Fractional calculus as a generalized kinetic model for biochemical methane potential tests.

This study presents a fractional calculus model as a generalized kinetic model for estimating the maximum methane yield and degradation kinetics in biomethane potential (BMP) assays, a key analytical method in anaerobic digestion research and application. The fractional model outperformed common first-order kinetic models by yielding superior data fitting and properly managing substrate heterogeneity. The fractional model showed robust performance in mono-digestion, co-digestion and pre-treatment BMP assays with or without presence of large tailing or sigmoidal patterns in the BMP curve. The main advantage of the fractional model over other models is its ability to capture the complexities of the methane production process without losing model accuracy. Assessment of the mathematical model revealed that for fractional orders greater than 0.8 the Mittag-Leffler sequence could be transformed into a more computationally efficient exponential function.

Temperature-driven carboxylic acid production from waste activated sludge and food waste: Co-fermentation performance and microbial dynamics.

This work aims to improve the continuous co-fermentation of waste activated sludge (WAS) and food waste (FW) by investigating the long-term impact of temperature on fermentation performance and the underpinning microbial community. Acidogenic co-fermentation of WAS and FW (70:30 % VS-basis) to produce volatile fatty acids (VFA) was studied in continuous fermenters at different temperatures (25, 35, 45, 55 °C) at an organic loading rate of 11 gVS/(L·d) and a hydraulic retention time of 3.5 days. Two batches of WAS (A and B) were collected from the same wastewater treatment plant at different periods to understand the impact of the WAS microbioota on the fermenters' microbial communities. Solubilisation yield was higher at 45 °C (575 ± 68 mgCOD/gVS) followed by 55 °C (508 ± 45 mgCOD/gVS). Fermentation yield was higher at 55 °C (425 ± 28 mgCOD/gVS) followed by 35 °C (327 ± 17 mgCOD/gVS). Temperature also had a noticeable impact on the VFA profile. At 55 °C, acetic (40 %) and butyric (40 %) acid dominated, while acetic (37 %), butyric acid (31 %), and propionic acid (17 %) dominated at 35 °C. At 45 °C, an accumulation of caproic acid was detected which did not occur at other temperatures. Each temperature had a distinct microbial community, where the WAS microbiota played an important role. The biomass mass-balance showed the highest growth of microorganisms (51 %) at 35 °C and WAS_B, where a consumption of acetic acid was observed. Therefore, at 35 °C, there is a higher risk of acetic acid consumption probably due to the proliferation of methanogens imported from WAS.

Struvite precipitation in wastewater treatment plants anaerobic digestion supernatants using a magnesium oxide by-product.

Struvite precipitation is a well-known technology to recover and upcycle phosphorus from municipal wastewater as a slow-release fertiliser. However, the economic and environmental costs of struvite precipitation are constrained by using technical-grade reagents as a magnesium source. This research evaluates the feasibility of using a low-grade magnesium oxide (LG-MgO) by-product from the calcination of magnesite as a magnesium source to precipitate struvite from anaerobic digestion supernatants in wastewater treatment plants. Three distinct LG-MgOs were used in this research to capture the inherent variability of this by-product. The MgO content of the LG-MgOs varied from 42 % to 56 %, which governed the reactivity of the by-product. Experimental results showed that dosing LG-MgO at P:Mg molar ratio close to stoichiometry (i.e. 1:1 and 1:2) favoured struvite precipitation, whereas higher molar ratios (i.e. 1:4, 1:6 and 1:8) favoured calcium phosphate precipitation due to the higher calcium concentration and pH. At a P:Mg molar ratio of 1:1 and 1:2, the percentage of phosphate precipitated was 53-72 % and 89-97 %, respectively, depending on the LG-MgO reactivity. A final experiment was performed to examine the composition and morphology of the precipitate obtained under the most favourable conditions, which showed that (i) struvite was the mineral phase with the highest peaks intensity and (ii) struvite was present in two different shapes: hopper and polyhedral. Overall, this research has demonstrated that LG-MgO is an efficient source of magnesium for struvite precipitation, which fits the circular economy principles by valorising an industrial by-product, reducing the pressure on natural resources, and developing a more sustainable technology for phosphorus recovery.

Impact of food waste composition on acidogenic co-fermentation with waste activated sludge.

