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Lactic acid can be derived from microbial fermentation and be used as a platform chemical in various industrial applications. This study aims to investigate the challenges involved in combining a low-cost, heterogeneous feedstock, such as a mixture of candy-waste and digestate, with an optimized downstream strategy to achieve maximum recovery of high-purity lactic acid, targeting low energy consumption. To achieve this goal, four membrane separation technologies, namely microfiltration, nanofiltration, monopolar, and bipolar electrodialysis, were combined to design two purification processes. Microfiltration served as the pre-purification step, followed by either process A, which combined nanofiltration and bipolar electrodialysis, or process B, a combination of monopolar and bipolar electrodialysis. The findings emphasized the importance of pH as a control factor. Nanofiltration at pH 2.8 and monopolar electrodialysis at pH 4.0 led to increased lactic acid recovery. Moreover, it was observed that process B resulted in 1.09-fold higher lactic acid recovery than process A. However, process A had a 1.19-fold lower specific energy consumption, and the presence of ions in the final solution was reduced by 5-fold. In both processes lactic acid was separated from sugars and organic acids. Overall, the findings of this study suggest that membrane separation technology is a viable method for separating lactic acid produced from a mixture of residual candy-waste and digestate. [Display omitted] • Nanofiltration at pH 2.8 was efficient for lactic acid recovery. • Monopolar electrodialysis at pH 4.0 led to the highest lactic acid recovery. • Monopolar electrodialysis led to higher lactic acid recovery than nanofiltration. • Nanofiltration was less energy-demanding than monopolar electrodialysis. • Lactic acid titer reached 115 g L-1 after fed-batch monopolar electrodialysis. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The mono-digestion of wastewater microalgae led to the highest methane yield. • High biogas yield was produced when the share of wastewater microalgae was 40% VS. • The utilisation of dissimilar substrates resulted in a robust co-digestion process. • The experimental data were verified by dynamic modelling using the BioModel. Abstract A mixture of piggery slurry and algal species (mainly composed of Nannochloropsis limnetica), grown in municipal wastewater, were used as substrates for biogas production. Mono- and co-digestion experiments were performed at batch and continuous reactor operation. The mono-digestion of wastewater microalgae led to the highest methane yield (408 ± 34 N mL/gVS). However, for manure-based biogas plants, a 60:40 v/v piggery slurry to wastewater microalgae ratio in terms of organic matter was identified as the most efficient mixture in batch assays (355 ± 27 N mL/gVS). The advantage of co-digestion was also evidenced under continuous reactor operation, which had markedly higher biogas production (23%, p < 0.05) compared to the mono-digestion of livestock manure. Moreover, it was demonstrated that the co-digestion process resulted in a more robust process as indicated by lower accumulation of acetate (i.e. presented during mono-digestion of piggery slurry) and propionate (i.e. recorded during mono-digestion of wastewater microalgae). The experimental data were compared with dynamic modelling (BioModel). A new set of biodegradability parameters was estimated and employed to improve the simulations of mono-digestion scenarios. Subsequently, the co-digestion scenario was used for model validation. Results obtained from simulations showed that the co-digestion can lead to relatively high methane productivity and prevent process instabilities. [ABSTRACT FROM AUTHOR]

Meadow grass has recently gained increased attention as a substrate for full-scale biogas reactors. However, to increase its biodegradability, pretreatment is needed. In the present work, different harvesting machines were compared in order to assess their effect on biogas production. Specifically, a Disc-mower, an Excoriator and a Chopper were used to define the most appropriate machinery in order to improve the energy output per hectare for full-scale biogas plants. Among the harvesters, Excoriator, a novel simultaneous harvest and mechanical treatment, was found to significantly increase the methane yield of meadow grass by 20% compared to a classical Disc-mower. The positive effect was also validated by three kinetic model equations. The modified Gompertz model was the most capable of determining the kinetics of anaerobic digestion process, pointing out also the superiority of Excoriator. The usage of the novel harvester was associated with increased energy output, either for electrical/thermal energy generation or for transport fuel production, compared to the alternative machineries. Moreover, it was shown that the co-digestion of harvested biomass with different types of manure can enhance the bioenergy output of a full-scale biogas plant in a range of 12%–23%. [ABSTRACT FROM AUTHOR]

