The impact of increasing organic loading in two phase digestion of food waste

This paper examines the impact of increasing organic loading in a two phase anaerobic digestion system treating commercial food waste. The first phase is a series of sequentially fed leach bed reactors (LBRs). The second phase is an upflow anaerobic sludge bed (UASB). Leachate from the leach beds, form the influent to the UASB. Effluent from the UASB is re-circulated over the leach beds. Flow rates corresponded to 1 volume of leachate per effective LBR volume per day. The theoretical organic loading rate (OLR) of the UASB is based on the conversion of volatile solids (VS) in the LBR to chemical oxygen demand (COD). The experiment was set up such that the theoretical OLR would rise from 7.1 to 8.8 to 11.8 kg COD m⁻³ day⁻¹.The system operated effectively at the lowest organic loading rate producing 384 L CH₄ kg VS⁻¹ which corresponded to 72% of the value obtained in a BMP test. COD conversion efficiency was recorded at 75%. The accumulation of COD over the life of the experiment led to a situation whereby the volumetric OLR (product of COD concentration in the leachate by the flow rate) was over twice the theoretical OLR at the end of the experiment (24.3 kg VS m⁻³ day⁻¹ versus 11.8 kg VS m⁻³ day⁻¹). At the highest loading rate total ammonia nitrogen (TAN) reached levels of 4500 mg L⁻¹ with pH levels of 8.15. This resulted in significant reduction of methane production.     

Thermophilic hydrogen and methane production from sugarcane stillage in two-stage anaerobic fluidized bed reactors

This study evaluated the feasibility of H₂ and CH₄ production in two-stage thermophilic (55 °C) anaerobic digestion of sugarcane stillage (5,000 to 10,000 mg COD.L⁻¹) using an acidogenic anaerobic fluidized bed reactor (AFBR-A) with a hydraulic retention time (HRT) of 4 h and a methanogenic AFBR (AFBR-S) with HRTs of 24 h–10 h. To compare two-stage digestion with single-stage digestion, a third methanogenic reactor (AFBR-M) with a HRT of 24 h was fed with increasing stillage concentrations (5,000 to 10,000 mg COD.L⁻¹). The AFBR-M produced a methane content of 68.4 ± 7.2%, a maximum yield of 0.30 ± 0.04 L CH₄.g COD⁻¹, a production rate of 3.78 ± 0.40 L CH₄.day⁻¹.L⁻¹ and a COD removal of 73.2 ± 5.0% at an organic loading rate (OLR) of 7.5 kg COD.m⁻³.day⁻¹. In contrast, the two-stage AFBR-A system produced a hydrogen content of 23.9 ± 5.6%, a production rate of 1.30 ± 0.16 L H₂.day⁻¹.L⁻¹ and a yield of 0.34 ± 0.08 mmol H₂.g CODap⁻¹. Additionally, the decrease in the HRT from 18 h to 10 h in the AFBR-S favored a higher methane production, improving the maximum methane content (74.5 ± 6.0%), production rate (5.57 ± 0.38 L CH₄.day⁻¹.L⁻¹) and yield (0.26 ± 0.06 L CH₄.g COD⁻¹) at an OLR of 21.6 kg COD.m⁻³.day⁻¹ (HRT of 10 h) with a total COD removal of 70.1 ± 7.1%. Under the applied COD of 10,000 mg L⁻¹, the two-stage system showed a 52.8% higher energy yield than the single-stage anaerobic digestion system. These results show that, relative to a single-stage system, two-stage anaerobic digestion systems produce more hydrogen and methane while achieving similar treatment efficiencies.     

Response surface design to study the influence of inoculum,particle size and inoculum to substrate ratio on the methane production from Ulex sp.