The impact of food waste (FW) composition on co-fermentation performance was studied to elucidate if adjusting FW composition can be used to drive the fermentation yield and profile, which is relevant for biorefinery applications. First, the impact of individual FW components (i.e., fruit, vegetables, pasta, rice, meat, fish, and cellulose) was assessed. Subsequently, the effect of mixing a protein-rich component and a carbohydrate-rich component was studied (i.e., fish/fruit and fish/cellulose, and meat/rice and meat/vegetable). All experiments were carried out in mesophilic batch assays using waste activated sludge (WAS) as main substrate, the same mixture ratio (70 % WAS +30 % FW on VS basis), and no pH control. Results showed that each FW component had a distinct effect on VFA yield and profile, with protein-rich components reaching the highest VFA yields; 502 and 442 mgCOD/gVS for WAS/Fish and WAS/Meat, respectively. A positive interaction on VFA yield was observed when mixing a protein-rich and a carbohydrate-rich component. This interaction was not proportional to the co-substrates proportion in the mixtures. On the other hand, the VFA profile was clearly driven by the components in the mixture, including both WAS and FW composition. Overall, these results indicate that predicting the VFA yield of WAS/FW co-fermentation is not just related to FW composition, but FW composition could be used to adjust the VFA profile to a certain extent.

Gamma distribution function to understand anaerobic digestion kinetics: Kinetic constants are not constant.

The Gamma model is a novel approach to characterise the complex degradation dynamics taking place during anaerobic digestion. This three parameters model results from combining the first-order kinetic model and the Gamma distribution function. In contrast to conventional models, where the kinetic constant is considered invariant, the Gamma model allows analysing the variability of the kinetic constant using a probability density function. The kinetic constant of mono-digestion and co-digestion batch tests of different wastes were modelled using the Gamma model and two common first-order models: one-step one-fraction model and one-step two-fraction model. The Gamma distribution function approximates three distinct probability density functions, i.e. exponential, log-normal, and delta Dirac. Specifically, (i) cattle paunch and pig manure approximated a log-normal distribution; (ii) cattle manure and microalgae approximated an exponential distribution, and (iii) primary sludge and cellulose approximated a delta Dirac distribution. The Gamma model was able to characterise two distinct waste activated sludge, one approximated to a log-normal distribution and the other to an exponential distribution. The same cellulose was tested with two different inocula; in both tests, the Gamma distribution function approximated a delta Dirac function but with a different kinetic value. The potential and consistency of Gamma model were also evident when analysing pig manure and microalgae co-digestion batch tests since (i) the mean k of the co-digestion tests were within the values of the mono-digestion tests, and (ii) the profile of the density function transitioned from log-normal to exponential distribution as the percentage of microalgae in the mixture increased.

Ammonia recovery from acidogenic fermentation effluents using a gas-permeable membrane contactor.

A gas-permeable membrane (GPM) contactor was used to recover ammoniacal nitrogen from a synthetic and a biowaste fermentation broth under different pH (from 6 to 11) and temperatures (35 and 55 °C). Ammonia mass transfer constant (Km) increased as pH and temperature increased. For synthetic broth, pH 10 provided the best results, when considering the Km (9.2·10-7 m·s-1) and the reagents consumption (1.0 mol NaOH·mol-1 TAN and 0.6 mol H2SO4·mol-1 TAN). Biowaste fermentation generated a broth with a high concentration of ammoniacal nitrogen (4.9 g N·L-1) and volatile fatty acids (VFA) (41.1 g COD·L-1). Experiments using the biowaste broth showed a lower Km (5.0·10-7 m·s-1 at pH 10) than the synthetic broth, related to the solution matrix and other species interference. VFAs were not detected in the trapping solution. Overall, these results show that GPM is a suitable technology to efficiently separate ammoniacal nitrogen and VFA from fermentation broths.

Potential of anaerobic co-fermentation in wastewater treatments plants: A review.

