As a lignocellulose-based substrate for anaerobic digestion, rice straw is characterized by low density, high water absorbability, and poor fluidity. Its mixing performances in digestion are completely different from traditional substrates such as animal manures. Computational fluid dynamics (CFD) simulation was employed to investigate mixing performances and determine suitable stirring parameters for efficient biogas production from rice straw. The results from CFD simulation were applied in the anaerobic digestion tests to further investigate their reliability. The results indicated that the mixing performances could be improved by triple impellers with pitched blade, and complete mixing was easily achieved at the stirring rate of 80 rpm, as compared to 20–60 rpm. However, mixing could not be significantly improved when the stirring rate was further increased from 80 to 160 rpm. The simulation results agreed well with the experimental results. The determined mixing parameters could achieve the highest biogas yield of 370 mL (g TS)−1 (729 mL (g TSdigested)−1) and 431 mL (g TS)−1 (632 mL (g TSdigested)−1) with the shortest technical digestion time (T80) of 46 days. The results obtained in this work could provide useful guides for the design and operation of biogas plants using rice straw as substrates.

Anaerobic co-digestion of cattle manure with NaOH-treated corn stover for biogas production was investigated. Four cattle manure to corn stover ratios (CM/CS) of 1:1, 1:2, 1:3, and 1:4, and three feeding concentrations (FC) of 50, 65, and 80 g L−1 were used. The results showed that the co-digestion with CM/CS ratio of 1:3 obtained the highest total biogas production of 20.5 L, methane yield of 194 mL CH4 g−1 VS, and TS and VS reductions of 45.0% and 53.0% at the FC of 65 g L−1. Therefore, the FC of 65 g L−1 and CM/CS ratio of 1:3 was selected as the optimal one. Compared to the single digestion, 4.9−7.4% more biogas productions were obtained at the same FC of 65 g L−1 due to the synergistic effect. The synergistic effect is mainly attributed to more balanced nutrients and increased buffering capacity. It indicated that co-digestion of cattle manure with NaOH-treated corn stover could be one of the options for efficient biogas production and waste treatment.

Kitchen waste (KW), cattle manure (CM), and the mixture of KW and CM were anaerobically digested. The performances of single digestion with KW or CM and of codigestion with KW and CM were investigated and compared. Two loading rates of 10 and 20 g volatile solid (VS) L−1 were used for KW, CM, and their mixture digestion, respectively. NaOH was used as supplementary for KW, and sulfuric acid pretreatment was used for CM to explore the effects of alkalinity and acidification on methane production to verify the roles of codigestion. Scanning electron microscopy (SEM) was used to analyze the structural changes of CM fibers. The results showed that the codigestion of KW and CM increased methane yield by 44% as compared to the single digestion of KW, and the increase could be attributed to the synergistic effect in the codigestion process. A 32% more methane yield was achieved for the KW with NaOH addition than raw KW, which was due to increased alkalinity and buffering capacity. The methane yield and VS reduction for acid-pretreated CM were 116 and 74% higher than raw CM. SEM analysis showed that the structural changes of CM fibers were helpful for methane production. The results showed that codigestion could obtain better and stable performances and might be one of many options for efficient biogas production.

This study was conducted to investigate the changes of main compositions and extractives and their effects on biogas yield enhancement. Four NaOH doses (4%, 6%, 8%, and 10%) and four loading rates (35, 50, 65, and 80 g/L) were used. The rice straw was first pretreated by NaOH in solid-state conditions and anaerobically digested. The main compositions and extractives were then analyzed. The results showed that, compared to the untreated rice straw, 3.2%−58.1% more biogas yields were obtained with 4%−10% NaOH-treated rice straws. Hemicellulose, cellulose, and lignin were decomposed by 35.2%−54.2%, 14.2%−16.4%, and 8.0%−44.5%, respectively, for 4%, 6%, 8%, and 10% NaOH-treated rice straws. Considerable fractions of them were converted to relatively readily biodegradable substances, as indicated by increases of 80.3%−173.6% cold-water extractives and 80.4%−152.8% hot-water extractives. Some irresistible substances were removed, as represented by a 30.9%−51.8% decrease of benzene−ethanol extractives. The chemical structures of hot-water and benzene−ethanol extractives were also changed obviously. It was also found that the soluble sugar contents in the 6% NaOH-treated rice straw were twice that of the untreated one. The results specified that NaOH pretreatment was one of efficient methods to enhance biogas production of rice straw, and the changes of main compositions and extractives made important contributions to the enhancement.