Bioaugmentation with hydrolytic microbes was applied to improve the methane yield of bioreactors fed with agricultural wastes. The efficiency of Clostridium thermocellum and Melioribacter roseus to degrade lignocellulosic matter was evaluated in batch and continuously stirred tank reactors (CSTRs). Results from batch assays showed that C. thermocellum enhanced the methane yield by 34%. A similar increase was recorded in CSTR during the bioaugmentation period; however, at steady-state the effect was noticeably lower (7.5%). In contrast, the bioaugmentation with M. roseus did not promote markedly the anaerobic biodegradability, as the methane yield was increased up to 10% in batch and no effect was shown in CSTR. High-throughput 16S rRNA amplicon sequencing was used to assess the effect of bioaugmentation strategies on bacterial and archaeal populations. The microbial analysis revealed that both strains were not markedly resided into biogas microbiome. Additionally, the applied strategies did not alter significantly the microbial communities. [ABSTRACT FROM AUTHOR]

The effect of various micro-aeration strategies on the anaerobic digestion (AD) of wheat straw was thoroughly examined using a mixture of inocula, containing compost and well digested sludge from biogas plant. The aim was to determine the most efficient oxygen load, pulse repetition and treatment duration, resulting in the highest methane production. The oxygen load had the largest impact on the biodegradability of straw, among the examined variables. More specifically, a micro-aeration intensity of 10 mL O 2 /g VS was identified as the critical threshold above which the AD performance was more susceptible to instability. The highest enhancement in biogas production was achieved by injecting 5 mL O 2 /g VS for a consecutive 3-day treatment period, presenting a 7.2% increase compared to the untreated wheat straw. Nevertheless, the results from optimisation case study indicated a higher increase of 9% by injecting 7.3 mL O 2 /g VS, distributed in 2 pulses during a slightly shorter treatment period (i.e. 47 h). [ABSTRACT FROM AUTHOR]

Mechanical pretreatment is considered to be a fast and easily applicable method to prepare the biomass for anaerobic digestion. In the present study, the effect of mechanical pretreatment on lignocellulosic silages biodegradability was elucidated in batch reactors. Moreover, co-digestion of the silages with pig manure in continuously fed biogas reactors was examined. Metagenomic analysis for determining the microbial communities in the pig manure digestion system was performed by analysing unassembled shotgun genomic sequences. A comparative analysis allowed to identify the microbial species firmly attached to the digested grass particles and to distinguish them from the planktonic microbes floating in the liquid medium. It was shown that the methane yield of ensiled grass was significantly increased by 12.3% due to mechanical pretreatment in batch experiments. Similarly, the increment of the methane yield in the co-digestion system reached 6.4%. Regarding the metagenomic study, species similar to Coprothermobacter proteolyticus and to Clostridium thermocellum , known for high proteolytic and cellulolytic activity respectively, were found firmly attached to the solid fraction of digested feedstock. Results from liquid samples revealed clear differences in microbial community composition, mainly dominated by Proteobacteria . The archaeal community was found in higher relative abundance in the liquid fraction of co-digestion experiment compared to the solid fraction. Finally, an unclassified Alkaliphilus sp. was found in high relative abundance in all samples. [ABSTRACT FROM AUTHOR]

Trace metals are indispensable for the metalloenzymes involved for efficient H 2 /CO 2 biomethanation. In-depth understanding of trace metals requirements of methanogens cannot only lead to robust and stable operation, but also can enhance CH 4 productivity. In this study, the effect of Fe, Mn, Co, Ni, Cu, Zn, Se, Mo and W on the H 2 /CO 2 biomethanation was examined. In a period of nutrient deprivation, suppression of hydrogenotrophic archaea was revealed at higher feed gas rates, leading to a significant drop in CH 4 content and a concomitant rise in acetic acid concentration. After nutrient supplementation, the increase in Co, Ni and Fe concentrations were pivotal factors for enhanced methanogenic activity, leading to a CH 4 content of more than 98% in the outlet biogas and a sharp drop in acetic acid concentration. Moreover, mathematical modeling was employed to simulate the influence of the most prominent element, iron, on the biomethanation process. The amended BioModel demonstrated that Fe limitation suppresses hydrogenotrophic archaea which consequently leads to H 2 accumulation and growth of homoacetogenic bacteria. [Display omitted] • Effect of nutrients deficiency on H 2 /CO 2 biomethanation was experimentally assessed. • Effect of iron on the biomethanation process was modeled. • Lack of iron impaired hydrogenotrophs and enhanced homoacetogenic activity. • Nutrients supplementation using digested manure avoided acetate accumulation. • Nutrients replenishment recovered hydrogenotrophs and decreased homoacetogenesis. [ABSTRACT FROM AUTHOR]

