Determining Optimal Temperature Combination for Effective Pretreatment and Anaerobic Digestion of Corn Stalk.

Temperature is one of the important factors affecting both chemical pretreatment and anaerobic digestion (AD) process of corn stalk (CS). In this work, the combined ways between pretreatment temperature (40 °C and 60 °C) and AD temperature (35 °C and 55 °C) were selected to investigate the AD performance for sodium hydroxide (NaOH) pretreated CS. Three organic loading rates (OLRs) of 1.6, 1.8 and 2.0 g·L-1·d-1 were studied within 255 days using continuously stirred tank reactors (CSTR). The results revealed that biogas yields of CS after pretreated were higher than that of untreated groups by 36.79-55.93% and 11.49-32.35%, respectively. When the temperature of NaOH pretreatment changed from 40 °C to 60 °C, there was no significant difference in enhancing the methane yields during the three OLRs. The mesophilic AD (MAD) of CS pretreated with 2% NaOH under 40 °C and 60 °C conditions produced 275 and 280 mL·gvs-1 methane yield at OLR of 1.6 g·L-1·d-1. However, as the OLR increased, the methane yield of CS under thermophilic AD (TAD) condition was further higher than under MAD condition. Furthermore, from the perspectives of energy balance and economic analysis, AD of 40 °C-treated CS recovered more energy and TAD is less expensive. Therefore, temperature of 40 °C was considered as an appropriate for pretreatment whether in mesophilic or thermophilic AD system. On the other hand, TAD was chosen as the optimal AD temperatures for higher OLRs.

Anaerobic digestion of urea pretreated water hyacinth removed from Lake Abaya; bio-methane potential, system stability, and substance conversion.

Water hyacinth (Eichhornia crassipes) is characterized as an aquatic plant that can grow very rapidly and freely float over water, and covers the top surface of water. It prevents the penetration of sunlight and reduces nutrients and oxygen from water bodies, and can adversely affect the aquatic ecosystem; however, it can be used in the biogas production as the sole feedstock for anaerobic digestion due to high contents of carbohydrates and biodegradable matter. Thus, this study was aimed to remove water hyacinth from Lake Abaya as a renewable energy resource for biogas production through anaerobic digestion. A lab-scale anaerobic batch reactor was applied to assess its biogas and methane generation potential. The results showed that the total solids of 91.9% at dry basis and volatile solids of 84.82% of TS. The biogas and methane production potential of 474.92 mL g-1 VS and 213.92 mL g-1 VS was recorded during 45 days HRT at 37 ± 1 °C. The daily methane production discloses accelerated increment starting from day 8 up to the peak point on day 12 (2170 mL d-1). The biochemical conversion of substances in water hyacinth to biogas was recorded as follows: total volatile solids (96.1%), cellulose (92.5%), hemicellulose (88.2%), and chemical oxygen demand (99.3%). Based on results, E. crassipes can be the sole feedstock for the anaerobic reactor to produce biomethane, while the effluents can be applied as organic fertilizers.

Enhanced hydrolysis and acidification strategy for efficient co-digestion of pretreated corn stover with chicken manure: Digestion performance and microbial community structure.

The effect of enhanced hydrolysis and acidification (EHA) strategy on co-digestion performance of pretreated corn stover (CS) with chicken manure (CM) was investigated. The EHA process was applied to the CS pretreated with KOH and liquid fraction of digestate (LFD), prior to anaerobic digestion. The results showed that the efficiencies of hydrolysis and acidification for the pretreated CS group were significantly higher than the CS group. The maximum cumulative biomethane yield of 240.5 mL·gVS-1 and 242.0 mL·gVS-1 were obtained for the KOH CS group and LFD CS group during the EHA process at 1 day, showing 26.6% and 27.4% improvement over that of the control, respectively. T90 was shortened by 38.2%-44.1% and 17.7%-38.2%, correspondingly. The synergistic effects and hydrolysis kinetics were also enhanced by the EHA process. The communities of bacteria (Firmicutes, Proteobacteria, and Bacteroidetes) and archaea (Methanosaeta, Methanobacterium, and Methanosarcina) were enriched by the EHA process, and their interactions contributed to the improved digestion performance. Therefore, the EHA process was recommended for efficient biomethane conversion in co-digestion of CS and CM.