Ulex europaeus is one of the world worst invaders vegetal species and its suitability for biogas production is significant. The effect of three factors affecting the Biochemical Methane Potential (BMP, expressed as volume of CH₄ per mass of volatile solids of waste) and the biodegradability rate (k, expressed in volume of CH₄ per mass of VS and time) of U. europaeus was assessed by a Central Composite Face Centred Design. The BMP varied from 153 L kg⁻¹ to 308 L kg⁻¹. Inoculum to substrate ratio (ISR) and the type of inoculum had high influence on the final results. k varied from 14 L kg⁻¹ d⁻¹ to 49 L kg⁻¹ d⁻¹. The conditions that simultaneously maximized the BMP and k were an inoculum consisting in 55% (v) of granular sludge and 45 % (v) of suspended sludge from a sludge digester, an ISR of 4 g g⁻¹, and a particle size of 1.9 mm. Considering the average biomass production in shrub land areas, the potential energy production from U. europaeus is estimated in (36.9 ± 19.3) GJ ha⁻¹ yr⁻¹. For example, in Europe, a maximum energy supply of 7 EJ yr⁻¹ could be achieved from potentially harvestable shrub land areas.     

Improvement of biohydrogen production from brewery wastewater: Evaluation of inocula,support and reactor

This work explores the production of biohydrogen from brewery wastewater using as inoculum a culture produced by natural fermentation of synthetic wastewater and Klebsiella pneumoniae isolated from the environment. Klebsiella pneumoniae showed good performance as inoculum, as evaluated using assays of between 9 and 16 cycles, with durations of 12 and 24 h, carbohydrate concentrations from 2.79 to 7.22 g L⁻¹, and applied volumetric organic loads from 2.6 to 12.6 g carbohydrate L⁻¹ day⁻¹. The best results were achieved with applied volumetric organic loads of 12.6 g carbohydrate L⁻¹ day⁻¹ and cycle length of 12 h, resulting in mean volumetric productivity of 0.88 L H₂ L⁻¹ day⁻¹, maximum molar flow of 10.80 mmol H₂ h⁻¹, and mean yield of 0.70 mol H₂ mol⁻¹ glucose consumed. The biogas H₂ content was between 18 and 42%, while the mean organic compounds removal and carbohydrate conversion efficiencies were 23 and 81%, respectively. The inoculum produced by natural fermentation was not viable.     

Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm³) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH₄ and H₂S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH₄ potential than dairy manure on a volatile solids basis. While no synergistic effect on CH₄ production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH₄ production. Initial H₂S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H₂S concentrations (0.10–0.14%) and higher CH₄ content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH₄ yield (372 ± 12 L kg⁻¹ VS). The co-digestion mixture containing 40% radish had a lower specific CH₄ yield (345 ± 2 L kg⁻¹ VS) but also showed significantly less H₂S production at start-up and high quality biogas (58% CH₄). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH₄ production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH₄ production of manure digesters in the fall/winter.     

Hydrogen–methane production from swine manure: Effect of pretreatment and VFAs accumulation on gas yield

A two-phase anaerobic process to produce hydrogen and methane from swine manure was investigated, using pretreated sludge with heat, acid and alkali treatment as inoculum. The relative order of pretreatment methods of H₂ productivity effectiveness and CH₄ productivity effectiveness produced by the residua of the first phase was heat treatment > alkali treatment > acid treatment. When the inoculum sludge was heat-treated at 80°C for 30 min, the H₂ and CH₄ production rate was the highest of 36.6, 201.7 ml (g TS)_added⁻¹. There were significant correlations between biogas production and accumulation of acetic acid and butyric acids. When propionic acid and total VFA concentrations reached about 2850 mg L⁻¹ and 10.0 g L⁻¹, respectively, the average H₂ production rate and H₂ content decreased from 7.6 ml d⁻¹(g VS)_added⁻¹ and 55.3% to 1.4 ml d⁻¹(g VS)_added⁻¹ and 43.2%, respectively. The activity of methanogenic bacteria was inhibited to a significant extent when the total VFA concentration was above 10.0 g L⁻¹, but this inhibitory effect weakened when the VFA concentration fell to 6200–8500 mg L⁻¹. Correspondingly, average CH₄ production rate increased from 4.0 ml d⁻¹(g TS)_added⁻¹ to 12.5 ml d⁻¹(g TS)_added⁻¹. Propionic acid was degraded rapidly only when acetic and butyric acid concentrations dropped to 2500 mg L⁻¹ and 1000 mg L⁻¹, respectively.     

