Thermophilic Solid-state Anaerobic Digestion Research Articles

Review on solid-state anaerobic digestion of lignocellulosic biomass and organic solid waste.

Sustainable management of organic solid wastes especially the municipal solid waste (MSW) is essential for the realization of various sustainable development goals (SDGs). Resource recovery centric waste processing technologies generate valorizable products to meet the operations and maintenance (O&M) costs while reducing the GHG emissions. Solid-state anaerobic digestion (SSAD) of organic solid wastes is a biomethanation process performed at a relatively higher total solids (TS) loading in the range of 10-45%. SSAD overcomes various limitations posed by conventional anaerobic slurry digesters such as higher degradable matter per unit volume of the bioreactor resulting in a smaller footprint, low freshwater consumption, low wastewater generation, simple upstream and downstream processes, relatively lower operation, and maintenance costs. This review elucidates the recent developments and critical assessment of different aspects of SSAD, such as bioreactor design, operational strategy, process performances, mass balance, microbial ecology, applications, and mathematical models. A critical assessment revealed that the operating scale of SSAD varies between 1000 and 100,000 ts/year at organic loading rate (OLR) of 2-15g volatile solids (VS)/L·day. The SSAD experiences process failures due to the formation of volatile fatty acids (VFAs), biogas pockets and clogging of the digestate outlet. Acclimatization of microbes accelerates the startup phase, steady-state performances, and the enrichment of syntrophic microbes with 10-50 times greater population of cellulolytic and xylanolytic microbes in thermophilic SSAD over mesophilic SSAD. Experimental limitations in the accurate determination of rate constants and the oversimplification of biochemical reactions result in an inaccurate prediction by the models.

Thermophilic Solid-State Anaerobic Digestion of Corn Straw, Cattle Manure, and Vegetable Waste: Effect of Temperature, Total Solid Content, and C/N Ratio.

Thermophilic solid-state anaerobic digestion (SS-AD) of agricultural wastes, i.e., corn straw, cattle manure, and vegetable waste, was carried out in this study. The effects of temperature (40-60°C), initial solid content (ISC, 17.5-32.5%), and C/N ratio (15-32 : 1) on biogas production were evaluated using a Box-Behnken experimental design (BBD) combined with response surface methodology (RSM). The results showed that optimization of process parameters is important to promote the SS-AD performance. All the factors, including interactive terms (except the ISC), were significant in the quadratic model for biogas production with SS-AD. Among the three operation parameters, the C/N ratio had the largest effect on biogas production, followed by temperature, and a maximum biogas yield of 241.4 mL gVS−1 could be achieved at 47.3°C, ISC = 24.81%, and C/N = 22.35. After 20 d of SS-AD, the microbial community structure under different conditions was characterized by high-throughput sequencing, showing that Firmicutes, Bacteroidetes, Chloroflexi, Synergistetes, and Proteobacteria dominated the bacterial community, and that Firmicutes had a competitive advantage over Bacteroidetes at elevated temperatures. The biogas production values and relative abundance of OPB54 and Bacteroidia after 20 d of SS-AD can be fitted well using a quadratic model, implying that OPB54 and Bacteroidia play important roles in the methanogenic metabolism for agricultural waste thermophilic SS-AD.

Change to biogas production in solid-state anaerobic digestion using rice straw as substrates at different temperatures

This work investigated the effects of different temperatures on methane production, kinetics, and microbial communities during solid-state anaerobic digestion (SS-AD) using rice straw. The results indicated that thermophilic anaerobic digestion led to the faster methane production (13.74 L/kg) and a shorter biogas production cycle (34 days) than mesophilic anaerobic digestion (5.48 L/kg, 58 days). SS-AD under thermophilic conditions resulted in more intense lignocellulose degradation and better fitting results. The species of microorganisms did not differ when the temperature was altered; however, the abundances of various phyla, particularly Firmicutes, differed. Overall, the findings suggested that thermophilic SS-AD had higher methanogenic efficiency and dramatically altered the structure of the microbial community during solid-state anaerobic digestion. Moreover, a potential effective strategy for agricultural waste management by SS-AD was proposed.

