Anaerobic digestion for treatment of stillage from cellulosic bioethanol production

Modification of bioethanol production in an innovative pneumatic digester with non-thermal cold plasma detoxification

An anaerobic pneu-mechanical digester (PD) was designed to ferment lignocellulosic compounds. So, wheat and rice straws were pretreated using an ultrasound-acid, and then thermal-acid hydrolysis was conducted. Hydrolysis optimization was performed using the response surface method and the optimal points for time, temperature, and acid concentration were 45 min, 148.4 °C, and 2.04 % v/v, respectively. Cold plasma was then used as detoxification to reduce the amount of inhibitory compounds and acids. This method was capable of reducing the amounts of acetic acid, formic acid and furfural by 73, 83 and 68 % in hydrolyzed biomass, respectively. The biomass was fermented in a PD for 20 days and compared with a conventional digester (CD). The obtained results showed that the PD could increase the efficiency of bioethanol by 37 % in the detoxified state and 22 % in the non-detoxified state after 20 days of fermentation compared to the CD. Moreover, H₂S, CO and O₂ were measured during fermentation process. In PD, the amount of H₂S and O₂ was lower than CD, but CO was significantly higher in the PD.

Valorization of Distillery Stillage for Bioenergy Production: A Review

In alcohol distilleries, the amount of distillery stillage generated can be up to 15 times larger than the amount of alcohol produced. The stillage has high concentrations of organics and nitrogen, a low pH, and a dark brown color. Currently, stillage is mainly used for soil fertilization. For this purpose, it requires thickening and is used seasonally, which creates storage problems and transport costs. To reduce environmental pollution, physicochemical and biological processes have been employed for the treatment of distillery stillage. However, according to bioeconomy principles, the stillage should be transformed into value-added products. Therefore, this review paper focuses on methods of stillage processing that enable energy recovery. Due to its high content of organic compounds, stillage is often used as a raw material for biogas production. Accordingly, anaerobic digestion of stillage is discussed, including an overview of the bioreactors used and the effects of operational parameters on organics removal and biogas production. The necessity of integrating anaerobic stillage treatment with other treatment processes is presented. As complex compounds that are present in the stillage (mainly polyphenols and melanoidin) are difficult to biodegrade and have antibacterial activities, the effect of their recovery on biogas production is described. Next, the possibility of converting distillery stillage to bioethanol and biohydrogen is presented. In addition, bioelectrochemical treatment of distillery stillage using microbial fuel cells is discussed. For all these treatment methods, current challenges and opportunities are given.

Formulating a fully converged biorefining chain with zero wastewater generation by recycling stillage liquid to dry acid pretreatment operation

Huge wastewater generation is the major challenge of biorefinery technology for production of cellulosic ethanol. This study designed and verified a method for completely recycling of wastewater stream (the stillage liquid from the beer column) in cellulosic ethanol production by dry biorefining processing. When the stillage liquid was directly recycled to dry acid pretreatment operation, ethanol production gradually reduced after two recycles primarily because the inorganic compounds accumulated by around 139%. To ultimately solve this technical barrier, the stillage liquid was evaporated and condensed into distillated water, then recycled to the pretreatment for complete dry biorefining process. This strategy supported a stable cellulosic ethanol production, and the overall mass and heat balance confirmed that only 65% of the lignin residue consumption was used for wastewater evaporation with 35% surplus for electricity generation. This study provided a fully converged biorefining process with a closed-loop wastewater recycling.