The biogas yield of rice straw during anaerobic digestion can be substantially increased through solid-state sodium hydroxide (NaOH) pretreatment. This study was conducted to explore the mechanisms of biogas yield enhancement. The chemical compositions of the pretreated rice straw were first analyzed. Fourier transform infrared (FTIR), hydrogen-1 nuclear magnetic resonance spectroscopy ( 1H NMR), X-ray diffraction (XRD), and gas permeation chromatography (GPC) were then used to investigate the changes of chemical structures and physical characteristics of lignin, hemicellulose, and cellulose. The results showed that the biogas yield of 6% NaOH-treated rice straw was increased by 27.3−64.5%. The enhancement of the biogas yield was attributed to the improvement of biodegradability of the rice straw through NaOH pretreatment. Degradation of 16.4% cellulose, 36.8% hemicellulose, and 28.4% lignin was observed, while water-soluble substances were increased by 122.5%. The ester bond of lignin−carbohydrate complexes (LCCs) was destroyed through the hydrolysis reaction, releasing more cellulose for biogas production. The linkages of interunits and the functional groups of lignin, cellulose, and hemicellulose were either broken down or destroyed, leading to significant changes of chemical structures. The original lignin with a large molecular weight and three-dimensional network structure became one with a small molecular weight and linear structure after NaOH pretreatment. The cellulosic crystal style was not obviously changed, but the crystallinity of cellulose increased. The changes of chemical compositions, chemical structures, and physical characteristics made rice straw become more available and biodegradable and thus were responsible for the enhancement of the biogas yield.

Sodium hydroxide (NaOH) was used to pretreat corn stover in solid state at ambient temperature to improve biodegradability and anaerobic biogas production. Four NaOH doses of 4%, 6%, 8%, and 10% on dry matter basis of corn stover were applied. The untreated and NaOH-treated corn stovers were then anaerobically digested at four loading rates of 35, 50, 65, and 80 g L −1, respectively. The biodegradability and the changes of the main compositions of the corn stovers were analyzed. The results showed that 6% NaOH-treated corn stover digested at the loading rate of 65 g L −1 achieved 48.5% more biogas production and 71.0% more bioenergy gain, as compared to the untreated corn stover. Therefore, the NaOH dose and the loading rate were determined as the optimal parameters for pretreatment and anaerobic digestion, respectively. Under the optimal conditions, the total solids (TS) and volatile solids (VS) reductions and the biogas production based on TS digested (B/TS d) were increased by 23.6%, 56.3%, and 20.1%, respectively. The digestion time DT 90, defined as the time reaching 90% total biogas production, was shortened by 24.1%. The results indicated remarkably improved biodegradability of the NaOH-treated corn stover. The improved biodegradability made more substrate available to be digested and resulted in the increase of biogas production. The main compositions of the NaOH-treated corn stovers were changed obviously. The contents of lignin, cellulose, and hemicellulose decreased by 4.3%−39.2%, but the hot-water extractives increased by 64.8%. Both changes had positive effects on the biodegradability improvement. The results from this study proved that NaOH pretreatment was one of efficient methods to improve biodegradability and enhance biogas production of corn stover.

Reverse osmosis system with the disc-tube module (DT-RO) was applied to treat landfill leachate on full scale at the Changshengqiao Sanitary Landfill, Chongqing City, China. In the first six-mouth operation phase, the treatment performance of DT-RO system had been excellent and stable. The removal rate of chemical oxygen demand (COD), total organic carbon (TOC), electrical conductivity (EC), and ammonia nitrogen (NH3-N) reached 99.2–99.7%, 99.2%, 99.6%, and over 98%, respectively. The rejection of Ca2+, Ba2+, and Mg2+ was over 99.9%, respectively. Suspended solid (SS) was not detected in product water. Effective methods had been adopted to control membrane fouling, of which chemical cleaning is of utmost importance to guarantee the long smooth operation of the DT-RO system. The DT-RO system is cleaned in turns with Cleaner A and Cleaner C. At present, the 1st stage cleaning cycle by Cleaner A and Cleaner C is conducted every 100 and 500 h, respectively, depending on raw the water quality.

Aqueous ammonia was used to pretreat wheat straw to improve biodegradability and provide nitrogen source for enhancing biogas production. Three doses of ammonia (2%, 4%, and 6%, dry matter) and three moisture contents (30%, 60%, and 80%, dry matter) were applied to pretreat wheat straw for 7 days. The pretreated wheat straws were anaerobically digested at three loading rates (50, 65, and 80 g·L−1) to produce biogas. The results indicated that the wheat straw pretreated with 80% moisture content and 4% ammonia achieved the highest methane yield of 199.7 ml·g−1 (based on per unit volatile solids loaded), with shorter digestion time (T80) of 25 days at the loading rate of 65 g·L−1 compared to untreated one. The main chemical compositions of wheat straw were also analyzed. The cellulose and hemicellulose contents were decomposed by 2%-20% and 26%-42%, respectively, while the lignin content was hardly removed, cold-water and hot-water extracts were increased by 4%-44%, and 12%-52%, respectively, for the ammonia-pretreated wheat straws at different moisture contents. The appropriate C/N ratio and decomposition of original chemical compositions into relatively readily biodegradable substances will improve the biodegradability and biogas yield.