Transient performance during start-up of a two-phase anaerobic digestion process demonstration unit treating carbohydrate-rich waste with biochar addition

The paper reports an experimental investigation into the transient performance during the start-up of a pilot-scale two-phase anaerobic digestion (TPAD) process demonstration unit (PDU) treating food waste with biochar addition. Hydrogen (H₂) was produced in the first phase (R1) and methane (CH₄) was produced in the second phase (R2). A fed-batch operation strategy was applied to the start-up of both phases, followed by semi-continuous operation. Production rates and yields of H₂ and CH₄ and volatile fatty acids (VFA) were measured while the pH and temperature were monitored throughout the process. Fed-batch operation allowed microbe enrichment and gradual VFA production in both phases, which was observed to be efficient in starting up the TPAD PDU. Under semi-continuous operation, R1 produced biogas with composition up to 49% of H₂ and at a yield of 46 L H₂.kg ⁻¹ VS. CH₄ composition and yield reached up to 59% and 301 L CH₄.kg⁻¹ VS in the R2.     

Techno-economic anaerobic co-digestion feasibility study for two-phase olive oil mill pomace and pig slurry

Anaerobic mono- (AmoD) and co-digestion (AcoD) of two-phase olive oil mill pomace (TPOP) and pig slurry (PS) at different ratios were studied in a semi-continuous stirred tank reactor (sCSTR) at mesophilic temperature (37 °C). The methane yields for AcoD experiments ranged from 150.9 to 274.3 L CH₄ kg VS⁻¹ d⁻¹. The maximum chemical oxygen demand (COD) removal rate (59.60%) was achieved for TPOP/PS 80:20 w/w substrate composition at OLR 3.68 g VS L_reactor⁻¹ d⁻¹ and 24 days HRT. Despite the energy production from food waste is not a new process, the industrial implementation and application continue to be a challenge. Hence, the present paper also reports on the economic feasibility study of a full scale anaerobic co-digestion plant to treat 7000 t year⁻¹ and 1750 t year⁻¹ of TPOP and PS, respectively. The results of this study are compared against a conventional PS mono-digestion plant. Net present values (NPV) and payback time periods (PBT) were reported to be 782,493 € and 135,701 € and 6.7 and 9.2 years for AcoD and AmoD plants, respectively. Finally, the sensibility analysis concluded that AcoD configuration is less dependent on energy and compost sale prices variations.     

A bench scale study of fermentative hydrogen and methane production from food waste in integrated two-stage process

A two-stage fermentation process combining hydrogen and methane production for the treatment of food waste was investigated in this paper. In hydrogen fermentation reactor, the indigenous mixed microbial cultures contained in food waste were used for hydrogen production. No foreign inoculum was used in the hydrogen fermentation stage, the traditional heat treatment of inoculum was not applied either in this bench scale experiment. The effects of the stepwise increased organic loading rate (OLR) and solid retention time (SRT) on integrated two-stage process were investigated. At steady state, the optimal OLR and SRT for the integrated two-stage process were found to be 22.65 kg VS/m³ d (160 h) for hydrogen fermentation reactor and 4.61 (26.67 d) for methane fermentation reactor, respectively. Under the optimum conditions, the maximum yields of hydrogen (0.065 m³ H₂/kg VS) and methane (0.546 m³ CH₄/kg VS) were achieved with the hydrogen and methane contents ranging from 29.42 to 30.86%, 64.33 to 71.48%, respectively. Biodegradability analysis showed that 5.78% of the influent COD was converted to the hydrogen in H₂-SCRD and 82.18% of the influent COD was converted to the methane in CH₄-SCSTR under the optimum conditions.     