Thermophilic solid state anaerobic digestion of switchgrass for liquid digestate reuse and organic fertilizer production

AbstractTwo solid state anaerobic digesters (SSADs), 15 L each, were set up for co-digestion of switchgrass with primary digestate of a liquid anaerobic digester (LAD) and the recirculating leachate. Both the LAD and two SSADs were operated at 50°C. The results showed that the bioreactors were not started up stably until day 16 and day 47 for reactors A and B, respectively. The supplement of LAD digestate or injection of sodium hydroxide (NaOH) into the recirculating leachate readily reinitiated the biogas production to normal daily high rates of the two individual SSADs, one on day 16 and the other on day 47. In contrast to reactor A, there was a longer lag phase for bioreactor B, however, it showed 46.2% [77.9 vs 53.3 L kg−1 volatile solid (VS)] more cumulative biogas yields, and higher reduction rate of total solid, VS, cellulose and hemicellulose of 29.5, 31, 40.6 and 15%, respectively, which was likely due to optimized pH and NaOH pretreated switchgrass during start-up period. Methane contents of biogas increased gradually and stabilized at 50% for both reactors, indicating a normal operation of anaerobic digestion lasted for at least 100 days. The determined parameters of digested residues met China organic fertilizer standard (NY 525-2012) except for high moisture and low total nutrient contents. Therefore, the product of SSAD has the potential value of organic fertilizer. It is concluded that the LAD digestate can be reused as inoculums by co-digestion with agricultural residues for biogas and organic fertilizer production in SSAD.

Impact of turning waste on performance and energy balance in thermophilic solid-state anaerobic digestion of agricultural waste

Mixing is an important operation in solid-state anaerobic digestion (SS-AD) to improve the mass transfer of the solid phase. This study proposed simple turning by loader in common garage-type digester without commonly used mixer or percolation system (simplified SS-AD). In simplified-SS-AD, turning is conducted in open condition. Thus, oxidation of anaerobic sludge during turning would influence digestion performance. Therefore, in this study, the effect of turning wastes by mixing during digestion on a simplified SS-AD fed with rice straw and pig urine was investigated. Four different mixing frequency levels—no mixing (M0) and mixing once a day (M-1/1), once every 3 days (M-1/3) and once a week (M-1/7)—were conducted. Methane yields of M0, M-1/3 and M-1/7 were comparable with each other. Methane yield and lag period of M-1/1 were approximately 61% and 155% of M0 (351.2 mL/g VS and 4.7 days), respectively. Furthermore, the chemical oxygen demand (COD) of acetate accumulated in the digestate of M-1/1 was comparable to the difference in the COD of methane production between M-1/1 and the other treatments. Mixing every day also resulted in a higher oxidation-reduction potential and carbon dioxide content. These findings suggest that methanogenesis was inhibited in M-1/1 by frequent mixing in the atmosphere. Net energy analysis of SS-AD plant operation showed that M0 can obtain the highest net energy gain, whereas net energy production of M-1/7 was reduced by rewarming after mixing. Therefore, no mixing is the most effective approach for the proposed simplified process.

Comparison of leachate percolation and immersion using different inoculation strategies in thermophilic solid-state anaerobic digestion of pig urine and rice straw

Heterogeneous distribution of substrate and microorganisms and low mass transfer limit methane production dramatically in solid-state anaerobic digestion (SS-AD). To overcome this challenge, this study determined the optimal inoculation strategy (complete premix/slurry application) for reusing solid digestate as inoculum and the optimal leachate circulation method (percolation/immersion) using batch digestion. Initially, percolation and immersion (1 h per 3 days) were compared and the result shows that immersing rice straw into leachate was superior to leachate percolation in methane production. Effect of the immersion period (24, 48 and 72 h) in each circulation cycle on methane production was then evaluated for each inoculation strategy. Methane production increased until the immersion period up to 24 h and then decreased, while the average cumulative methane yield with an immersion period of 24 h was (180 mL/g volatile solids). Slurry application with an immersion period 24 h is recommended as the optimum operating condition.

Enhancement of the solid-state anaerobic digestion of rice straw by liquor supplementation

Solid-state anaerobic digestion (SS-AD) has gained attention in recent years because it is robust, cheap and easy to operate. However, at high loads, the SS-AD is often plagued by acidification and poor methane yields. Mesophilic and thermophilic SS-AD at the different substrate to inoculum ratios (S/I) aided by liquor supplementation was studied in batch mode. Results demonstrated that the liquor supplementation reinvigorated the failed or inhibited SS-AD processes giving methane yields as high as 260.3 ± 5 mL CH4/g VS and improved process stability evidenced low VFA accumulation (˂1 g/L). The achieved methane yield was 85% of the biochemical methane potential achieved under unlimiting batch assay. This study, therefore, revealed that the methane yield in an SS-AD of lignocellulosic biomass can be significantly boosted at high loads by simply adding recycled water or sludge supernatant especially in an inhibited SS-AD process.