Effect of ultrasonic application during KOH pretreatment and anaerobic process on digestion performance of wheat straw.

The digester performance was enhanced by ultrasonic application during pretreatment and the anaerobic digestion (AD) process. Two setups (with and without ultrasonic application) were applied during pretreatment and AD, to untreated and potassium hydroxide (KOH) pretreated wheat straw. The results confirmed that the ultrasonic application enhanced the hydrolysis process during pretreatment. The highest total volatile fatty acid (TVFA) (3012 ± 18 mg L-1) production and overall lignin, hemicellulose, and cellulose (LHC) reductions (22.4 ± 0.5%) were obtained from ultrasonic assisted KOH pretreated (KOHU) samples, after 36 hours of pretreatment. Similarly, the SEM analysis showed obvious disruption in the outer structure of KOHU samples. However, the ultrasonic application during AD showed significant improvement in biodegradation rate, biogas and biomethane production. Obvious reduction in sonication time (76%) along with enhanced biogas (570 ± 9 mL gm-1 VS) and biomethane (306 ± 12 mL gm-1 VS) production were observed from KOH pretreated digesters, when ultrasonication was applied during AD. Moreover, the biodegradation rate reached up to 76% along with highest total solid (TS) and volatile solid (VS) reductions from digesters with single KOH pretreatment and ultrasonic influence during AD. Finally, the digester effluent ranged in between the stable values, confirming the completion of the AD process. These results suggested that ultrasonic application was more effective when applied during AD due to the higher liquid to solid ratio. The reduction in energy input can be beneficial for commercial applications and to recognize the better stage for ultrasonic application for enhanced biomethane yield.

Mass conversion pathway during anaerobic digestion of wheat straw.

A material flow analysis (MFA) method was employed to investigate elemental flow direction during the anaerobic digestion (AD) of wheat straw (WS) pretreated with potassium hydroxide. A lab-scale batch AD experiment conducted at 35 ± 1 °C was investigated to realize carbon conversion in biogas, liquid and solid digestates. The results showed that the highest growth rate of carbon conversion in biogas was observed from the fourth day to the twenty-fourth day, which accounted for 70.64%. The cumulative biogas production of WS was 531 mL g-1 VS, along with a high volatile solids degradation rate (55.0%). The MFA results indicated that the flow mass fractions of carbon in biogas, liquid and solid digestates were 49.96%, 5.61% and 44.43%, respectively. The flow mass fraction of nitrogen in liquid and solid digestates was 45.65% and 54.35%, respectively. This study can provide a theoretical basis for elemental flow in each product from biogas projects.

Effects of different potassium and nitrogen pretreatment strategies on anaerobic digestion performance of rice straw.

The effects of different potassium and nitrogen pretreatment strategies on the anaerobic digestion (AD) performance of rice straw (RS) were investigated. KOH, NH3·H2O and KOH + NH3·H2O combined pretreatments were applied. The results showed that KOH + NH3·H2O combined pretreatment achieved the highest biomethane production and TS (TS: total solid) removal rate of 274 mL g VS-1 and 43.9%, which were 6.2-75.8% and 4.3-29.5% higher than that of single alkali pretreatments and untreated RS, respectively. The NH3·H2O groups improved the process stability, which maintained the NH3-N concentration in the range of 265-580 mg L-1. It was also found that Bacteroidetes and Firmicutes were the dominant bacterial at phyla level, and the populations of acetate methanogen (Methanosarcina and Methanosaeta) were enriched in the AD system by KOH + NH3·H2O pretreatment. Furthermore, the cost of pretreatment agents can be recovered by the increased digestate nutritional value due to the K and N remaining in the digestate after AD. The results indicated that the KOH + NH3·H2O combined pretreatment might be a promising method for efficient AD of straw in future industrial applications.

