of methanogenic propionate-degrading culture, which alleviated volatile fatty acids (VFA) accumulation, especially that of acetate and propionate. Metagenomic sequencing analysis indicated that the increased proportion of propionate-oxidizing bacteria, syntrophic butyrate-oxidizing bacteria, and acetoclastic methanogens in bioaugmentation reactors may be responsible for better AD performance.

Effect of Hydrogen Addition on the Energetic and Ecologic Parameters of an SI Engine Fueled by Biogas

The global policy solution seeks to reduce the usage of fossil fuels and greenhouse gas (GHG) emissions, and biogas (BG) represents a solutions to these problems. The use of biogas could help cope with increased amounts of waste and reduce usage of fossil fuels. Biogas could be used in compressed natural gas (CNG) engines, but the engine electronic control unit (ECU) needs to be modified. In this research, a spark ignition (SI) engine was tested for mixtures of biogas and hydrogen (volumetric hydrogen concentration of 0, 14, 24, 33, and 43%). In all experiments, two cases of spark timing (ST) were used: the first for an optimal mixture and the second for CNG. The results show that hydrogen increases combustion quality and reduces incomplete combustion products. Because of BG’s lower burning speed, the advanced ST increased brake thermal efficiency (BTE) by 4.3% when the engine was running on biogas. Adding 14 vol% of hydrogen (H2) increases the burning speed of the mixture and enhances BTE by 2.6% at spark timing optimal for CNG (CNG ST) and 0.6% at the optimal mixture ST (mixture ST). Analyses of the rate of heat release (ROHR), temperature, and pressure increase in the cylinder were carried out using utility BURN in AVL BOOST software.

Influence of carbon type and carbon to nitrogen ratio on the biochemical methane potential, pH, and ammonia nitrogen in anaerobic digestion

Organic waste used as a feedstock in the anaerobic digestion (AD), it includes carbon and nitrogen. Carbon and nitrogen have an effect on the various digestive characteristics during AD, however, the study is rare about those of the interaction. This study investigates the influence of carbon type and carbon to nitrogens (C/N ratios) on the AD characteristics of organic waste. Experimental treatments involved a combination of three carbon types with three C/N ratios. The AD tests were carried out using a 125-mL serum bottle at a constant temperature of 37°C and moisture 95% for 18 days. Degradation pattern shows the difference among three-carbon treatments, the starch group was faster than other groups. Maximum methane production date was similar between starch (9.96 ± 0.05 day) and xylan group (10.0 ± 0.52 day), those of the cellulose group (14.6 ± 1.80 day) was slower than other groups (p < 0.05). The lag phase was only affected by the carbon type (p < 0.05). Ammonia nitrogen was mainly affected by nitrogen concentration regardless of carbon type (p < 0.05). This study showed that xylan is useful as feedstock in order to decrease the lag phase, and it showed that ammonia was independently affected by the nitrogen concentration.

Influences of Temperature and Substrate Particle Content on Granular Sludge Bed Anaerobic Digestion

Influences of temperature (25–35 °C) and substrate particulate content (3.0–9.4 g total suspended solids (TSS)/L) on granular sludge bed anaerobic digestion (AD) were analyzed in lab-scale reactors using manure as a substrate and through modeling. Two particle levels were tested using raw (RF) and centrifuged (CF) swine manure slurries, fed into a 1.3-L lab-scale up-flow anaerobic sludge bed reactor (UASB) at temperatures of 25 °C and 35 °C. Biogas production increased with temperature in both high- and low-particle-content substrates; however, the temperature effect was stronger on high-particle-content substrate. RF and CF produced a comparable amount of biogas at 25 °C, suggesting that biogas at this temperature came mainly from the digestion of small particles and soluble components present in similar quantities in both substrates. At 35 °C, RF showed significantly higher biogas production than CF, which was attributed to increased (temperature-dependent) disintegration of larger solid particulates. Anaerobic Digestion Model No.1 (ADM1) based modeling was carried out by separating particulates into fast and slow disintegrating fractions and introducing temperature-dependent disintegration constants. Simulations gave a better fit for the experimental data than the conventional ADM1 model.