Wheat straw biodegradability during anaerobic digestion was improved by treatment with potassium hydroxide (KOH) to decrease digestion time and enhance biomethane production and fertility value. KOH concentrations of 1% (K1), 3% (K2), 6% (K3) and 9% (K4) were tested for wheat straw pretreatment at ambient temperature with a C:N ratio of 25:1. 86% of total solids (TS), 89% of volatile solids (VS) and 22% of lignocellulose, cellulose and hemicellulose (LCH) (22%) were decomposed effectively with the wheat straw pretreated by 6% KOH. Enhanced biogas production and cumulative biomethane yield of 258 ml·(g VS)− 1 were obtained increased by 45% and 41% respectively, compared with untreated wheat straw. Pretreated wheat straw digestion also yielded a digestate with higher fertilizer values potassium (138%), calcium (22%) and magnesium (16%). These results show that TS, VS and LCH can be effectively removed from wheat straw pretreated with KOH, improving biodegradability biomethane production and fertilizer value.Improvement on wheat straw biodegradability during AD was investigated by using potassium hydroxide (KOH) to decrease digestion time and enhanced biomethane yield and fertility value. 86% of TS, 89% of VS and 22% of LCH were decomposed effectively in wheat straw pretreated with 6% KOH. The total biogas production and cumulative biomethane yield was enhanced by 45% and 41% respectively, compared with untreated wheat straw. Digestion of pretreated wheat straw also yielded a digestate with higher fertilizer value, potassium (138%), calcium (22%) and magnesium (16%).Download full-size image

•Serial configurations for anaerobic digestion of corn stover were conducted.•Methane yield obtained from serial systems was 8.3–14.6% higher than single system.•Serial systems improve substance conversion and more stable in process performance.•HRT30 + 10 d serial system shows best methane production and conversions.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 40 days, and distribution of HRT are 10 + 30 days (HRT10 + 30 d), 20 + 20 days (HRT20 + 20 d), and 30 + 10 days (HRT30 + 10 d) were compared to a conventional one-step CSTR at the same HRT of 40 d. The results showed that in HRT10 + 30 d serial system, the process became very unstable at organic load of 50 gTS·L−1. The HRT20 + 20 d and HRT30 + 10 d serial systems improved methane production by 8.3–14.6% compared to the one-step system in all loads of 50, 70, 90 gTS·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 + 10 d system showed the best biogas production and conversions among all systems.

•Three lipases were applied to hydrolyze the floatable grease in the food waste.•Animal fat and vegetable oil are applied as substrates for anaerobic digestion.•Three lipids are hydrolyzed in the conditions of 24 h, 1000–1500 μL and 40–50 °C.•The digestion time was shortened by 10–40 d for three lipids treated by lipases.•The biomethane production rate of the hydrolysis products was enhanced 26.9–157.7%.Three lipases were applied to hydrolyze the floatable grease (FG) in the food waste for eliminating FG inhibition and enhancing digestion performance in anaerobic process. Lipase-I, Lipase-II, and Lipase-III obtained from different sources were used. Animal fat (AF) and vegetable oil (VO) are major crude lipids in Chinese food waste, therefore, applied as substrates for anaerobic digestion tests. The results showed that Lipase-I and Lipase-II were capable of obviously releasing long chain fatty acid in AF, VO, and FG when hydrolyzed in the conditions of 24 h, 1000–1500 μL and 40–50 °C. Compared to the untreated controls, the biomethane production rate were increased by 80.8–157.7%, 26.9–53.8%, and 37.0–40.7% for AF, VO, and FG, respectively, and the digestion time was shortened by 10–40 d. The finding suggests that pretreating lipids with appropriate lipase could be one of effective methods for enhancing anaerobic digestion of food waste rich in crude lipid.