Anaerobic co-digestion of cattle manure and liquid fraction of digestate (LFD) pretreated corn stover: Pretreatment process optimization and evolution of microbial community structure.

Liquid fraction of digestate (LFD) was used to pretreat corn stover to enhance the biomethane production of anaerobic co-digestion (AcoD) with cattle manure. The effects of LFD concentration and water content (WC) for pretreatment on co-digestion performance and microbial community structure were investigated in a batch system. Results showed that the cumulative biomethane yield (CBY) for co-digestion was improved by 16.85%-41.78% compared with the control. The highest biomethane yield of 238.25 mL g VS-1 was obtained at 85% WC for pretreatment and a 5 M LFD concentration, and this yield was 41.78% higher than that in the control. The LFD pretreatment enriched the dominant bacterial phyla (Firmicutes and Bacteroidetes), but had little influence on the prevalent archaeal genus (Euryarchaeota). This study demonstrated that LFD pretreatment can greatly enhance the biomethane yield of co-digestion of corn stover and cattle manure under optimal parameters.

Evaluation of system stability and anaerobic conversion performance for corn stover using combined pretreatment.

A novel pretreatment method combining freezing-thawing and ammonia was developed, and the system stability and anaerobic conversion performance of corn stover (CS) were investigated. The CS was pretreated by water freezing-thawing (water-FT) and ammonia freezing-thawing (ammonia-FT), and then pretreated and untreated CS were anaerobically digested in three continuously stirred tank biodigesters (CSTBRs) at three organic loading rate (OLR) of 1.6, 1.8 and 2.0 gVS L-1 d-1. The analyses showed that pH value and volatile fatty acids (VFAs) concentration of three systems were in reasonable ranges of 6.8-7.5 and 160-330 mg L-1, respectively. Total ammonia nitrogen (TAN) in R3 system ranged from 600 mg L-1 to 1300 mg L-1, which was conducive to maintain system stability. The removal amount of soluble chemical oxygen demand (SCOD) of CS by ammonia-FT pretreatment was 17.3-20.5% higher than that of water-FT pretreatment. The results also showed that CS pretreated by ammonia-FT achieved 27.8-32.4% and 13.9-16.1% more methane yield (250-267 mL gvs-1) than those of untreated and water-FT pretreatment, respectively. Correspondingly, ammonia-FT pretreated CS in R3 obtained the highest conversion rates of 47.7%, 56.9%, 42.7%, and 48.8% for TS, VS, cellulose, and hemicellulose, respectively, and the lowest cost was of 0.0336 USD m3 CH4-1 and the highest energy recovery of 2428.1 kWh kgvs-1. The study demonstrated that ammonia-FT pretreatment is one of efficient and cost effective method for stable CS bioconversion.

Effects of adding osmoprotectant on anaerobic digestion of kitchen waste with high level of salinity.

Anaerobic digestion of kitchen waste can be inhibited by a high concentration of Na+ in the substrate. The aim of this study was to determine the extent to which the effect of salinity during KW anaerobic digestion could be reduced by adding osmoprotectant. The results show that when Na+ concentrations were increased from 0 to 20 g/L, the yield of cumulative methane production decreased from 623.97 to 0 mL/g volatile solids (VS). Adding 2.0 and 2.5 g/L glycine betaine (GB) to reactors during anaerobic digestion of KW with concentrations of 5 and 10 g Na+/L were determined to be the optimal dosages to alleviate Na+ inhibition; this resulted in 29.07% and 63.49% improvements in methane yield respectively. The maximum soluble chemical oxygen demand reductions that resulted from adding 2.0 and 2.5 g GB/L to kitchen waste anaerobic digestion with 5 and 10 g Na+/L were 90.00% and 82.92%, respectively. Adding GB was helpful to both the production and degradation of acetic and butyric acids, which enhanced the rate of conversion of raw material to methane. Archaeal community changes between the medium and high salt concentration groups were revealed by high-throughput sequencing and by adding GB. The most abundant archaeal phyla in all samples were Euryarchaeota and Crenarchaeota. The hydrogenotrophic methanogens showed more salt tolerance than acetoclastic methanogens. The hydrogenotrophic pathway was predominant for methanogenesis of high-level Na+ inhibition anaerobic digestion.