Biohythane production from Chlorella sp. biomass by two-stage thermophilic solid-state anaerobic digestion

Two-stage thermophilic solid state anaerobic digestion (SS-AD) processes for biohythane production from Chlorella sp. biomass was investigated. The hydrogen and methane yield from fresh Chlorella sp. biomass was 124.9 mL H2 gVS−1 and 230.1 mL CH4 gVS−1, respectively, at 16% TS with biodegradation efficiency of 34%. Hydrogen and methane production from hydrothermal pretreated Chlorella sp. biomass was 190.0 mL H2 gVS−1 and 319.8 mL CH4 gVS−1, respectively at 16% TS with biodegradation efficiency of 47%. Pretreatment of Chlorella sp. biomass with hydrothermal follow by two-stage thermophilic SS-AD improved 150% biogas production comparing with fresh biomass. Clostridium sp. was the main bacteria responsible for hydrogen production. Clostridium sp. and Ruminococcus sp. was main bacteria in methane stage, while Methanoculleus sp. and Methanothermobacter sp. was dominated archaea in methane production process. Chlorella sp. biomass with hydrothermal pretreatment has great potential for hydrogen and methane production via two-stage thermophilic SS-AD.

Sequential batch thermophilic solid-state anaerobic digestion of lignocellulosic biomass via recirculating digestate as inoculum – Part II: Microbial diversity and succession

This study aimed to investigate the effect of recirculation of digestate as inoculum on the microbial communities in thermophilic solid-state anaerobic digestion (SS-AD) of yard trimmings. The SS-AD consisted of 4 consecutive runs (30days/run), with digestate from the previous run being used as the inoculum of the subsequent run. Bacterial and archaeal communities (day 0, 4, 8, 12, 20, and 30) were examined using Illumina sequencing of 16S rRNA genes.The results revealed substantial microbial succession toward increased diversity until run 3. The proportions of Firmicutes that contained cellulolytic bacteria doubled, which might explain the concomitantly increased cellulose degradation and volatile fatty acids (VFAs). Clostridia and Thermotogae appeared to correlate with VFAs. The VFA accumulation likely induced dynamic shifts of methanogens, particularly to hydrogenotrophic Methanothermobacter, implying that non-acetoclastic oxidative pathway dominated during the steady-state thermophilic SS-AD. This study suggested that recirculating SS-AD digestate might be an effective way for inoculation.

Sequential batch thermophilic solid-state anaerobic digestion of lignocellulosic biomass via recirculating digestate as inoculum – Part I: Reactor performance

Sequential batch thermophilic (55°C) solid-state anaerobic digestion (SS-AD) of yard trimmings was evaluated at a total solids (TS) content of 22% via recirculating digestate as the inoculum. The substrate-to-inoculum (S/I) ratio of 1 (TS basis) was favored over 2 and 3 due to significantly higher methane yield and volumetric productivity. At an S/I ratio of 1, sequential batch SS-AD gradually reached steady state by 3 runs (30days/run) with increases in both methane yields (up to 11.5%) and cellulose degradation (up to 55%), indicating that recirculated digestate could be a feasible inoculum to establish long term stable SS-AD of lignocellulosic biomass. The initial sharp increases of volatile fatty acids during runs 2–4 indicated faster hydrolysis of organic matter than during run 1, suggesting that microbes were probably more acclimated due to digestate recirculation. At steady state, 51% (w/w) of the digestate was recirculated as the inoculum.