Fermentation (not anaerobic digestion) is an emerging biotechnology to transform waste into easily assimilable organic compounds such as volatile fatty acids, lactic acid and alcohols. Co-fermentation, the simultaneous fermentation of two or more waste, is an opportunity for wastewater treatment plants (WWTPs) to increase the yields of sludge mono-fermentation. Most publications have studied waste activated sludge co-fermentation with food waste or agri-industrial waste. Mixing ratio, pH and temperature are the most studied variables. The highest fermentation yields have been generally achieved in mixtures dominated by the most biodegradable substrate at circumneutral pH and mesophilic conditions. Nonetheless, most experiments have been performed in batch assays which results are driven by the capabilities of the starting microbial community and do not allow evaluating the microbial acclimation that occurs under continuous conditions. Temperature, pH, hydraulic retention time and organic load are variables that can be controlled to optimise the performance of continuous co-fermenters (i.e., favour waste hydrolysis and fermentation and limit the proliferation of methanogens). This review also discusses the integration of co-fermentation with other biotechnologies in WWTPs. Overall, this review presents a comprehensive and critical review of the achievements on co-fermentation research and lays the foundation for future research.

Nitrogen recovery from pig slurry by struvite precipitation using a low-cost magnesium oxide.

Ammonia nitrogen management is a recurrent problem in intensive livestock areas. Struvite precipitation stands as a mature technology to recover ammonia nitrogen and prevent associated environmental problems. However, the feasibility of struvite technology to recover ammonia nitrogen from pig manure is limited by the reagents cost. This research aimed to optimise the formulation of a stabilizing agent (SA) synthesised using an industrial low-grade MgO by-product (LG-MgO) and phosphoric acid for efficient TAN recovery via struvite precipitation. Experimental results showed that the H3PO4/LG-MgO ratio controls the magnesium phosphate mineral phase of the SA (bobierrite and/or newberyite). Newberyite-rich SA showed the highest TAN removal efficiency from pig manure (66-73%) compared to the SA formed by a mixture of newberyite and bobierrite (51-59%) and by bobierrite (26%). Particle size reduction of LG-MgO did not improve the SA's TAN removal efficiency, although XRD patterns showed that the precipitates from the TAN removal experiments contained some unreacted newberyite. The economic analysis showed that the higher reactivity of the SA formulated using higher H3PO4/LG-MgO ratios compensated reagent costs. The SA synthesised with a H3PO4/LG-MgO ratio of 0.98 showed the most economical treatment cost, which was estimated at 7.5 € per kg of ammonia nitrogen from pig manure. Finally, the optimum SA was successfully synthesised in a 200-L pilot plant, with a TAN removal capacity only 10% lower than the one synthesised at lab-scale.

Assessing the potential of waste activated sludge and food waste co-fermentation for carboxylic acids production.

This study investigated waste activated sludge (WAS) and food waste (FW) co-fermentation in batch assays to produce carboxylic acids. Three mixtures (50%, 70% and 90% WAS in VS basis) were studied under different conditions: with and without extra alkalinity, and with and without WAS auto-hydrolysis pre-treatment. All tests were carried out at 35 °C, without pH adjustment and without external inoculum. Experimental results showed that co-fermentation yields, including volatile fatty acids and lactic acid, were always higher than WAS and FW mono-fermentation yields (ca. 100 and 80 mgCOD/gVS, respectively). Co-fermentation yields increased as the proportion of FW in the mixture increased, indicating that the improvement was primarily due to a higher FW degradation under co-fermentation conditions. The maximum co-fermentation yield was on average 480 mgCOD/gVS for the WAS/FW_50/50 mixture. The importance of pH on co-fermentation performance was evident in the experiments carried out with extra alkalinity, which showed that the proportion of WAS in the mixture should be high enough to keep the pH above 5.0. However, fermenters operational conditions should also prevent the enrichment of acetic acid consuming microorganisms. WAS auto-hydrolysis pre-treatment did not enhance co-fermentation yields but showed minor kinetic improvements. Regarding the product profile, butyric acid was enriched as the proportion of FW in the mixture increased and the concomitant pH decreased to the detriment of propionic acid. Propionic acid prevailed under neutral pH in the WAS mono-fermentation and the WAS/FW_90/10 mixture.

Humic acid inhibition of hydrolysis and methanogenesis with different anaerobic inocula.