Continuous dynamics in anaerobic reactor during bioconversion of rice straw: Rate of substance utilization, biomethane production and changes in microbial community structure.

Anaerobic digestion (AD) technology is one of the best eco-friendly waste-to-energy processes. Hence the trend of biogas production shows considerable ranges of fluctuations in entire digestion period. This study analyzes the daily substance conversion dynamics, biomethane production and changes in microbial community structure. The results show that in the first peak of biogas production during 1st up to 4th days of the digestion period, CO2 was the dominant component. However, the daily methane production (DMP) reveal accelerated increment starting from day 8 up to the peak point on day 13 (462.11 mL/d). The concentration of acetic acid covered 21.80% up to 62.00% of the total VFAs in the first 10 days of digestion period. On the other hand, the accumulation of propionic acid is in the range of 1735.70 mg/L- 2893.12 mg/L in between day 5-15, which is beyond the inhibition level (1000 mg/L) but the system didn't stop biogas production. The distribution of bacterial family such as Clostridiaceae_1, Prevotellaceae, Enterobacteriaceae and Peptostreptococcaceae are the dominant group at early stage as compared to composition in remaining stages except Enterobacteriaceae which have marginally high abundance in lowest biogas production point. The archaeal genus Methanosaeta is dominant among the samples collected at early stages (65.66%-77.22%). However, the Methanobacterium is predominant (34.88%-59.40%) in samples obtained at late stages of AD period. On the other hand, the distribution of Methanosarcina is comparable in the first three samples (S1(16.60%), S2(22.21%) and S3(20.38%)) than the stable stage (S4 (6.7%)). This study demonstrates the detailed conditions at fluctuating and constant biogas production periods, which would benefit future researchers working in similar area.

A novel strategy for efficient anaerobic co-digestion based on the pretreatment of corn stover with fresh vinegar residue.

A novel method was advanced for efficient anaerobic co-digestion by using fresh vinegar residue (FVR) as acidifier for pretreating corn stover (CS). FVR acted as one substrate as well as an acidifier by the acids contained in FVR. It was found that the organic acids in FVR could efficiently enhance the hydrolysis of lignocellulose in CS. The biomethane production from co-digestion of FVR and CS pretreated reached 140.48 L/kg VS, which was 35.7% higher than that of unpretreated mixture substrates. The highest biomethane production was obtained when pretreatment was conducted at 150 °C. The increase of biomethane production was contributed to the improved hydrolysis of CS due to the acidic pretreatment. Pretreatment and co-digestion could improve the asynchronism and generate synergistic effect. The study provides one novel method for efficient biomethane conversion from FVR and CS.

Effect of hydrothermal pretreatment severity on the pretreatment characteristics and anaerobic digestion performance of corn stover.

The pretreatment characteristics and anaerobic digestion (AD) performance of corn stover were investigated at different hydrothermal pretreatment (HP) severity levels. The pretreatment of corn stover with and without ammonia was performed at different temperatures (100, 150, and 200 °C) and with a holding time of 5-30 mins. Results showed that after HP, the highest volatile fatty acid (VFAs) concentration was 10533.94 mg/L at a HP severity of 7.27, which was 1.72-3.35 times greater than those of others HP severity levels. Similarly, the highest removal rates of hemicellulose and lignin were 95.41% and 13.85% for HP severity levels of 6.81 and 1.98, respectively. The maximum biogas and methane increasing yield at 100 °C holding 30 min without ammonia were 16.26% and 22.74% greater than that of the untreated, respectively. These results suggest that an appropriate HP intensity can promote biogas production.