•Digestibility of different ammonia additions and moisture contents investigated.•Changes of main compositions before and after ammonia pretreatment explored.•Pretreatment time and VS conversion rate were studied in batch tests.The effect of ammonia pretreatment on the anaerobic digestibility of corn stover was investigated. Corn stover with different moisture contents (30%, 50%, 70%, and 90%) was pretreated with three concentrations of ammonia (2%, 4%, and 6%) at 35 ± 2 °C for the following batch digestion. Results showed that the reagent of 4% ammonia and 70% moisture content could achieve the highest anaerobic digestibility. In comparison with the untreated, the time needed to produce 90% of the maximum digester gas production (T90) shortened from 52 d to 37 d. The total biogas production and the unit volatile solids (VS) biogas yield were 20,740 ml and 427.1 ml respectively, both 26.70% higher than the untreated. It was found that the digesters with high moisture contents of 70% and 90% were more stable and had shorter acidification periods relative to the low moisture contents of 30% and 50%. The decreases in cellulose, hemicelluloses and lignin indicated that ammonia pretreatment could destroy the lignocellulose (LCH) structure and furthermore enhance the biogas production. Following anaerobic digestion, 80.6% of cellulose and 68.52% of hemicelluloses were consumed where there was 4% ammonia and 70% moisture content, indicating why these conditions produced the highest level of biogas.

•Co-digestion of fruit/vegetable waste (FVW) and kitchen waste (KW) were studied.•FVW to KW ratio of 5:8 was suitable for co-digestion and methane production.•Two-phase digestion showed better buffer ability to high organic loading rate (OLR).•Two-phase digestion was more stable as OLR ⩽ 3.0 g (VS) L−1 d−1 in pilot-scale system.•It was profitable for pilot-scale food waste digestion at OLR of 3.0 g (VS) L−1 d−1.The anaerobic digestion performances of kitchen waste (KW) and fruit/vegetable waste (FVW) were investigated for establishing engineering digestion system. The study was conducted from lab-scale to pilot-scale, including batch, single-phase and two-phase experiments. The lab-scale experimental results showed that the ratio of FVW to KW at 5:8 presented higher methane productivity (0.725 L CH4/g VS), and thereby was recommended. Two-phase digestion appeared to have higher treatment capacity and better buffer ability for high organic loading rate (OLR) (up to 5.0 g (VS) L−1 d−1), compared with the low OLR of 3.5 g (VS) L−1 d−1 for single-phase system. For two-phase digestion, the pilot-scale system showed similar performances to those of lab-scale one, except slightly lower maximum OLR of 4.5 g (VS) L−1 d−1 was allowed. The pilot-scale system proved to be profitable with a net profit of 10.173 $/ton as higher OLR (⩾3.0 g (VS) L−1 d−1) was used.

CO2 removal from biogas by water washing system was investigated with various parameters, including liquid/gas ratio, pressure, temperature, and CO2 content. The results indicate that CO2 removal ratio could reach 34.6%–94.2% as liquid/gas ratio increased from 0.14 to 0.50. Increasing pressure (from 0.8 to 1.2 MPa) could improve gas purification with a constant inflow rate of gas. Temperature played a key role in the process and lower temperature in absorption tower was beneficial for reducing CO2 content. CO2 removal ratio could reach 24.4%–83.2% when CO2 content in the simulated gas was 25%–45%. The lowest CO2 content after absorption was 2.6% at 1.2 MPa with 400 L·h-1 gas flow and 200 L·h-1 water flow, which meets the requirement of CO2 content in natural gas for vehicle fuel.CO2 removal from biogas by a pilot water washing system was investigated at various parameters, including liquid/gas ratio, pressure, temperature and initial CO2 content. Water was used as the absorbent, which could be recycled in the process through the regeneration. At certain conditions, a large amount of CO2 (25%–45%) in biogas could be decreased to less than 3%, which meets the requirement of CO2 content in natural gas for vehicle fuel.Download full-size image

Ignition-assisting agents for densified corn stover briquette fuel (DCBF) were developed, and their ignition and emission characteristics were investigated using type LLA-6 household cooking stove. Three waste liquid fuels, waste engine oil (E), diesel oil (D), and industrial alcohol (A), were used as raw materials to make 25 ignition-assisting agents by mixing at different ratios. Their ignition performance was evaluated in terms of ignition time and cost. It was found that ignition-assisting agents ED15 (a mix of E and D at volume ratio of 1?5) and DA51 (a mix of D and A at volume ratio of 5?1) presented better ignition results with shorter ignition time (40-53 s) and lower cost (6.1 and 5.3 cents) at the dosages of 9 ml and 8 ml, respectively. The emission of O2, CO, CO2, NOx, and SO2, the temperature in fume gas, and combustion efficiency were investigated for ED15 and DA51. The results show that the emission of ED15 with the dosage of 9 ml is lower than that of DA51 with the dosage of 8 ml in the ignition process. ED15 at the dosage of 9 ml achieves satisfactory combustion efficiency and emits less pollutant, so it is recommended for practical application. The study will provide a cost-effective and environmentally friendly approach to fast ignite DCBF and break the barrier to the practical application of DCBF.