Biohydrogen and biomethane production from cassava wastewater in a two-stage anaerobic sequencing batch biofilm reactor

The objective of the present study was to determine the energetic potential from cassava starch wastewater in a two-stage system (BioH₂ + BioCH₄) composed by anaerobic sequencing batch biofilm reactors (AnSBBR). Included in this general objective, the behavior of the methanogenic AnSBBR regarding organic matter removal and biomethane production will be investigated. The acidogenic AnSBBR was operated with organic loading rate (OLR) of 14 gCarb.L⁻¹.d⁻¹, influent concentration of 5 gCarb.L⁻¹ and cycle time of 4 h. The methanogenic AnSBBR was submitted to OLR increase (3.7–12 gCOD.L⁻¹.d⁻¹), provided by arrangements between influent concentration (2.8; 4.0 and 6.0 gCOD.L⁻¹) and cycle time (6; 8 and 12 h). For the evaluated condition, the acidogenic reactor presented productivity of 0.7 LH₂.L⁻¹.d⁻¹ and yield of 1.1 molH₂.kg⁻¹Carb. The methanogenic reactor presented stable methane production (%CH₄ > 78) during the 260-days operating period. The maximum methane productivity (2.71 LCH₄.L⁻¹.d⁻¹) and yield (0.263 LCH₄.g⁻¹COD) were obtained at OLR of 12 gCOD.L⁻¹.d⁻¹ and cycle time of 6 h. The estimated energy production rate in the two-stage system (BioH₂ + BioCH₄) was 105.2 kJ.L⁻¹.d⁻¹.     

Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing

Anaerobic treatment of solid wastes from potato processing was studied in completely stirred tank reactors (CSTR) at 55 °C. Special attention was paid to the effect of increased organic loading rate (OLR) on the biogas yield in long-term experiments. Both biogas yield and CH₄ in the biogas decreased with the increase in OLR. For OLR in the range of 0.8 gl⁻¹ d⁻¹–3.4 gl⁻¹ d⁻¹, biogas yield and CH₄ obtained were 0.85 l g⁻¹–0.65 l g⁻¹ and 58%–50%, respectively. Biogas yield y as a function of maximum biogas yield y_m, reaction rate constant k and HRT are described on the basis of a mass balance in a CSTR and a first order kinetic. The value of y_m can be obtained from curve fitting or a simple batch test and k results from plotting y/(y_m−y) against 1/OLR from long-term experiments. In the present study values for y_m and k were obtained as 0.88 l g⁻¹ and 0.089 d⁻¹, respectively. The simple model equations can apply for dimensioning completely stirred tank reactors (CSTR) digesting organic wastes from food processing industries, animal waste slurries or biogas crops.     

Anaerobic digestion of grain stillage at high organic loading rates in three different reactor systems

In this study the anaerobic digestion of grain stillage in three different reactor systems (continuous stirred tank reactor, anaerobic sequencing batch reactor, fixed bed reactor) with and without immobilization of microorganisms was investigated to evaluate the performance during increase of the organic loading rate (OLR) from 1 to 10 g of volatile solids (VS) per liter reactor volume and day and decrease of the hydraulic retention time (HRT) from 40 to 6 days. No significant differences have been observed between the performances of the three examined reactor systems. The changes in OLR and HRT caused a reduction of the specific biogas production (SBP) of about 25% from about 650 to 550 L kg⁻¹ of VS but would also diminish the necessary digester volume and investment costs of about 75% compared to the state of the art.     