Natural freezing-thawing pretreatment of corn stalk for enhancing anaerobic digestion performance.

Natural freezing-thawing (NFT) was proposed as a low energy input and alternative pretreatment method for high biomethane production from corn stalk (CS) by anaerobic digestion (AD). The CS was pretreated by freezing-thawing in winter season using different pretreatment time periods (7d, 14d, 21d and 28d) and solid-to-liquid ratios (1:2, 1:4, 1:6, 1:8 and 1:10). The results showed that CS pretreated for 21d coupled with a solid-to-liquid ratio of 1:6 achieved the best result among all pretreatment conditions. In this case, the biomethane yield and VS removal rate of CS reached the highest values of 253 mL·gvs-1 and 58.6%, respectively, which were 40.5% and 27.4% higher than that of the untreated. It was also found that the predominant bacterial and archaeal at genus level in AD were Clostridium_sensu_stricto_1 (36.1%) and Methanobacterium (54.0%), respectively. This study provided that NFT is a simple pretreatment strategy for efficient AD bioconversion of CS to biomethane.

Anaerobic digestion performance and microbial community structure of corn stover in three-stage continuously stirred tank reactors.

A new three-stage anaerobic digestion (TSAD) system combining the two-stage and serial continuously stirred tank reactor (CSTR) was developed for the high-efficiency anaerobic digestion (AD) of corn stover. At the same hydraulic retention time of 50 d and organic loading rate (OLR) of 1.8 g TS L-1 d-1, TSAD achieved a 33.2-50.5% higher methane yield than that of the traditional one-stage and two-stage AD. Moreover, the TSAD process showed higher buffering ability and system stability, relieving the negative impact of serial CSTR at high OLR. It was also found that the hydrogenotrophic methanogen Methanobacteriaceae and multi-function methanogen Methanosarcinaceae were dominant, and the populations of Ruminococcaceae and Syntrophomonadaceae with the function of acetogenesis were enriched in TSAD. The results demonstrated that TSAD could be a high efficient system for converting corn stover into bioenergy.

Modification of corn stover for improving biodegradability and anaerobic digestion performance by Ceriporiopsis subvermispora.

Ceriporiopsis subvermispora was used to modify corn stover for improving the biodegradability and biomethane yield. Corn stover was incubated with C. subvermispora for 5-90 days then anaerobically digested. It was found that the corn stover modified for 15 days achieved the highest biomethane yield of 235 mL·g-1 VS, which was an increase of 15.2% over that of the non-modified one. The mechanism analyses indicated that the improvement resulted from the combined roles of degradation selectivity, destruction of lignocellulosic structures, and linkages. The analyses showed that C. subvermispora has a high relative selectivity of lignin degradation. The structure of the lignin and the linkages among lignin and hemicellulose and cellulose were broken obviously by acetyl group removal, and the enzymatic hydrolysis of cellulose was increased by 35.61%. The finding indicated that C. subvermispora modification is one of the effective methods for enhancing biomethane yield of corn stover.

Enhancing methane production of excess sludge and dewatered sludge with combined low frequency CaO-ultrasonic pretreatment.

