Leach Bed Reactor Research Articles

Evaluation of Semi-dry Mesophilic Anaerobic Co-digestion of Corn Stover and Vegetable Waste by a Single-Phase Process

The performance of semi-dry anaerobic co-digestion of corn stover and vegetable waste was investigated using batch reactors and leaching bed reactors (LBRs). Six mixing ratios of corn stover and vegetable waste were employed to investigate optimal methane production using batch reactors, and process stability was assessed using the two LBRs. Kinetic assessment, methane production, volatile solid (VS) reduction, and digestate characteristics were used to evaluate the performance of anaerobic co-digestion. After 50 days of anaerobic co-digestion in the batch reactor, methane yields reached 314.5–323.4 mL/g VS, and increase in methane production rate indicated that synergistic effects occurred during anaerobic co-digestion. The corn stover and vegetable waste with a total solids (TS) ratio of 14:1 yielded the highest methane production efficiency and rate, as well as VS removal efficiency. However, serious acidification at the initial stage of digestion in the LBR experiments suggests that the scaling-up process from batch reactor to LBR was unstable, which may be attributed to high TS concentration and substrate characteristics.

Effect of macro- and micro-nutrients addition during anaerobic mono-digestion of grass silage in leach-bed reactors

ABSTRACT The effect of macro- (NH4Cl) (set I) and micro-nutrients (Fe, Ni, Co and Mo) (set II) addition on chemical oxygen demand (COD) solubilisation during anaerobic mono-digestion of grass silage was investigated in two sets of leach bed reactor experiments at 35°C. Results showed that addition of NH4Cl and micro-nutrients improved COD solubilisation by 18% (0.56 g SCOD g−1 volatile solids) and 7% (0.45 g SCOD g−1 VS), respectively than control. About 20–50% of the added micro-nutrients were bioavailable in the produced leachates, while the rest (50–80%) were adsorbed onto the grass silage. Results of biological methane potential assays showed that, specific methane yields of grass silage were improved by 17% (0.36 ± 0.02 m3 CH4 kg−1 VSadded) when NH4Cl was supplemented while Fe, Ni, Co and Mo addition improved methane yields by 15% (0.33 ± 0.005 m3 CH4 kg−1 VSadded) when compared to control.

Control of lactic acid production during hydrolysis and acidogenesis of food waste

Lactate accumulation occurs frequently during the hydrolysis and acidogenesis of food waste and produces an unfavorable substrate for anaerobic digestion. The objective of the present study was to reduce lactic acid production during the hydrolysis and acidogenesis of food waste in leachate bed reactor for establishment of the two-phase anaerobic digestion system. The results showed that the hydrolysis and acidogenesis of food waste in batch feeding mode underwent two consecutive stages, namely lactic acid fermentation and mixed acid fermentation. In the lactic acid fermentation stage, lactate constituted 74.4–96.8% of the total organic acids in the leachate. However in semi-continuous mode the content of lactate in the leachate could be reduced less than 0–2% for leach bed reactors operated at feeding loads of 50–150g/d although lactate accumulation occurred at a feeding load of 200g/d. Furthermore the organic acid shifted to acetate and butyrate, providing ideal substrates for anaerobic digestion.

Influence of acidogenic headspace pressure on methane production under schematic of diversion of acidogenic off-gas to methanogenic reactor

This study investigated the effects of 12.6psi (T1), 6.3psi (T2), 3.3psi (T3) and ambient (T4) headspace pressure on the metabolic pathways in the acidogenic leach bed reactor (LBR) and overall methane recovery during two-phase anaerobic digestion of food waste. Diversion of biogas from LBR enhanced COD and soluble product generation in T2, T3 and T4 whereas, high pressure (T1) resulted in comparatively higher lactate production and low protein degradation. A pressure of 3–6psi (T2 and T3) improved the production of COD by ∼22–36%, soluble products by ∼9–43%, volatile solid reduction by ∼14–19%, and CH4 production by ∼10–31% compared with control. Besides, ∼3–6psi headspace pressure positively influenced the composition of soluble products resulting in enhanced methane recovery adding advantage to the two-phase system. A headspace pressure of ∼3–6psi is recommended to enhance the hydrolysis-acidogenesis; however, the actual hydrogen concentration should be considered.