High rate low solids methane fermentation of sorghum, corn and cellulose

Sorghum, sorghum/alpha-cellulose mixture, and corn were anaerobically digested at 55°C at effluent solids contents of 8–12% total solids (TS), using trace nutrient supplementation. Volatile solids (VS) loading rates at much higher levels than conventional maxima were maintained without volatile fatty acid (VFA) accumulation. Semi-continuously fed digesters with organic loading rates (OLR) up to 12 gVS kg⁻¹ d⁻¹ produced methane at rates up to 3.3 L kg⁻¹d⁻¹. Continuous feeding of corn at an OLR of 18 gVS kg⁻¹ d⁻¹ resulted in a methane production rate of 5.4 L kg⁻¹d⁻¹. VS removal efficiencies at maximum OLRs were 60% (sorghum) and 67% (corn). At an OLR of 4 gVS kg⁻¹ d⁻¹ sorghum alone as a feedstock led to excess ammonia-N accumulation. Excess ammonia did not accumulate at sorghum loading rates of 8 and 12 gVS kg⁻¹ d⁻¹ nor with a sorghum/alpha-cellulose mix loaded at 8 gVS kg⁻¹ d⁻¹. Instantaneous gas production rates were directly related to feedstock cell soluble content, with peak instantaneous biogas production rates from corn (OLR of 8 gVS kg⁻¹ d⁻¹ approaching 25 L kg⁻¹ d⁻¹ following a three-day feeding.     

Effects of the reduction of the hydraulic retention time to 1.5 days at constant organic loading in CSTR,ASBR, and fixed-bed reactors – Performance and methanogenic community composition

The hydraulic retention time (HRT) is one of the key parameters in biogas processes and often it is postulated that a minimum HRT of 10–25 days is obligatory in continuous stirred tank reactors (CSTR) to prevent a washout of slow growing methanogens. In this study the effects of the reduction of the HRT from 6 to 1.5 days on performance and methanogenic community composition in different systems with and without immobilization operated with simulated thin stillage (STS) at mesophilic conditions and constant organic loading rates (OLR) of 10 g L⁻¹d⁻¹ of volatile solids were investigated. With the reduction of the HRT process instability was first observed in the anaerobic sequencing batch reactor (ASBR) (at HRT of 3 days) followed by the CSTR (at HRT of 2 days). The fixed bed reactor (FBR) was stable until the end of the experiment, but the reduction of the HRT to 1.5 days caused a decrease of the specific biogas production to about 450 L kg⁻¹ of VS compared to about 600 L kg⁻¹ of VS at HRTs of 4–5 days. Methanoculleus and Methanosarcina were the dominant genera under stable process conditions in the CSTR and the ASBR and members of Methanosaeta and Methanospirillum were only present at HRT of 4 days and lower. In the effluent of the FBR Methanosarcina spp. were not detected and Methanosaeta spp. were more abundant then in the other reactors.     

Electrochemical behavior of polyamides with cyclic disulfide structure and their application to positive active material for lithium secondary battery

Polyamides (DTA-I, DTA-II, and DTA-III) containing cyclic disulfide structure were prepared by condensation between 1,2-dithiane-3,6-dicarboxylic acid (DTA) and alkyl diamine, NH₂–(CH₂)_n–NH₂ (DTA-I; n=4, DTA-II; n=6, DTA-III; n=8) and their application to positive active material for lithium secondary batteries was investigated. Cyclic voltammetry (CV) measurements under slow sweep rate (0.5 mV s⁻¹) with a carbon paste electrode containing the polyamide (DTA-I, DTA-II, or DTA-III) were performed. The results indicated that the polyamides were electroactive in the organic electrolyte solution (propylene carbonate (PC)-1,2-dimethoxyethane (DME), 1:1 by volume containing lithium salt, such as LiClO₄). The responses based on the redox of the disulfide bonds in the polyamide were observed.Test cells, Li/PC-DME (1:1. by volume) with 1 mol dm⁻³ LiClO₄/the polyamide cathode, were constructed and their performance was tested under constant current charge/discharge condition. The average capacity of the test cells with the DTA-III cathode was 64.3 Ah kg⁻¹ of cathode (135 Wh kg⁻¹ of cathode, capacity (Ah kg⁻¹) of the cathode×average cell voltage (2.10 V)). Performance of the cell with linear polyamide containing disulfide bond (–CO–(CH₂)₂–S–S–(CH₂)₂–CONH–(CH₂)₈–NH–, GTA-III) was also investigated and the average capacity was 56.8 Ah kg⁻¹ of cathode (100 Wh kg⁻¹ of cathode, capacity (Ah kg⁻¹) of the cathode×average cell voltage (1.76 V)). Cycle efficiency of the test cell with the DTA-III cathode was higher than that with the GTA-III cathode.     