There is increasing evidence that humic acid (HA) is hampering the performance of anaerobic digesters treating animal manures and thermally-hydrolysed waste activated sludge. In the present study, HA inhibition and inhibition resilience was examined for hydrolysis (carbohydrate and protein) and acetotrophic methanogenesis with four distinct full-scale anaerobic inocula. The aim was to further understand HA inhibition and to explore potential relationships between microbial factors and inhibition resilience. For two of the four tested inocula, cellulose degradation showed a start-up delay that lengthened as HA concentration increased from 0 to 2 g L-1. This inhibition was reversible because, after the initial delay, subsequent hydrolysis rates and methane yields were not significantly influenced by HA concentration. Cellulose hydrolysis results at HA concentrations below 2 g L-1 support a threshold inhibition mechanism, i.e. HA complexes with hydrolytic enzymes preventing them from binding with cellulose, but once all the HA had been complexed, enzymes subsequently released are free to bind with cellulose. Inocula with higher cellulose hydrolytic activity were less affected by HA inhibition, suggesting a potential link between HA inhibition resilience and microbial activity. However, above 5 gHA L-1, cellulose hydrolysis rates decreased with increasing HA concentration; indicating that the mechanisms of inhibition may change depending on some threshold HA concentration. Protein hydrolysis and acetotrophic methanogenesis were less susceptible to HA inhibition than cellulose hydrolysis, since signs of inhibition were only observed above 5 gHA L-1. Acetotrophic methanogenesis was partially inhibited at 10 gHA L-1 and completely inhibited at 20 gHA L-1. These results further support that HA inhibition is selective towards particular enzymes.

Characterising and modelling free ammonia and ammonium inhibition in anaerobic systems.

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH3) and ammonium ion (NH4+), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH3 and NH4+ inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NH4HCO3 and NH4Cl without pH adjustment, rather than by using NH4Cl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R2 > 0.99) the joint inhibition of NH3 and NH4+ on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R2 ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KImin) and upper limit (concentration where inhibition is complete; KImax). The threshold inhibition function also identifies the 50% inhibition concentration (KI50) at the midpoint of KImin and KImax. Finally, experimental and model results show that at pH 7.3-7.7 and TAN concentrations above 2000 mgN·L-1, both NH3 and NH4+ contribute significantly to overall inhibition.

Deterministic mechanisms define the long-term anaerobic digestion microbiome and its functionality regardless of the initial microbial community.

The impact of the starting inoculum on long-term anaerobic digestion performance, process functionality and microbial community composition remains unclear. To understand the impact of starting inoculum, active microbial communities from four different full-scale anaerobic digesters were each used to inoculate four continuous lab-scale anaerobic digesters, which were operated identically for 295 days. Digesters were operated at 15 days solid retention time, an organic loading rate of 1 g COD Lr-1 d-1 (75:25 - cellulose:casein) and 37 °C. Results showed that long-term process performance, metabolic rates (hydrolytic, acetogenic, and methanogenic) and microbial community are independent of the inoculum source. Digesters process performance converged after 80 days, while metabolic rates and microbial communities converged after 120-145 days. The convergence of the different microbial communities towards a core-community proves that the deterministic factors (process operational conditions) were a stronger driver than the initial microbial community composition. Indeed, the core-community represented 72% of the relative abundance among the four digesters. Moreover, a number of positive correlations were observed between higher metabolic rates and the relative abundance of specific microbial groups. These correlations showed that both substrate consumers and suppliers trigger higher metabolic rates, expanding the knowledge of the nexus between microorganisms and functionality. Overall, these results support that deterministic factors control microbial communities in bioreactors independently of the inoculum source. Hence, it seems plausible that a desired microbial composition and functionality can be achieved by tuning process operational conditions.

Relationship between microbial community, operational factors and ammonia inhibition resilience in anaerobic digesters at low and moderate ammonia background concentrations.

The relationship between anaerobic digestion operational conditions and (i) microbial community, (ii) acetoclastic methanogenic activity and (iii) free ammonia (NH3) inhibition resilience was investigated. Thirteen inocula were obtained from full and pilot scale digesters fed with different substrates, digester configurations, operating temperatures and NH3 concentrations (0.1-241 mgN·L-1). Substrate type and temperature were the primary factors influencing microbial community composition. Methanogenic activity ranged from 0.04 to 0.14 gCOD-CH4·g-1VS·day-1, and was significantly correlated with archaeal relative abundance and archaeal community PC2. The variability of NH3 resilience among inocula was moderate, with inhibition threshold values (KI50) ranging between 32 and 175 mgNH3-N·L-1. No microbial or operational factors correlated with NH3 resilience. However, the slopes of inhibition threshold curves were influenced by some environmental factors, namely substrate type, digester temperature and NH3 concentration. Overall, these results indicate that low and moderate background NH3 concentrations is not a key determinant of microbial community nor NH3 resilience.