Two-stage thermophilic anaerobic co-digestion of corn stover and cattle manure to enhance biomethane production

Two-stage thermophilic anaerobic co-digestion of cattle manure and corn stover was conducted to increase biomethane production. The first stage pre-digestion of corn stover was studied based on the following treatment variables: corn stover to liquid fraction of digestate (CS:LFD) ratio (1:7, 1:10, 1:13, 1:14), digestion temperature (55 °C, 60 °C) and digestion time (3, 7, 14 days). The reduction in lignin, cellulose and hemicellulose (LCH) was between 3.97% and 11.98%, which increased the biodegradability of corn stover. Corn stover pre-digested with a CS:LFD ratio of 1:10 at 55 °C for a period of 3 and 7 days was subjected to anaerobic co-digestion with cattle manure. The highest biomethane yield was observed on day 21 with a value of 357.41 mL/g volatile solids (VS) for untreated corn stover, 446.84 mL/g VS for corn stover pre-digested for 3 days and 518.58 mL/g VS for corn stover pre-digested for 7 days with LFD. The VS conversion efficiency for co-digestion of cattle manure with untreated corn stover, corn stover pre-digested for 3 days and 7 days was 42.8%, 43.3% and 51.8%, respectively, on day 21, which was higher than that (34.0%) of cattle manure only.

A start-up of psychrophilic anaerobic sequence batch reactor digesting a 35 % total solids feed of dairy manure and wheat straw

Zero liquid discharge is currently an objective in livestock manure management to minimize water pollution. This paper reports the start-up phase of a novel psychrophilic (20 °C) dry anaerobic digestion of dairy manure with bedding fed at 35 % total solids and an organic loading rate of 3.0 g total chemical oxygen demand kg⁻¹ inoculum day⁻¹ in anaerobic sequence batch reactors. The specific methane (CH₄) yield ranged from 165.4 ± 9.8 to 213.9 ± 13.6 NL CH₄ kg⁻¹ volatile solids (VS) with an overall average of 188 ± 17 NL CH₄ kg⁻¹ VS during 11 successive start-up cycles (231 days) and a maximum CH₄ production rate of 10.2 ± 0.6 NL CH₄ kg⁻¹ VS day⁻¹. The inoculum-to-substrate (VS-based) ratio ranged from 4.06 to 4.47. Although methanogenesis proceeded fairly well the hydrolysis seemed to be the rate limiting step. It is possible start up psychrophilic dry anaerobic digestion of cow feces and wheat straw at feed TS of 35 % within 7–10 successive cycles (147–210 days).

High rate psychrophilic anaerobic digestion of high solids (35%) dairy manure in sequence batch reactor

Zero liquid discharge is increasingly adopted as an objective for waste treatment process. The objective of this study was to increase the feed total solids (TS) and the organic loading rate (OLR) fed to a novel psychrophilic (20°C) dry anaerobic digestion (PDAD). Duplicate laboratory-scale bioreactors were fed cow feces and wheat straw (35% TS in feed) at OLR of 6.0 g TCOD kg(-1) inoculum d(-1) during long-term operation (147 days consisting of 7 successive cycles). An overall average specific methane yield (SMY) of 151.8±7.9 N L CH4 kg(-1) VS fed with an averaged volatile solids removal of 42.4±4.3% were obtained at a volatile solids-based inoculum-to-substrate ratio (ISR) of 2.13±0.2. The operation was stable as indicated by biogas and VFAs profiles and the results were reproducible in successive cycles; a maximum SMY of 163.3±5.7 N L CH4 kg(-1) VS fed was obtained. Hydrolysis was the reaction limiting step. High rate PDAD of 35% TS dairy manure is possible in sequential batch reactor within 21 days treatment cycle length.

Impact of organic loading rate on the performance of psychrophilic dry anaerobic digestion of dairy manure and wheat straw: long-term operation

Development of efficient processes for valorising animal wastes would be a major advancement in cold-climate regions. This paper reports the results of long term (315 days experiment) of novel psychrophilic (20°C) dry anaerobic digestion (PDAD) of cow feces and wheat straw in laboratory scale sequence batch reactor operated at increasing organic loading rate. The PDAD process fed with a mixture of feces and straw (TS of 27%) over a treatment cycle length of 21 days at organic loading rate (OLR) 4.0, 5.0 and 6.0 g TCOD kg(-1) inoculum d(-1) (of 2.9 ± 0.1, 3.7 ± 0.1, and 4.4 ± 0.1g VS kg(-1) inoculum d(-1), respectively) resulted in average specific methane yield (SMY) of 187.3 ± 18.1, 163.6 ± 39.5, 150.8 ± 32.9 N L CH4 kg(-1)VS fed, respectively. PDAD of cow feces and wheat straw is possible with VS-based inoculum-to-substrate ratio of 1.4 at OLR of 6.0 g TCOD kg(-1) inoculum d(-1). Hydrolysis was the limiting step reaction.