Evaluation of Filamentous Fungi and Yeasts for the Biodegradation of Sugarcane Distillery Wastewater

Sugarcane Distillery Spent Wash (DSW) is among the most pollutant industrial effluents, generally characterized by high Chemical Oxygen Demand (COD), high mineral matters and acidic pH, causing strong environmental impacts. Bioremediation is considered to be a good and cheap alternative to DSW treatment. In this study, 37 strains of yeasts and filamentous fungi were performed to assess their potential to significantly reduce four parameters characterizing the organic load of vinasses (COD, pH, minerals and OD_475nm). In all cases, a pH increase (until a final pH higher than 8.5, being an increase superior to 3.5 units, as compared to initial pH) and a COD and minerals removal could be observed, respectively (until 76.53% using Aspergillus terreus var. africanus and 77.57% using Aspergillus niger). Depending on the microorganism, the OD_475nm could decrease (generally when filamentous fungi were used) or increase (generally when yeasts were used). Among the strains tested, the species from Aspergillus and Trametes genus offered the best results in the depollution of DSW. Concomitant with the pollutant load removal, fungal biomass, with yields exceeding 20 g·L^-1, was produced.

Comparison of furfural and biogas production using pentoses as platform

Coffee cut-stems (CCS), a biomass with high lignocellulosic content, is a coffee crop waste after bean harvesting. The main application of this material is as fuelwood for farmers, disregarding their carbohydrate content for biotechnological processes. In these terms, this work aims to compare three process scenarios for the experimental valorization of C5 fraction from CCS to produce biogas and furfural with and without the ethanol production from remaining C6 fraction under biorefinery concept. Therefore, an experimental stage was performed to obtain these products, based on a previous diluted acid pretreatment. The hydrolysate fraction was used to produce furfural and biogas, achieving yields of 0.34 g of furfural/g xylose and 81.1 mL of CH₄ per gram of volatile solids. Concerning the solid fraction after acid pretreatment, it was used to produce ethanol with a previous enzymatic hydrolysis. After fermentation, 0.47 g of ethanol/g of glucose (92% of the theoretical yield) was obtained. These experimental results were fed to simulation models in order to compare three scenarios in technical, economic and environmental terms. As the main results, from technical point of view, the biogas production presents the lowest energy requirements. From the economic perspective, the furfural production presents a prefeasibility at the base scale of processing (e.g., 12.5 ton h^-1). Meanwhile, the biogas scenario needs a processing capacity >22.5 ton h^-1 to achieve the economic prefeasibility. In the biorefinery case, the positive economic performance is found at processing scales above 83 ton h^-1. This work concludes that the C5 sugars platform is identified as a potential alternative for the generation of furfural and biogas, however, in this case a multiproduct biorefinery system is not always the best option to valorize biomass given the very high scale required and the economic indicators.

Treatment and resource recovery options for first and second generation bioethanol spentwash - A review

A decline in the availability of fossil fuel resources coupled with deleterious environmental concerns has prompted further research into biofuels. Conventional bioethanol production via a first-generation approach may soon become superseded through integration with lignocellulosic feedstocks. However, the underlying concerns pertaining to the disposal of high-strength liquid waste (i.e. spentwash) remain both unchanged and constitute a substantial cost to bioethanol manufacturers. Therefore this review details current efforts in the literature to elucidate various approaches for spentwash treatment and investigate the potential for resource recovery. Insight into the composition of distillery wastewater is given in the lead-up to a thorough discussion encompassing the origin, transformation and characterisation of the highly problematic melanoidin compounds entrained within this effluent. Close examination of advanced organic characterisation methods used by researches yields further insight into the nature of spentwash dissolved organic matter (DOM). Employment of both biological and physio-chemical treatment schemes to alleviate the environmental footprint of such high-strength wastewater are also reviewed. Opportunities to dramatically improve the economic viability of biofuel production by exploiting the potential for resource recovery in the form of energy, organic/inorganic constituents and effluent reuse are discussed. Overall, the review culminates by highlighting recommendations for future work to accelerate the onset of an environmentally benign bio-refinery.