Thermophilic solid-state anaerobic digestion of solid waste residues from palm oil mill industry for biogas production

Thermophilic solid-state anaerobic digestion (SS-AD) of biomass from palm oil mill industry, including empty fruit bunches (EFB), oil palm fronds (OPF) and oil palm trunks (OPT) were investigated under different feedstock-to-inoculum ratios (F:I), carbon-to-nitrogen ratios (C:N) and total solids contents (TS). The optimal condition for biogas production was achieved at F:I ratio of 2:1, C:N ratio of 40:1 and TS content of 16%. The highest methane yield from EFB, OPF and OPT were 223, 207 and 161m3 CH4 tonne−1 VS, respectively. Bacterial community structure in thermophilic SS-AD of oil palm biomass was dominated by Thermoanaerobacterium sp. and Clostridium sp. Archaeal community structure was dominated by Methanosaeta sp. and Methanoculleus sp. The highest methane production of 45m3 CH4 tonne−1 fresh biomass was achieved from EFB. Solid waste residues from palm oil mill industry have great potential for methane production via thermophilic SS-AD.

Effect of Feedstock Components on Thermophilic Solid-State Anaerobic Digestion of Yard Trimmings

Solid-state anaerobic digestion of yard trimmings, composed of wood chips, lawn grass, and maple leaves, was conducted at 22% total solids at 55 °C for 45 days. Results showed digestion of mixed feedstocks generated earlier peaks and more methane than digestion of single components. The favorable peaking time (14 days) and methane yield (143 L/kg of VS) were achieved with equal fractions of the three components, increasing the methane yield by 80–200% compared to digestion of single components. Concentrations of volatile fatty acids and ammonia varied with component ratios and correlated with system performance. While organic components in mixtures were degraded differently, they were mostly degraded within 24 days and agreed with the methane yield. A mixture design model was established to predict the methane yield, and results showed that all interactions were synergistic, with the ternary interaction having the strongest effect. The model was verified using experimental data, which showed good agreement.

Thermophilic Solid-State Anaerobic Digestion of Alkaline-Pretreated Corn Stover

Solid-state anaerobic digestion (SS-AD) of corn stover (CS) was conducted at mesophilic (37 °C) and thermophilic (50 °C) conditions with/without solid-state NaOH pretreatment. Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to help with the understanding of the changes of the physicochemical structure of pretreated CS. Additionally, thermophilic SS-AD of pretreated CS and chicken manure (CM) was investigated. First-order, Cone, and modified Gompertz models were used to evaluate the methane production. Results showed that solid-state NaOH pretreatment could partly dissolve lignin and hemicelluloses and significantly increase the internal surface area of CS, which could make it more readily biodegradable. At a substrate/inoculum (S/I) ratio of 3, mesophilic SS-AD of CS failed because of the accumulation of organic acids. However, under thermophilic SS-AD conditions, biogas and methane yields of pretreated CS were found to be 386.3 and 194.8 mL g–1 of VSadded, respectively, which were 29.4 and 40.1% higher than those of untreated CS. Thermophilic solid-state co-digestion of pretreated CS with CM showed no beneficial effects for enhancing the biogas and methane yields, because of the accumulation of volatile fatty acids and ammonia. Methane production could well be explained by the Cone and modified Gompertz models compared to the first-order model, because higher R2 values were obtained. On the basis of the results of the Cone model, the first-order rate constant (k) of thermophilic SS-AD of CS and CM decreased from 0.132 to 0.039 day–1 with the increasing portion of CM. Through the modified Gompertz model, a higher lag phase time (λ) and lower maximal methane production rate (μm) were found, with the content of CM increasing under thermophilic solid-state co-digestion conditions. These results collectively suggested that thermophilic SS-AD of pretreated CS could be a promising way to produce biogas in the future.

Reactor performance and microbial community dynamics during solid-state anaerobic digestion of corn stover at mesophilic and thermophilic conditions

Reactor performance and microbial community dynamics were investigated during solid state anaerobic digestion (SS-AD) of corn stover at mesophilic and thermophilic conditions. Thermophilic SS-AD led to faster and greater reductions of cellulose and hemicelluloses during the first 12days compared to mesophilic SS-AD. However, accumulation of volatile fatty acids (VFAs) was 5-fold higher at thermophilic than mesophilic temperatures, resulting in a large pH drop during days 6–12 in the thermophilic reactors. Culture-based enumeration revealed 10–50 times greater populations of cellulolytic and xylanolytic microbes during thermophilic SS-AD than mesophilic SS-AD. DGGE analysis of PCR amplified 16S rRNA genes showed dynamic shifts, especially during the thermophilic SS-AD, of bacterial and archaeal communities over the 38days of SS-AD as a result of acclimation of the initial seed microbial consortia to the lignocellulosic feedstock. The findings of this study can guide future studies to improve efficiency and stability of SS-AD.