Mild alkaline pre-treatments loosen fibre structure enhancing methane production from biomass crops and residues

Three ligno-cellulosic substrates representing varying levels of biodegradability (giant reed, GR; fibre sorghum, FS; barley straw, BS) were combined with mild alkaline pre-treatments (NaOH 0.05, 0.10 and 0.15 N at 25 °C for 24 h) plus untreated controls, to study pre-treatment effects on physical-chemical structure, anaerobic digestibility and methane output of the three substrates. In a batch anaerobic digestion (AD) assay (58 days; 35 °C; 4 g VS l⁻¹), the most recalcitrant substrate (GR) staged the highest increase in cumulative methane yield: +30% with NaOH 0.15 N over 190 ml CH₄ g⁻¹ VS in untreated GR. Conversely, the least recalcitrant substrate (FS) exhibited the lowest gain (+10% over 248 ml CH₄ g⁻¹ VS), while an intermediate behaviour was shown by BS (+15% over 232 ml CH₄ g⁻¹ VS). Pre-treatments speeded AD kinetics and reduced technical digestion time (i.e., the time needed to achieve 80% methane potential), which are the premises for increased production capacity of full scale AD plants. Fibre components (cellulose, hemicellulose and acid insoluble lignin determined after acid hydrolysis) and substrate structure (Fourier transform infra-red spectroscopy and scanning electron microscopy) outlined reductions of the three fibre components after pre-treatments, supporting claims of loosened binding of lignin with cellulose and hemicellulose. Hence, mild alkaline pre-treatments were shown to improve the biodegradability of ligno-cellulosic substrates to an extent proportional to their recalcitrance. In turn, this contributes to mitigate the food vs. fuel controversy raised by the use of whole plant cereals (namely, maize) as feedstocks for biogas production.     

Biomethane Production from co-fermentation of agricultural wastes

Biomethane (CH₄) was recovered from co-digestion process of waste glycerol and banana wastes. The wastes used contain waste glycerol with varying concentrations from 7.5 to 90 g L⁻¹ and banana peel in the range 2.5–10 %w·v⁻¹. The co-substrate mixture ratio was implemented in 0.5 L batch reactor operated at 37 °C and pH 7 for 120 h. The composition of biogas gas and liquid samples (COD, VFA, pH, alkalinity) were analyzed every 12 and 24 h, respectively. The optimum condition to produce CH₄ was found at 7.5 g L⁻¹ waste glycerol mixed with 7.5% banana peel. The highest CH₄ yield and CH₄ production potential were 0.281 m³ kg⁻¹ COD and 652 mL, respectively.     

Mesophilic and thermophilic co-fermentation of cattle excreta and olive mill wastes in pilot anaerobic digesters

Cattle excreta and two-phase olive mill wastes (TPOMW) were codigested at a 3:1 ratio in two 75 L continuous stirred tank reactors at 37 °C and 55 °C to analyse their biogas production. The contribution of each residue to the total gas production at 37 °C was evaluated in reactors digesting either 3:1 excreta:water or 3:1 water:TPOMW. The mesophilic co-fermentation of cattle excreta with TPOMW at an organic loading rate (OLR) of 5.5 g COD L^?1 d^?1 rendered 1096 mL biogas L^?1 sludge d^?1. This was 337% higher than that of excreta alone. The methane yield resulting from the codigestion was 179 L CH₄ kg^?1 VS loaded, of which 42% was attributed to the quarter of the reactor corresponding to TPOMW. Under thermophilic conditions, the codigestion yielded 17.3% more methane than mesophilically. In the reactor digesting TPOMW alone (OLR = 3.8 g COD L^?1 d^?1) the ratio VFA/alkalinity exceeded 0.8 after 21 d, leading to its acidification and inhibition of methanogenesis. Farm-scale digestion of animal excreta and TPOMW should be promoted in Mediterranean countries as an environmentally sound option for waste recycling and renewable energy production.     