Biochemical methane potential (BMP) tests: Reducing test time by early parameter estimation.

Biochemical methane potential (BMP) test is a key analytical technique to assess the implementation and optimisation of anaerobic biotechnologies. However, this technique is characterised by long testing times (from 20 to >100days), which is not suitable for waste utilities, consulting companies or plants operators whose decision-making processes cannot be held for such a long time. This study develops a statistically robust mathematical strategy using sensitivity functions for early prediction of BMP first-order model parameters, i.e. methane yield (B0) and kinetic constant rate (k). The minimum testing time for early parameter estimation showed a potential correlation with the k value, where (i) slowly biodegradable substrates (k≤0.1d-1) have a minimum testing times of ≥15days, (ii) moderately biodegradable substrates (0.1

Indigenous microbial capability in solid manure residues to start-up solid-phase anaerobic digesters.

Batch solid-phase anaerobic digestion is a technology for sustainable on-farm treatment of solid residues, but is an emerging technology that is yet to be optimised with respect to start-up and inoculation. In the present study, spent bedding from two piggeries (site A and B) were batch digested at total solids (TS) concentration of 5, 10 and 20% at mesophilic (37°C) and thermophilic (55°C) temperatures, without adding an external inoculum. The results showed that the indigenous microbial community present in spent bedding was able to recover the full methane potential of the bedding (140±5 and 227±6L CH4 kgVSfed-1 for site A and B, respectively), but longer treatment times were required than for digestion with an added external inoculum. Nonetheless, at high solid loadings (i.e. TS level>10%), the digestion performance was affected by chemical inhibition due to ammonia and/or humic acid. Thermophilic temperatures did not influence digestion performance but did increase start-up failure risk. Further, inoculation of residues from the batch digestion to subsequent batch enhanced start-up and achieved full methane potential recovery of the bedding. Inoculation with liquid residue (leachate) was preferred over a solid residue, to preserve treatment capacity for fresh substrate. Overall, the study highlighted that indigenous microbial community in the solid manure residue was capable of recovering full methane potential and that solid-phase digestion was ultimately limited by chemical inhibition rather than lack of suitable microbial community.

The effect of magnesium as activator and inhibitor of anaerobic digestion.

Anaerobic digestion stands as a key technology in the emerging green energy economy. Mg(2+) has been identified as an important element to improve digesters methane production; however the inhibition risk that high Mg(2+) concentrations can cause to the AD process must also be considered when dosing Mg reagents and wastes containing Mg(2+). Despite its importance, Mg(2+) stimulation and inhibition mechanisms as well as threshold values are scarce in the literature. This research paper investigates the impact (stimulation and inhibition) of Mg(2+) on pig manure anaerobic digestion. Mathematical modelling was used to better understand the interaction between substrate, inoculum and magnesium, where Mg(2+) inhibition was modelled by a n-component non-competitive inhibition function. Modelling was done on absolute curves rather than specific methane productions curves (new approach) to account for the lower background methane production of the inoculum as the Mg(2+) concentration increased. Results showed that no stimulation or inhibition occurred between 40 (native concentration) and 400mgMg(2+)L(-1), while minor and major inhibition were observed at 750 and 1000mgMg(2+)L(-1), and at 2000 and 4000mgMg(2+)L(-1), respectively. Mg(2+) half maximal inhibition concentration was estimated at 2140mgMg(2+)L(-1) with an inhibition order of 2. The latter indicates that Mg(2+) inhibition is a progressive rather than a steep inhibition mechanism.

Impact of paper and cardboard suppression on OFMSW anaerobic digestion.

Mechanical-biological treatment plants treat municipal solid waste to recover recyclable materials, nutrients and energy. Waste paper and cardboard (WP), the second main compound in municipal solid waste (∼30% in weight basis), is typically used for biogas generation. However, its recovery is gaining attention as it can be used to produce add-value products like bioethanol and residual derived fuel. Nevertheless, WP suppression or replacement will impact anaerobic digestion in terms of biogas production, process stability and digestate management. Two lab-scale reactors were used to assess the impact of WP in anaerobic digestion performance. A control reactor was only fed with biowaste (BioW), while a second reactor was fed with two different mixtures of BioW and WP, i.e. 85/15% and 70/30% (weight basis). Results indicate that either replacing half of the WP by BioW or removing half of the WP has little impact on the methane production. When removing half of the WP, methane production could be sustained by a larger waste biodegradability. The replacement of all WP by BioW increased the reactor methane production (∼37%), while removing all WP would have reduced the methane production about 15%. Finally, replacing WP loading rate by BioW led to a system less tolerant to instability periods and with poorer digestate quality.