Life Cycle Assessment for Bioethanol Production from Oil Palm Frond Juice in an Oil Palm Based Biorefinery

A study was conducted to estimate the possible environmental impacts arising from the generation of bioethanol from oil palm frond sugar juice in a theoretical oil palm based biorefinery model. A life cycle assessment (LCA) with the gate-to-gate approach was performed with the aid of SimaPro version 8.0 whereby ten impact categories were evaluated. The scope included frond collection and transportation, frond sugar juice extraction, and bioethanol fermentation and purification. Evaluation on the processes involved indicated that fermentation contributed to the environmental problems the most, with a contribution range of 52% to 97% for all the impact categories. This was due to a substantial usage of nutrient during this process, which consumes high energy for its production thus contributing a significant burden to the surrounding. Nevertheless, the present system offers a great option for biofuel generation as it utilizes sugar juice from the readily available oil palm waste. Not only solving the issue of land utilization for feedstock cultivation, the enzymatic saccharification step, which commonly necessary for lignocellulosic sugar recovery could also be eliminated.

The use of sugar beet pulp stillage for co-digestion with sewage sludge and poultry manure

The anaerobic mesophilic co-digestion of sugar beet pulp stillage with poultry manure and municipal sewage sludge was investigated in this study. The sugar beet pulp stillage (SBPS) mono-digestion failed owing to an accumulation of volatile fatty acids, leading to a pH value lower than 5.5. A 20% addition of poultry manure to stillage allowed for stable digestion performance despite high volatile fatty acid (total volatile fatty acids) concentrations of 5500-8500 g m^-3 with propionic acid being the predominant one and constituting 72%-76% total volatile fatty acids. For this mixture, the maximum methane production of 418 dm³ kgVS_fed^-1 was achieved when the reactor was operated at a solids retention time of 20 days and an organic loading rate of 4.25 kgVS m^-3 d^-1. The co-digestion of stillage with 60% municipal sewage sludge gave the average methane yield of around 357 dm³ kgVS_fed^-1 for all operational conditions applied, however, the methane percentage of biogas (up to 70%) was far greater than the corresponding values obtained for sugar beet pulp stillage-poultry manure co-digestion. Neither ammonia nor volatile fatty acids destabilised the biogas production, and the volatile fatty acid profile showed the dominance of acetic acid (72%-82% total volatile fatty acids) followed by propionic and butyric acids.

Microwave-assisted pretreatment of maize distillery stillage with the use of dilute sulfuric acid in the production of cellulosic ethanol

This study aimed to evaluate the effectiveness of microwave-assisted pretreatment in production of cellulosic bioethanol from maize distillery stillage. High glucose concentration (104.4 ± 0.4 mg/g dry weight) and the highest yield of enzymatic cellulose hydrolysis (75.8 ± 0.9%) were obtained for microwave pretreatment characterized by the following parameters: 300 W, 54 PSI, 15 min. Such pre-treatment parameters allowed the obtaining of not only a high concentration of glucose, but also a low concentration of fermentation inhibitors, i.e. 5-hydroxymethylfurfural (6.8 ± 0.4 mg/g of DW) and furfural (6.0 ± 1.2 mg/g dry weight). Optimal dose of yeast, Saccharomyces cerevisiae strain Ethanol Red which gave a high attenuation was 2 g/L of cellulose fermentation medium. Detoxification of cellulose hydrolysates with activated carbon enabled achieving high fermentation yield (approximately 77% of the theoretical yield). Microwave processing can be an effective pretreatment method in production of cellulosic ethanol from maize distillery stillage, but this process requires careful selection of parameters.

Efficiency of dilute sulfuric acid pretreatment of distillery stillage in the production of cellulosic ethanol

The aim of this study was to examine suitability of distillery stillage of various origins subjected to dilute sulfuric acidic pretreatment for production of cellulosic ethanol. Optimal conditions for dilute acid pretreatment of: rye and wheat distillery stillage 121 °C, 0.2 M H₂SO₄, 60 min; maize stillage 131 °C, 0.2 M H₂SO₄, 60 min. The highest efficiency of enzymatic hydrolysis was achieved for rye and wheat stillage using 1 g of DW and the concentration of cellulolytic enzyme of 24% w/w, and for maize stillage 3 g of DW and enzyme concentration of 24% w/w. The use of rye and wheat stillage for production of ethanol does not require a detoxification process and enables full attenuation of glucose after 48 h of the process. However, the use of maize stillage as a raw material must be preceded by a detoxification process that guarantees a reduction of 5-hydroxymethylfurfural concentration in the fermentation medium.