Acidogenic digestion of food waste in a thermophilic leach bed reactor: Effect of pH and leachate recirculation rate on hydrolysis and volatile fatty acid production

The effect of pH control (4, 5, 6, 7) on volatile fatty acids (VFA) production from food waste was investigated in a leach bed reactor (LBR) operated at 50°C. Stabilisation of pH at 7 resulted in hydrolysis yield of 530g soluble chemical oxygen demand (sCOD)/kg total volatile solids (TVS) added and VFA yield of 247gCOD/kg TVS added, which were highest among all pH tested. Butyric acid dominated the VFA mix (49–54%) at pH of 7 and 6, while acetate composed the primary VFA (41–56%) at pH of 4 and 5. A metabolic shift towards lactic acid production was observed at pH of 5. Improving leachate recirculation rate further improved the hydrolysis and degradation efficiency by 10–16% and the acidification yield to 340gCOD/kgTVS added. The butyric acid concentration of 16.8g/L obtained at neutral pH conditions is among the highest reported in literature.

Assessment of Solute Transfer Between Static and Dynamic Water During Percolation Through a Solid Leach Bed in Dry Batch Anaerobic Digestion Processes

The aim of this work was to characterize solute transfer between static and dynamic water during percolation through a solid leach bed reactor. A new experimental procedure was set up to measure the solute exchange rate between macro-and micro-porosity. Tracer tests were performed in closed-circuit recirculation experiments. The water behavior was modeled by a multiphase flow model in a double porosity medium using a previously published methodology. The solute exchange rate between static and dynamic water was described by first-order kinetics. The methodology was applied to wheat straw and solid cow manure beds. The solute exchange rate (hs was 0.054 and 0.324 h−1) for wheat straw and solid cow manure, respectively. The measured data was used to improve the prediction capacity of a CFD tool. The results of this work could be used to develop appropriate leachate recirculation strategies to optimize full-scale dry batch anaerobic digestion processes.

Conversion of vegetable wastes to organic acids in leaching bed reactor: Performance and bacterial community analysis

Vegetable wastes (VWs), which contained abundant solid content, were digested in a leaching bed reactor (LBR). The tested VWs for acidification in the LBR included cabbage, potato, and tomato vine. The digestion in the LBR was carried out at the same organic loading rate of 25g volatile solid (VS) per liter. It was found that cabbage and potato produced much more volatile fatty acids (VFAs) than tomato vine, probably because readily degradable components were more abundant in cabbage and potato. The retention time to digest each feedstock was different. It took 4, 5, and 8 days for cabbage, potato, and tomato vine to completely disappear in the reactor, respectively. The profiles of the VFAs generated from the three VWs were examined. In all cases, the predominant VFAs were acetic acid and butyric acid, although the relative abundance of individual VFA varied across the tested leachate samples. The bacterial community compositions of the leachates were analyzed by high throughput sequencing, and it was found that the feedstock strongly affected the bacterial community structure in the acidogenic process. All leachates had distinct bacterial community structure, although they did share a common set of core communities that included Proteobacteria, Firmicutes, and Bacteroidetes.

Comparison of the mesophilic and thermophilic anaerobic digestion of spent cow bedding in leach-bed reactors

Anaerobic digestion of spent cow bedding in batch leach-bed reactors (LBRs) was compared in mesophilic and thermophilic conditions for the first time. Results show that the use of thermophilic conditions enhanced only the degradation kinetics of easily-degradable matter during the first days of the digestion, whereas similar methane yields (80% of the Biomethane Potential) were reached after 42days at both temperatures. Therefore, the anaerobic digestion in LBRs of spent cow bedding, a substrate rich in slowly-degradable compounds, was not improved in term of methane production considering the overall digestion time. Moreover, the high initial biogas production rate in thermophilic reactors was found to significantly reduce the energetic performance of the cogeneration unit at industrial scale, leading to a 5.9% decrease in the annual electricity production when compared to a mesophilic one.

Leachate flush strategies for managing volatile fatty acids accumulation in leach-bed reactors

In anaerobic leach-bed reactors (LBRs) co-digesting an easily- and a slowly-degradable substrate, the importance of the leachate flush both on extracting volatile fatty acids (VFAs) at the beginning of newly-started batches and on their consumption in mature reactors was tested. Regarding VFA extraction three leachate flush-rate conditions were studied: 0.5, 1 and 2Lkg−1TSd−1. Results showed that increasing the leachate flush-rate during the acidification phase is essential to increase degradation kinetics. After this initial phase, leachate injection is less important and the flush-rate could be reduced. The injection in mature reactors of leachate with an acetic acid concentration of 5 or 10gL−1 showed that for an optimized VFA consumption in LBRs, VFAs should be provided straight after the methane production peak in order to profit from a higher methanogenic activity, and every 6–7h to maintain a high biogas production rate.