High rate psychrophilic anaerobic digestion of undiluted dairy cow feces

Novel high rate psychrophilic (20°C) anaerobic digestion (PAD) of undiluted cow feces (11.5-13.5% total solids) was demonstrated using sequence batch reactor in long-term operation with successive cycles of 21days treatment cycle length (TCL). At organic loading rates (OLR) 9.0, 10.0, 11.0 and 12.0g TCOD kg(-1) inoculum d(-1) average specific methane yield (SMY) was 154.0±11.7, 152.1±12.2, 126.0±2.8 and 116.0±6.1NL CH4 per kg of VS fed, respectively. Volatile solids removal averaged around 31.7±3.3%, 32.2±1.0%, 27.9±2.2% and 23.4±0.5%, respectively. Substrate-to-inoculum ratio (SIR; wet-mass basis) ranged between 1.17±0.06 and 1.43±0.05. Concentration of volatile fatty acids in the bioreactors during the TCL indicated that hydrolysis was the rate limiting reaction. High rate PAD of undiluted cow feces is possible at OLR (g TCOD kg(-1) inoculum d(-1)) 9.0 and 10.0 with a TCL of 21days; however, OLR of 11.0 and 12.0 are also possible but require longer TCL to maintain the SMY.

Psychrophilic dry anaerobic digestion of dairy cow feces: long-term operation

This paper reports experimental results which demonstrate psychrophilic dry anaerobic digestion of cow feces during long-term operation in sequence batch reactor. Cow feces (13-16% total solids) has been anaerobically digested in 12 successive cycles (252 days) at 21 days treatment cycle length (TCL) and temperature of 20 °C using psychrotrophic anaerobic mixed culture. An average specific methane yield (SMY) of 184.9 ± 24.0, 189.9 ± 27.3, and 222 ± 27.7 (N)L CH4 kg(-1) of VS fed has been achieved at an organic loading rate of 3.0, 4.0, and 5.0 g TCOD kg(-1) inoculum d(-1) and TCL of 21 days, respectively. The corresponding substrate to inoculum ratio (SIR) was 0.39 ± 0.06, 0.48 ± .02, 0.53 ± 0.05, respectively. Average methane production rate of 10 ± 1.4(N)L CH4 kg(-1) VS fed d(-1) has been obtained. The low concentration of volatile fatty acids indicated that hydrolysis was the reaction limiting step.

Chemical characteristics of biomass from nature conservation management for methane production

The aim of the current study was to assess the biochemical methane potential (BMP) of different functional groups harvested from different semi-natural grassland types that are valuable for nature conservation purposes. Ensiling of particular biomass did not significantly influence its methane yield, however, the ranking of functional groups by their methane yield varied during the experiment. During the first days of the experiment, methane was released most rapidly by legumes and other forbs with higher N and P contents. At the end of the BMP experiment the quantity of methane produced was higher in grasses and sedges/rushes with lower K, Mg and lignin content. Hence, measurement of feedstock chemical composition is an essential input to develop suitable technology for anaerobic digestion of late harvested biomass from semi-natural grasslands.

Asparagus stem as a new lignocellulosic biomass feedstock for anaerobic digestion: increasing hydrolysis rate, methane production and biodegradability by alkaline pretreatment

Recently, anaerobic digestion of lignocellulosic biomass for methane production has attracted considerable attention. However, there is little information regarding methane production from asparagus stem, a typical lignocellulosic biomass, by anaerobic digestion. In this study, alkaline pretreatment of asparagus stem was investigated for its ability to increase hydrolysis rate and methane production and to improve biodegradability (BD). The hydrolysis rate increased with increasing NaOH dose, due to higher removal rates of lignin and hemicelluloses. However, the optimal NaOH dose was 6% (w/w) according to the specific methane production (SMP). Under this condition, the SMP and the technical digestion time of the NaOH-treated asparagus stem were 242.3 mL/g VS and 18 days, which were 38.4% higher and 51.4% shorter than those of the untreated sample, respectively. The BD was improved from 40.1% to 55.4%. These results indicate that alkaline pretreatment could be an efficient method for increasing methane production from asparagus stem.