Kinetic modelling and synergistic impact evaluation for the anaerobic co-digestion of distillers' grains and food waste by ethanol pre-fermentation

The anaerobic digestion of food waste (FW) often leads to acidification inhibition owing to rapid biodegradation, resulting in system instability. In this study, distillers' grains (DG) and food waste were mixed in accordance with volatile solid (VS) ratios of 0.9:0.1, 0.85:0.15, 0.8:0.2, and 0.7:0.3. The experimental groups adopted yeast to conduct ethanol pre-fermentation and then inoculated sludge to perform anaerobic digestion, while the control groups conducted anaerobic digestion without pre-treatment. Results showed that the experimental groups had lower propionic acid concentrations; higher alkalinities, pH values and methane production rates and shorter stagnation periods than the control groups regardless of the mixing ratio. Specifically, at the DG/FW ratio of 0.7:0.3, compared with the control group, the propionic acid concentration was reduced by 59.6%, the alkalinity was increased by 41.7%. Even under high organic loading, the propionic acid and VFA did not accumulate in the system after ethanol pre-fermentation, and the anaerobic digestion system remained stable. At DG/FW ratios of 0.9:0.1 and 0.85:0.15, a synergistic effect was observed during the co-digestion of DG and FW. And, the synergistic effect of EP was relatively high, especially when the DG/FW ratio was 0.9:0.1, and methane yield increased by 26.8%.

Removal of sulfamethoxazole (SMX) and sulfapyridine (SPY) from aqueous solutions by biochars derived from anaerobically digested bagasse

This study explored the sorption of sulfamethoxazole (SMX) and sulfapyridine (SPY) onto biochars produced from raw and anaerobically digested bagasse. Initial evaluation of six bagasse biochars showed that digested bagasse biochar prepared at 600 °C (DBG600) was the best adsorbent to remove SMX and SPY. Further laboratory batch sorption experiments showed that DBG600 adsorbed SMX and SPY from aqueous solution with maximum adsorption capacity of 54.38 and 8.60 mg g^-1, respectively. Solution pH showed strong effect on the sorption ability of DBG600 to the two antibiotics, and the sorption decreased with increasing of solution pH. Experimental and model results suggested that adsorption of SMX and SPY onto DBG600 might be controlled by the π-π interaction.

Increasing the economic value of lignocellulosic stillage through medium-chain fatty acid production

BACKGROUND

Lignocellulosic biomass is seen as an abundant renewable source of liquid fuels and chemicals that are currently derived from petroleum. When lignocellulosic biomass is used for ethanol production, the resulting liquid residue (stillage) contains large amounts of organic material that could be further transformed into recoverable bioproducts, thus enhancing the economics of the biorefinery.

RESULTS

Here we test the hypothesis that a bacterial community could transform the organics in stillage into valuable bioproducts. We demonstrate the ability of this microbiome to convert stillage organics into medium-chain fatty acids (MCFAs), identify the predominant community members, and perform a technoeconomic analysis of recovering MCFAs as co-products of ethanol production. Steady-state operation of a stillage-fed bioreactor showed that 18% of the organic matter in stillage was converted to MCFAs. Xylose and complex carbohydrates were the primary substrates transformed. During the MCFA production period, the five major genera represented more than 95% of the community, including Lactobacillus, Roseburia, Atopobium, Olsenella, and Pseudoramibacter. To assess the potential benefits of producing MCFAs from stillage, we modeled the economics of ethanol and MCFA co-production, at MCFA productivities observed during reactor operation.