Evaluation of the anaerobic digestion of kitchen waste by thermal pretreatment in a batch leach bed reactor with down flow and the kinetics of methane yields

ABSTRACTThe aim of this study was to evaluate the effects of autoclave and microwave irradiation (MW) pretreatments on the solubilization, biogas and methane yields of kitchen waste (KW). Autoclave (for 30 min @ 120, 1 bar) and MW (for 400 s @ 80, 700 W) pretreatments were used to enhance anaerobic digestion of KW. The anaerobic digestion of KW in autoclave and MW pretreatments were compared with a control in a batch leach bed reactor with down flow (LBR DF). During autoclave and MW pretreatments, the biogas yields were respectively 25.41 and 41.43% higher than the control test and for methane were respectively 22.73 and 38.91% higher than the control test. The highest solubilization efficiency was obtained after MW pretreatment (89.21%), followed by after autoclave pretreatment (85.73%), and the lowest value was obtained with control (61.37%). Mathematical models fitted experimental data with R2>0.9798. The present study shows that autoclave and MW pretreatments can be used to improve biogas and methane yields and solubilization efficiency. Schnute and modified Gompertz models gave a good fit of the first stage of fermentation as well as the asymptotic phase.

Mesophilic anaerobic digestion of several types of spent livestock bedding in a batch leach-bed reactor: substrate characterization and process performance

Spent animal bedding is a valuable resource for green energy production in rural areas. The properties of six types of spent bedding collected from deep-litter stables, housing either sheeps, goats, horses or cows, were compared and their anaerobic digestion in a batch Leach-Bed Reactor (LBR) was assessed. Spent horse bedding, when compared to all the other types, appeared to differ the most due to a greater amount of straw added to the litter and a more frequent litter change. Total solids content appeared to vary significantly from one bedding type to another, with consequent impact on the methane produced from the raw substrate. However, all the types of spent bedding had similar VS/TS (82.3–88.9)%, a C/N well-suited to anaerobic digestion (20–28, except that of the horse, 42) and their BMPs were in a narrow range (192–239NmLCH4/gVS). The anaerobic digestion in each LBR was stable and the pH always remained higher than 6.6 regardless of the type of bedding. In contrast to all the other substrates, spent goat bedding showed a stronger acidification resulting in a methane production lag phase. Finally, spent bedding of different origins reached, on average, (89±11)% of their BMP after 60days of operation. This means that this waste is well-suited for treatment in LBRs and that this is a promising process to recover energy from dry agricultural waste.

Comparison of Reactor Configurations for Biogas Production from Rapeseed Straw

To investigate the effects of reactor configurations on the anaerobic digestion performance of agricultural residue rapeseed straw, semi-continuous, one-stage, continuously stirred tank reactors (CSTR) and batch, two-stage, leach bed-upflow anaerobic sludge blanket (UASB) reactors were operated at the same hydraulic retention time (HRT) (30 days) and target organic loading rate (OLR) (3.0 gVS/L.d). In the continuously loaded CSTR, the specific methane yields did not substantially change as the OLR increased from 1.5 gVS/L.d to 3.0 gVS/L.d during the four periods. Conversely, the specific methane yields in the batch-fed leach bed-UASB increased considerably with the increase in OLR. The leach bed reactor contributed 75% of the total yield, while the UASB reactor contributed only 25% of the total yield. The total specific methane yields were 108 mL/gVS and 160 mL/gVS for the leach bed-UASB and CSTR, respectively, while the volatile solid (VS) reductions of the rapeseed straw were 27.1% and 36.6%, respectively. The results indicated that the process performance was more efficient in the CSTR than in the leach bed-UASB for the digestion of rapeseed straw. Biogas production was clearly affected by the reactor configurations.

Effect of lime loading on the performance of simultaneous lime treatment and dry anaerobic digestion of smooth cordgrass

ABSTRACTThe effects of lime loading on simultaneous lime treatment and dry digestion (SLTDD) of smooth cordgrass (SC) were evaluated at 35°C by batch reactors and leaching bed reactors (LBRs). Biogas yields of 1.5%, 3%, and 5% (w/w) lime loadings decreased by 7.1%, 20%, and 75.7%, respectively, compared with no-lime treatment with 198.0 mL/g total solids (TS) by batch reactors. The LBRs with liquid recycling and pH adjustment enhanced biogas production with 148.1–236.1 mL/g TS. The inhibition occurred SLTDD may be ascribed to be high pH and temperature from lime hydration at the initial stage. The activity for methanogenic bacteria was more inhibited than other anaerobic bacteria.