Animal manure digestion is associated with limited methane production, due to the high content in fibers, which are hardly degradable lignocellulosic compounds. In this study, different mechanical and thermal alkaline pretreatment methods were applied to partially degradable fibers, separated from the effluent stream of biogas reactors. Batch and continuous experiments were conducted to evaluate the efficiency of these pretreatments. In batch experiments, the mechanical pretreatment improved the degradability up to 45%. Even higher efficiency was shown by applying thermal alkaline pretreatments, enhancing fibers degradability by more than 4-fold. In continuous experiments, the thermal alkaline pretreatment, using 6% NaOH at 55 °C was proven to be the most efficient pretreatment method as the methane production was increased by 26%. The findings demonstrated that the methane production of the biogas plants can be increased by further exploiting the fraction of the digested manure fibers which are discarded in the post-storage tank. [ABSTRACT FROM AUTHOR]

As the biogas sector is rapidly expanding, there is an increasing need in finding new alternative feedstock to biogas plants. Meadow grass can be a suitable co-substrate and if ensiled it can be supplied to biogas plants continuously throughout the year. Nevertheless, this substrate is quite recalcitrant and therefore efficient pretreatment is needed to permit easy access of microbes to the degradable components. In this study, different mechanical pretreatment methods were applied on ensiled meadow grass to investigate their effect on biomass biodegradability. All the tested pretreatments increased the methane productivity and the increase ranged from 8% to 25%. The best mechanical pretreatment was the usage of two coarse mesh grating plates. Additionally, simple analytical methods were conducted to investigate the possibility of rapidly determining the methane yield of meadow grass. Among the methods, electrical conductivity test showed the most promising calibration statistics ( R 2 = 0.68). [ABSTRACT FROM AUTHOR]

Competition for H 2 between homoacetogenic bacteria and methanogenic archaea is commonly faced in biological biogas upgrading process reducing the potential for high methane production capacities. In the present work, different feeding regimes were examined using anaerobic inocula that were adapted and non-adapted to the gaseous feedstock. Adapted inoculum could compensate the increased pressure at all examined levels and especially, produced above 95% of the theoretical methane without accumulating acetate at 0.2 atm. On the contrary, acetate accumulation was detected when a non-adapted inoculum was used. Microbial adaptation to elevated pressures can be applied as a means to improve production capacities. Thermodynamic analysis showed that hydrogenotrophic methanogenesis had always lower permissive H 2 partial pressures (3.20–7.46 Pa) compared to homoacetogenesis (0.43–0.85 Pa) at the examined pH (neutral vs alkaline) and pressure levels (i.e., 0.2–2.0 atm). An unstructured kinetic model was developed to study the influence of the examined variables in the hydrogenotrophic, homoacetogenesis and aceticlastic pathways. The kinetic parameters of the proposed model were estimated from experimental results and the goodness of the fitting was assessed by the coefficient of determination which indicated that the model describes efficiently the dynamics of the different compounds involved in the investigated pathways. [Display omitted] • Biological CO 2 and H 2 capturing is competitive between homoacetogens and methanogens. • Adaptation to substrate can avoid accumulation of acetate. • Methanogens had lower permissive H 2 partial pressures than homoacetogens. • Substrate competition between homoacetogens and methanogens was modeled. [ABSTRACT FROM AUTHOR]

Abstract: A precise and efficient antifoaming control strategy in bioprocesses is a challenging task as foaming is a very complex phenomenon. Nevertheless, foam control is necessary, as foam is a major operational problem in biogas reactors. In the present study, the effect of 14 chemical compounds on foam reduction was evaluated at concentration of 0.05%, 0.1% and 0.5% v/vsample, in raw and digested manure. Moreover, two antifoam injection methods were compared for foam reduction efficiency. Natural oils (rapeseed and sunflower oil), fatty acids (oleic, octanoic and derivative of natural fatty acids), siloxanes (polydimethylsiloxane) and ester (tributylphosphate) were found to be the most efficient compounds to suppress foam. The efficiency of antifoamers was dependant on their physicochemical properties and greatly correlated to their chemical characteristics for dissolving foam. The antifoamers were more efficient in reducing foam when added directly into the liquid phase rather than added in the headspace of the reactor. [Copyright &y& Elsevier]