Methane production of excess sludge (ES) and dewatered sludge (DS) were investigated with low frequency CaO-ultrasonic pretreatment. The results showed that the concentrations of SCOD and VFAs in pretreated ES (P-ES) and DS (P-DS) were 212.11% and 75.26%, 270.30% and 159.52% higher than those of untreated ES and DS, respectively. The contents of acetic acid and ethanol comprised 83.87%-92.88% of the total VFAs. The cumulative methane production (CMP) of P-ES and P-DS were 167.08 and 162.96 mL/g·VS, respectively, which resulted in 40.45% and 36.94% higher than those of untreated ones. The biodegradability of P-ES was 87.65%, which was close to theoretical value. Low frequency CaO-ultrasonic pretreatment could not only improve the performance of anaerobic digestion (AD), but also accelerate the decomposition rate of two kinds of sludge. Therefore, this study provided meaningful insight for exploring efficient pretreatment strategy to stabilize and enhance AD performance for practical application.

Enhancing anaerobic digestion performance and degradation of lignocellulosic components of rice straw by combined biological and chemical pretreatment.

In order to determine eco-friendly pretreatment method, the combination of different pretreatment reagents such as: CaO, ammonia solution (AS), liquid fraction of digestate (LFD), CaO-AS and CaO-LFD were used in this study. The features of physico-chemical structures and anaerobic digestion (AD) performance of rice straw were investigated using different combined biological and chemical pretreatment methods. The results showed that CaO-LFD bio-chemical pretreatment achieved the best effect among different pretreatment conditions. The removal rate of lignocellulosic components from CaO-LFD pretreated rice straw was 20.73% higher than that of the control sample. The ether and ester bonds between lignin and hemicellulose were ruptured during pretreatment. Moreover, the methane yield from CaO-LFD pretreated rice straw was 274.65 mL gVS-1, which was 57.56% more than the control. Compared with the untreated rice straw, T80 decreased by 42.86%. CaO-LFD combined pretreatment has advantages as both biological and chemical pretreatment, which complement each other to improve the degradation of the rice straw. Meantime, AD performance was improved and excellent economic viability was achieved. Therefore, this study provides sustainable insight for exploring efficient pretreatment strategy to stabilize and enhance AD performance for further application.

Biogas production and microbial community properties during anaerobic digestion of corn stover at different temperatures.

Temperature has different effects on anaerobic digestion (AD) of various biomasses, which could bring out changes in microbial communities. The relationship between microbial community and methane production at 35 °C (R35), 38 °C (R38), 41 °C (R41), and 44 °C (R44) was analyzed during AD of corn stover (CS). The results showed that the daily biogas and methane production from R44 were 16.6%-42.4% and 16.2%-40.6% higher than yields from R35, R38 and R41, respectively. The abundance of Bacteroidetes in R35, R38 and R41 was relatively close (30.70%-39.36%), which was low in R44 (16.00%). The abundance of Firmicutes in R35 was 32.30%, however, Firmicutes was the most dominant phylum at R44 (66.58%). The abundance of Miscellaneous_Crenarchaeotic_Group and Euryarchaeota were 54.63 ±â€¯6.47% and 44.43 ±â€¯6.73% across all digesters. This research demonstrated that among all temperatures studied, 44 °C could enhance the conversion efficiency of the substrates to methane and be recommended for better conversion of CS in AD process.

Serial completely stirred tank reactors for improving biogas production and substance degradation during anaerobic digestion of corn stover.

Several completely stirred tank reactors (CSTR) connected in series for anaerobic digestion of corn stover were investigated in laboratory scale. Serial anaerobic digestion systems operated at a total HRT of 40days, and distribution of HRT are 10+30days (HRT10+30d), 20+20days (HRT20+20d), and 30+10days (HRT30+10d) were compared to a conventional one-step CSTR at the same HRT of 40d. The results showed that in HRT10+30d serial system, the process became very unstable at organic load of 50gTS·L-1. The HRT20+20d and HRT30+10d serial systems improved methane production by 8.3-14.6% compared to the one-step system in all loads of 50, 70, 90gTS·L-1. The conversion rates of total solid, cellulose, and hemicellulose were increased in serial anaerobic digestion systems compared to single system. The serial systems showed more stable process performance in high organic load. HRT30+10d system showed the best biogas production and conversions among all systems.