Enhanced anaerobic digestion performance via combined solids- and leachate-based hydrolysis reactor inoculation

Suboptimal conditions in anaerobic digesters (e.g., presence of common inhibitors ammonia and salinity) limit waste hydrolysis and lead to unstable performance and process failures. Application of inhibitor-tolerant inocula improves hydrolysis, but approaches are needed to establish and maintain these desired waste-hydrolyzing bacteria in high-solids reactors. Herein, performance was compared for leach bed reactors (LBRs) seeded with unacclimated or acclimated inoculum (0–60% by mass) at start-up and over long-term operation. High quantities of inoculum (∼60%) increase waste hydrolysis and are beneficial at start-up or when inhibitors are increasing. After start-up (∼112days) with high inoculum quantities, leachate recirculation leads to accumulation of inhibitor-tolerant hydrolyzing bacteria in leachate. During long-term operation, low inoculum quantities (∼10%) effectively increase waste hydrolysis relative to without solids-derived inoculum. Molecular analyses indicated that combining digested solids with leachate-based inoculum doubles quantities of Bacteria contacting waste over a batch and supplies additional desirable phylotypes Bacteriodes and Clostridia.

Start-up of a sequential dry anaerobic digestion of paunch under psychrophilic and mesophilic temperatures

The present laboratory study evaluated the sequential leach bed dry anaerobic digestion (DAD) of paunch under psychrophilic (22°C) and mesophilic (40°C) temperatures. Three leach bed reactors were operated under the mesophilic temperature in sequence at a solid retention time (SRT) of 40d with a new batch started 27d into the run of the previous one. A total of six batches were operated for 135d. The results showed that the mesophilic DAD of paunch was efficient, reaching methane yields of 126.9–212.1mLg−1 volatile solid (VS) and a VS reduction of 32.9–55.5%. The average daily methane production rate increased from 334.3mLd−1 to 571.4mLd−1 and 825.7mLd−1 when one, two and three leach bed reactors were in operation, respectively. The psychrophilic DAD of paunch was operated under a SRT of 100d and a total of three batches were performed in sequence for 300d with each batch starting after completion of the previous one. Improvements in the methane yield from 93.9 to 107.3 and 148.3mLg−1 VS and VS reductions of 24.8, 30.2 and 38.6% were obtained in the consecutive runs, indicating the adaptation of anaerobic microbes from mesophilic to psychrophilic temperatures. In addition, it took three runs for anaerobic microbes to reduce the volatile fatty acid accumulation observed in the first and second trials. This study demonstrates the potential of renewable energy recovery from paunch under psychrophilic and mesophilic temperatures.

Innovative method for increased methane recovery from two-phase anaerobic digestion of food waste through reutilization of acidogenic off-gas in methanogenic reactor

In this study, the performance of a two-phase anaerobic digestion reactor treating food waste with the reutilization of acidogenic off-gas was investigated with the objective to improve the hydrogen availability for the methanogenic reactor. As a comparison a treatment without off-gas reutilization was also set up. Results showed that acidogenic off-gas utilization in the upflow anaerobic sludge blanket (UASB) reactor increased the methane recovery up to 38.6%. In addition, a 27% increase in the production of cumulative chemical oxygen demand (COD) together with an improved soluble microbial products recovery dominated by butyrate was observed in the acidogenic leach bed reactor (LBR) with off-gas reutilization. Of the increased methane recovery, ∼8% was contributed by the utilization of acidogenic off-gas in UASB. Results indicated that utilization of acidogenic off-gas in methanogenic reactor is a viable technique for improving overall methane recovery.

ORGANIC ACID PRODUCTION FROM THE ORGANIC FRACTION OF MUNICIPAL SOLID WASTE AND COW MANURE IN LEACHING BED REACTORS

This study was carried out to (1) investigate the potential of in-vessel solid-state anaerobic acidogenic co-digestion of high solids containing substrates, namely organic fraction of municipal solid waste (OFMSW) and cow manure and (2) examine the feasibility of using leaching bed reactors (LBR) for hydrolysis/liquefaction and acidification of OFMSW and cow manure mixtures for maximum total volatile fatty acid (VFA) and alcohol production. Results indicated that hydrolysis efficiency of 2025% of was achieved in the LBR containing 100 % OFMSW and 75 % OFMSW and 25% manure. The hydrolysis was mainly realized in the initial 20 days and the acid production was observed in the initial 25-30 day period. The maximum total VFA concentration (in terms of acetic acid) achieved was around 6500 mg/L. The main VFA were found as acetic acid and butyric acid, whereas the main alcohol produced was found as ethanol. The LBR were found as efficient reactors in terms of rapid hydrolysis and acidification of the feedstocks investigated. The produced VFA can be used as a raw material in many industrial sectors after concentration or as a feedstock for bioenergy (mainly biogas and biohydrogen) production.