Pilot-scale testing of a leachbed for anaerobic digestion of livestock residues on-farm.

A leachbed is a relatively simple anaerobic digester suitable for high-solids residues and on-farm applications. However, performance characteristics and optimal configuration of leachbeds are not well-understood. In this study, two 200 L pilot-scale leachbeds fed with spent straw bedding from pigs/swine (methane potential, B0 = 195-218 L CH4 kg(-1) VS fed) were used to assess the effects of leachate recirculation mode (trickling vs. flood-and-drain) on the digestion performance. Results showed comparable substrate solubilisation extents (30-45% of total chemical oxygen demand fed) and methane conversion (50% of the B0) for the trickling and flood-and-drain modes, indicating that digestion performance was insensitive to the mode of leachate flow. However, the flood-and-drain leachbed mobilised more particulates into the leachate than the trickling leachbed, an undesirable outcome, because these particulates were mostly non-biodegradable. Inoculation with solid residues from a previous leachbed (inoculum-to-substrate ratio of 0.22 on a VS basis) hastened the leachbed start-up, but methane recovery remained at 50% of the B0 regardless of the leachate recirculation mode. Post-digestion testing indicated that the leachbeds may have been limited by microbial activity/inhibition. The high residual methane potential of leachate from the trickling (residual Bo = 732 ± 7 L CH4 kg(-1) VS fed) and flood-and-drain leachbeds (582 ± 8 L CH4 kg(-1) VS fed) indicated an opportunity for further processing of leachate via a separate methanogenic step. Overall, a trickling leachbed appeared to be more favourable than the flood-and-drain leachbed for treating spent bedding at farm-scale due to easier operation.

Semi-aerobic fermentation as a novel pre-treatment to obtain VFA and increase methane yield from primary sludge.

There is a growing trend to consider organic wastes as potential sources of renewable energy and value-add products. Fermentation products have emerged as attractive value-add option due to relative easy production and broad application range. However, pre-fermentation and extraction of soluble products may impact down-stream treatment processes, particularly energy recovery by anaerobic digestion. This paper investigates primary sludge pre-fermentation at different temperatures (20, 37, 55, and 70°C), treatment times (12, 24, 48, and 72h), and oxygen availability (semi-aerobic, anaerobic); and its impact on anaerobic digestion. Pre-fermentation at 20 and 37°C succeeded for VFA production with acetate and propionate being major products. Pre-fermentation at 37, 55, and 70°C resulted in higher solubilisation yield but it reduced sludge methane potential by 20%. Under semi-aerobic conditions, pre-fermentation allowed both VFA recovery (43gCODVFAkg(-1)VS) and improved methane potential. The latter phenomenon was linked to fungi that colonised the sludge top layer during pre-fermentation.

Effect of moisture on pretreatment efficiency for anaerobic digestion of lignocellulosic substrates.

The present study evaluates the effect of moisture in low-temperature and ultrasound pretreatment on lignocellulosic substrates anaerobic biodegradability, where brewer's spent grain was used as model substrate. Besides moisture content, low-temperature pretreatment was also evaluated in terms of temperature (60-80°C) and exposure time (12-72 h). Likewise, ultrasonication was also evaluated in terms of specific energy (1000-50,000 kJ kg TS(-1)). In addition, the effect of substrate particle size reduction by milling pretreatment was also considered. The results clearly demonstrated that substrate moisture (total solid concentration) is a significant parameter for pretreatment performance, although it has been rarely considered in pretreatment optimisation. Specifically, moisture optimisation increased the methane yield of brewer's spent grain by 6% for low-temperature pretreatment (60°C), and by 14% for ultrasound pretreatment (1000 kJ kg TS(-1)) towards the control (without pretreatment). In both pretreatments, the experimental optimum total solid concentration was 100 gTS kg(-1). Thus, lowering substrate moisture, a strategy suggested attaining energetic pretreatment feasibility, needs to be analysed as another pretreatment variable since it might have limited correlation. Finally, a preliminary energetic balance of the pretreatments under study showed that the extra methane production could not cover the energetic pretreatment expenses.