Anaerobic fermentation of various organic wastes to produce valuable intermediate products and generate a mixture of combustible gases has been researched actively in the past decades. For one, marine biomass as an abundant organic residue of water-locked countries has recently gained increasing popularity. However, such biomass is known to contain high levels of salt, which can be detrimental to the machinery and affects the microbial activity involved in the process. To evaluate the latter, a comprehensive study of sodium chloride and marine salt as potential inhibitors of the anaerobic digestion process is hereby presented. The hypothesis was tested in three major steps. In the first step, batch toxicity assays showed that marine salt is already inhibitory to the anaerobic digestion microbiome, at a half-maximal inhibitory concentration of 2.8 g-Na L−1, compared to synthetic sodium that was inhibitory only at a concentration of 10.1 g-Na L−1. Next, experiments with a continuous reactor showed that the gradual addition of salt to a reactor leads to irreversible inhibition, while a similar reactor that is also spiked with salt might be recoverable to some extent. Finally, a bioconversion model, which for the first time was extended with a dynamic salt inhibition module, produced simulations that agreed well with the results of the continuous experiments, suggesting the presence of agile methanogenic archaea that can temporarily buffer the negative effect of increasing salt concentrations. The findings of this work may provide inputs to future anaerobic fermentation experiments with salts and could benefit from detailed microbial analyses. [Display omitted] • Anaerobic digestion of marine biomass is a field of increasing commercial interest. • A typical marine biomass is seaweed, which contains high concentrations of salts. • To assess the effect of salts on the anaerobic process, three tests were designed. • Marine salt was the most inhibitory, but the dosage strategy was equally important. • Model simulations supported the putative existence of agile methanogenic groups. [ABSTRACT FROM AUTHOR]

A primary target in lactic acid production is to reduce the production cost. Due to its high content of fermentable carbohydrates and low-cost, municipal biopulp can represent a promising feedstock to match this target. In addition to low-cost raw materials, a cost-effective downstream separation process (DSP) for the recovery of lactic acid is necessary to improve the overall competitiveness of the process. In the present study, lactic acid was produced from municipal biopulp obtaining a yield and titer of 82.6 ± 2.3%. g/g of total sugars and 16.1 ± 0.4 g/L, respectively. Two downstream processes (DSP1 and DSP2) were also investigated to recover lactic acid from the fermentation broth. DSP1 consisted of a pre-purification step (centrifugation, ultrafiltration, and activated carbon) followed by ion-exchange and vacuum distillation. DSP1 resulted in a recovery of 75.70 ± 1.5% and lactic acid purity of 72.50 ± 2.0%. In the DSP2, a nanofiltration unit was included after the pre-purification step, which resulted in a higher lactic acid purity of 82.0 ± 1.5% but compromising the recovery, i.e. 65.0 ± 1.5%. Overall, the results of the present study indicate the feasibility to use municipal biopulp as a low-cost feedstock for lactic acid production, and thus efforts should be focused to optimize the fermentation and DSP2 steps. ga1 • Municipal biopulp is a feasible feedstock for lactic acid production. • Two simple downstream processes to recover lactic acid were developed. • Highest lactic acid purity achieved was 82.05%. • No need of nutrients addition and sterilization. • Bioaugmentation strategy exhibited a positive effect on lactic acid titer and yield. [ABSTRACT FROM AUTHOR]

• LCA of biogas production from pretreated grass at harvesting time was evaluated. • Downstream strategies were combined heat and power and biogas upgrading. • Co-digestion of grass and manure provides insight into sustainable energy system. • Sustainability of system improved with biological biogas upgrading. • Transportation distance lower than 65 km contributes to sustainability of system. Biogas production from anaerobic digestion of grass was evaluated in this study taking into account two harvesting machines, a Disc-mower and an Excoriator, under diverse operating conditions. In addition, the application of generated biogas either in a Combined Heat and Power (CHP) plant for thermal and electrical energy production or as transportation fuel after upgrading (BGU) process was evaluated. Consequential Life Cycle Assessment (CLCA) with long term marginal data was employed. Lab-scale data as well as those obtained from the ecoinvent database were used to compile life cycle inventory data. The system boundary of the present study covered harvesting operation of grass, baling, transportation of bales, anaerobic digestion, use of digestate on farmlands, and downstream processes for biogas usage. Additionally, the system boundary was expanded to take into account the effect of substituting grass with straw in animal feeds. The results demonstrated that the environmental performance of grass-based biogas plants were highly dependent on selected downstream strategies. Furthermore, it was evident that mono-digestion of grass would not guarantee a long-term sustainable renewable energy system. Based on the results obtained, Excoriator at driving speed of 7.5 km/ha had the best environmental performance in all damage categories, i.e., "Human health", "Ecosystem quality", "Climate change", and "Resources". CHP had a greater environmental performance than water scrubbing BGU for the downstream strategies taken into account. The results from the sensitivity analysis proved that a specific methane yield lower than 329 mLCH 4 /gVS cannot ensure the achievement of an eco-friendly energy system from grass-based biogas plants. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rapeseed oil, oleic and octanoic efficiently suppressed foaming in biogas reactors. [•] Rapeseed oil was the most suitable antifoam for overloaded reactors. [•] Rapeseed oil showed synergistic effect on methane yield when digested with manure. [•] Tributylphosphate inhibited severely the anaerobic digestion process. [Copyright &y& Elsevier]