Biotic and abiotic roles of leachate recirculation in batch mode solid-state anaerobic digestion of cattle manure

Solid state anaerobic digestion, with leachate recirculation, is suitable for exploiting manure with a high solid content. The biotic and abiotic effects of the leachates were studied in lab-scale leach bed reactors (LBRs). LBRs were fed with cow manure and four leachates either biologically active or inert. The biotic impact of leachate was assessed by monitoring the microbial communities in the manure and in the leachates. LBRs with biologically active leachates, regardless to their origin, produced equivalent methane volumes (114.52±19.05 and 99.79±6.4NL/kgVS) while LBRs with inert leachates produced half less methane (60.22±5.71 and 58.87±13.2NL/kgVS) attesting to the biotic role of leachate. Moreover, its beneficial abiotic role is mainly due to its initial nutrient content, pH, and buffering capacity. The microbial community in the manure was strongly involved in methane production, and no transfer of microorganisms from the liquid phase was found (p<0.05).

Stabilization of fine fraction from landfill mining in anaerobic and aerobic laboratory leach bed reactors

Fine fraction (FF, <20mm) from mined landfill was stabilized in four laboratory-scale leach bed reactors (LBR) over 180days. The aim was to study feasibility of biotechnological methods to treat FF and if further stabilization of FF is possible. Four different stabilization methods were compared and their effects upon quality of FF were evaluated. Also during the stabilization experiment, leachate quality as well as gas composition and quantity were analyzed. The methods studied included three anaerobic LBRs (one without water addition, one with water addition, and one with leachate recirculation) and one aerobic LBR (with water addition). During the experiment, the most methane was produced in anaerobic LBR without water addition (18.0LCH4/kgVS), while water addition and leachate recirculation depressed methane production slightly, to 16.1 and 16.4LCH4/kgVS, respectively. Organic matter was also removed via the leachate and was measured as chemical oxygen demand (COD). Calculated removal of organic matter in gas and leachate was highest in LBR with water addition (59gCOD/kgVS), compared with LBR without water addition or with leachate recirculation (51gCOD/kgVS). Concentrations of COD, ammonium nitrogen and anions in leachate decreased during the experiment, indicating washout mechanism caused by water additions. Aeration increased sulfate and nitrate concentrations in leachate due to oxidized sulfide and ammonium. Molecular weight distributions of leachates showed that all the size categories decreased, especially low molecular weight compounds, which were reduced the most. Aerobic stabilization resulted in the lowest final VS/TS (13.1%), lowest respiration activity (0.9–1.2mgO2/gTS), and lowest methane production after treatment (0.0–0.8LCH4/kgVS), with 29% of VS being removed from FF. Anaerobic stabilization methods also reduced organic matter by 9–20% compared with the initial amount. Stabilization reduced the quantity of soluble nitrogen in FF and did not alter concentration of soluble and insoluble phosphorus, and insoluble nitrogen. All four stabilization methods decreased organic matter and thus are possible stabilization methods for FF, but aerobic treatment was the most efficient in this study.

Anaerobic Leaching-Bed Reactor Treating Food Waste for Organic Acid Production: Effect of Bulking Agent

Food waste was a troublesome organic waste stream, but a highly desirable substrate for anaerobic digestion to recover energy. Among the diverse reactor configurations, the leaching-bed reactor was reported to be best choice to treating the particular organic waste for platform compounds (volatile fatty acid). In this study, we carried out a series of experiments to investigate the bulking agent and pH control on process performance, and special focus was put on the biodegradability of bulking agent. The supplement of bulking agent greatly improved the leaching rate by 3.6 folds. Although the hydrolytic and acidogenic bacteria were resistant to low pH, the pH control (neutralization) caused a significantly increased volatile fatty acid (VFA) productions from 39.1 g COD/kg VSaddedto 183.4 g VFA/kg VSadded. Comparing with the undegradable bulking agent, the case with supplementing corncob as a bulking agent showed superior VFA yields (225 g COD/kg VSadded) which could be ascribed to the good adhesive properties for microorganisms and biodegradability. The ultimate analysis of the substrate (excluding the bulking agent) also showed that most of food waste was degraded with relative short reaction time. In addition, the TG/DTA and FTIR of residual corncob results indicated some components (cellulose, hemi-cellulose) were degraded, which could contribute to the additional VFA production (14 g COD/kg VSadded). This study suggested that the corncob could be considered as a good bio-compatible bulking agent for leaching-bed reactor, which not only increased the VFA productivity, but provided additional VFA production.