Enhanced substrate degradation and methane yield with maleic acid pre-treatments in biomass crops and residues

Organic acids are envisaged as alternative catalysts to strong mineral acids, in pre-treatment of ligno-cellulosic biomass for anaerobic digestion (AD). To evaluate this hypothesis, an untreated control and four pre-treatments (25 °C for 24 h) involving two levels of maleic acid (34.8 and 69.6 kg m⁻³), alone and combined with sulphuric acid (4 kg m⁻³), were studied in three agricultural substrates: Arundo (aka giant reed), Barley straw and B133 fibre sorghum. Methane production was assessed in a batch AD assay (35 °C for 51 days) with 4 g L⁻¹ of volatile solid (VS) load. Fibre composition and structure were investigated through chemical analysis and Fourier transform infrared (FTIR) spectrometry. Arundo and B133 that were the most and least recalcitrant substrate, respectively, staged the highest and lowest increase in methane with high maleic acid: +62% over 218 cm³ g⁻¹ of VS in untreated Arundo; +36% over 284 cm³ g⁻¹ of VS in untreated B133. Barley straw showed an intermediate behaviour (+41% over 269 cm³ g⁻¹ of VS). H₂SO₄ addition to maleic acid did not improve CH₄ output. The large increase in methane yield determined by pre-treatments was reflected in the concurrent decrease of fibre (between 14 and 39% depending on fibrous component). Based on FTIR spectra, bands assigned to hemicellulose and cellulose displayed lower absorbance after pre-treatment, supporting the hypothesis of solubilisation of structural carbohydrates and change in fibre structure. Hence, maleic acid was shown a suitable catalyst to improve biodegradability of ligno-cellulosic biomass, especially in recalcitrant substrates as Arundo.     

Associated effects of storage and mechanical pre-treatments of microalgae biomass on biomethane yields in anaerobic digestion

The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH₄) yields during anaerobic digestion. This study investigated the combined effects of two different biomass storage methods and physical pre-treatments on the anaerobic digestion for three different microalgae species. Acutodesmus obliquus, Chlorella vulgaris and Chlorella emersonii were cultivated in 80 L sleevebag photobioreactors (batch mode), and then subjected to different storage (cooling and freezing) and pre-treatment methods prior to anaerobic digestion using the biochemical methane potential (BMP) test. A. obliquus was selected to evaluate pre-treatment methods for further experimentation. Significantly higher CH₄ yields of cooled (4 °C) A. obliquus biomass were achieved through ultrasonication (+53% CH₄) and wet-milling (+51% CH₄). These methods were then applied in follow-up experiments to cooled (4 °C) biomass of C. emersonii and A. obliquus. Ultrasonication again led to significantly higher CH₄ yields for A. obliquus biomass (323 dm³ kg⁻¹ CH₄ yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH₄), and C. emersonii biomass (308 dm³ kg⁻¹; +35% CH₄). In a third experiment series, frozen A. obliquus and C. vulgaris biomass were thawed prior to pre-treatment and BMP-testing. Among all BMP tests, the highest CH₄ yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm³ kg⁻¹); pre-treatment did not enhance CH₄ yields for C. vulgaris, but for A. obliquus (ultrasonication +20%). Pre-treatment was more effective for cooled than freeze-thawed microalgal biomass and combined effects acted strain dependently.