CONCLUSIONS

The analysis predicts that production of MCFAs, ethanol, and electricity could reduce the minimum ethanol selling price from $2.15 to $1.76 gal-1 ($2.68 gal-1 gasoline equivalents) when compared to a lignocellulosic biorefinery that produces only ethanol and electricity.

Effect of acetic acid in recycling water on ethanol production for cassava in an integrated ethanol-methane fermentation process

Recently, the integrated ethanol-methane fermentation process has been studied to prevent wastewater pollution. However, when the anaerobic digestion reaction runs poorly, acetic acid will accumulate in the recycling water. In this paper, we studied the effect of low concentration of acetic acid (≤25 mM) on ethanol fermentation at different initial pH values (4.2, 5.2 or 6.2). At an initial pH of 4.2, ethanol yields increased by 3.0% and glycerol yields decreased by 33.6% as the acetic acid concentration was increased from 0 to 25 mM. Raising the concentration of acetic acid to 25 mM increased the buffering capacity of the medium without obvious effects on biomass production in the cassava medium. Acetic acid was metabolized by Saccharomyces cerevisiae for the reason that the final concentration of acetic acid was 38.17% lower than initial concentration at pH 5.2 when 25 mM acetic acid was added. These results confirmed that a low concentration of acetic acid in the process stimulated ethanol fermentation. Thus, reducing the acetic acid concentration to a controlled low level is more advantageous than completely removing it.

Microbial community analysis in a combined anaerobic and aerobic digestion system for treatment of cellulosic ethanol production wastewater

This study investigated the microbial diversity established in a combined system composed of a continuous stirred tank reactor (CSTR), expanded granular sludge bed (EGSB) reactor, and sequencing batch reactor (SBR) for treatment of cellulosic ethanol production wastewater. Excellent wastewater treatment performance was obtained in the combined system, which showed a high chemical oxygen demand removal efficiency of 95.8% and completely eliminated most complex organics revealed by gas chromatography-mass spectrometry (GC-MS). Denaturing gradient gel electrophoresis (DGGE) analysis revealed differences in the microbial community structures of the three reactors. Further identification of the microbial populations suggested that the presence of Lactobacillus and Prevotella in CSTR played an active role in the production of volatile fatty acids (VFAs). The most diverse microorganisms with analogous distribution patterns of different layers were observed in the EGSB reactor, and bacteria affiliated with Firmicutes, Synergistetes, and Thermotogae were associated with production of acetate and carbon dioxide/hydrogen, while all acetoclastic methanogens identified belonged to Methanosaetaceae. Overall, microorganisms associated with the ability to degrade cellulose, hemicellulose, and other biomass-derived organic carbons were observed in the combined system. The results presented herein will facilitate the development of an improved cellulosic ethanol production wastewater treatment system.

Engineered biochar from biofuel residue: characterization and its silver removal potential

A novel approach was used to prepare engineered biochar from biofuel residue (stillage from bagasse ethanol production) through slow pyrolysis. The obtained biochar was characterized for its physicochemical properties as well as silver sorption ability. Sorption experimental data showed that engineered biochar quickly and efficiently removed silver ion (Ag(+)) from aqueous solutions with a Langmuir maximum capacity of 90.06 mg/g. The high sorption of Ag(+) onto the biochar was attributed to both reduction and surface adsorption mechanisms. The reduction of Ag(+) by the biochar was confirmed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses of the postsorption biochar, which clearly showed the presence of metallic silver nanoparticles on the surface of the carbon matrix. An antimicrobial ability test indicated that silver-laden biochar effectively inhibited the growth of Escherichia coli, while the original biochar without silver nanoparticles promoted growth. Thus, biochar, prepared from biofuel residue materials, could be potentially applied not only to remove Ag(+) from aqueous solutions but also to produce a new value-added nanocomposite with antibacterial ability.