Energy generation from multifarious wastes of alcohol distillery raki production process: Kinetic modeling of methane production

Highlights

• This is the first study on anaerobic co-digestion of vinasse, pomace, and aniseed.

• The optimum substrate ratios were determined for co-digestion of alcohol distillery wastes.

• Vinasse improved the synergistic effect of co-digestion.

• Both of waste disposal and energy recovery were achieved for alcohol distillery wastes.

Abstract

Co-digestion is an efficient way to utilize different substrates to achieve better methane production and waste management. In this study, anaerobic co-digestion of vinasse (V), pomace (P) and aniseed (A) at 13 different ratio combinations were evaluated from the point of methane production performances. The methane production potentials of aniseed, vinasse, and pomace were determined as 444.1, 367.9, and 116.8 mL CH₄ g⁻¹ VS_added, respectively. The highest synergistic and antagonistic effects were obtained as 1.09 and 0.81 along with the 205.8 mL CH₄ g⁻¹ VS_added and 257.1 mL CH₄ g⁻¹ VS_added methane production potentials from the co-digested samples having ratios as A:V:P = 0:70:30 and A:V:P = 20:60:20, respectively. The co-digestion of aniseed with vinasse showed a positive impact on the methane production compared to pomace added co-digestion methane potentials. Addition of vinasse to pomace as a co-substrate improved the methane productions. The methane contents of produced biogas ranged between 56% and 67%. First Order, Cone, Modified Gompertz, and Reaction Curve models were applied to predict the methane production potentials. The R² values were calculated between 0.914 and 0.999 for the models. The ratio of A:V:P = 33:33:33 for the evaluation of alcohol distillery wastes from raki production by anaerobic co-digestion found as an appropriate option due to providing utilization of all wastes and adequate methane production. The findings of this study provided useful information for the evaluation of alcohol distillery wastes as a renewable energy source and gave a perspective about the practical application of the co-digestion process for industry as a waste management alternative.

Introduction

Alcohol distilleries produce different kinds and large amount of wastes during the ethanol production which has high pollutant properties. Due to this waste generation, alcohol distilleries are reported as one of the most 17 polluting industries [1]. Ethanol production from molasses generates wastewater defined as vinasse which is the main residue of the process. 8‐15 liter of vinasse per liter of produced ethanol is generated by ethanol production [2]. Vinasse is a complex material characterized by high chemical oxygen demand above 70 g COD L⁻¹, low pH value between 3.5‐5, dark brown color and high temperature [2], [3], [4], [5]. The discharge of vinasse without treatment can cause serious environmental problems. Besides the high organic content of vinasse, its dark color can decrease the light penetration and inhibit the photosynthesis, so it can deteriorate the aquatic life. Alcohol distilleries generate other wastes than vinasse, depending on the produced type of alcohol, such as aniseed and pomace from the production of different types of aniseed –flavored spirits belongs to Mediterranean culinary cultures. Grape is the main component of pastis, raki, ouzo, anis, sambuca and arak and after the utilization of grape for alcohol production, pomace is produced. Untreated pomace can generate several environmental hazards like surface and groundwater pollution and foul odors. Also untreated pomace can cause pestiferous diseases by attraction of flies and insects [6].

Aniseed is used as a flavor addition in raki and ouzo production due to its rich essential oils [7], [8]. Traditional raki production steps consist of preparation of grapes, fermentation of grapes, distillation, addition of aniseed, distillation, addition of sugar, and dilution. First three steps are called as production of suma (raisins, molasses and/or grape must, and suma itself is a grape based distillate containing a maximum 94.5% of ethanol). Raki is a very popular and most consumed aniseed spirit produced by double distillation with aniseed (Pimpinella anisum) of only suma or suma and ethanol mixture in Turkey. As a result of this popularity, huge amount of aniseed and pomace from suma production wastes are produced besides vinasse in Turkey during raki production.

Anaerobic digestion is a widespread process used for the stabilization of wastes that has high organic content. Anaerobic digestion has many advantages like less energy input, less sludge production and, low nutrient demand compared to aerobic processes. Also, production of renewable energy as biogas is possible with anaerobic digestion by the conversion of organic materials. The use of biogas to generate energy can significantly reduce the operational costs of anaerobic digestion. There are many studies in the literature evaluating the anaerobic digestion of vinasse. Fu et al. [9] investigated the methane potential of vinasse using two-stage anaerobic digestion. They found methane yields of vinasse as 274 mL g⁻¹ VS_substrate. Fu et al. [9] reported, two-stage anaerobic digestion improve the energy recovery from vinasse. Santos et al. [10] investigated the sugarcane vinasse treatment using two-stage anaerobic membrane bioreactor and found the process biologically stable and effective in vinasse treatment. Djalma Nunes Ferraz Júnior et al. [11] studied the thermophilic anaerobic digestion of sugarcane vinasse using UASB. They achieved a methane yield of 306 mL g⁻¹ COD_removed with the two-stage UASB. Due to high content of readily biodegradable components in vinasse, initial AD stage of vinasse can easily be acidified which can lead to unstable fermentation [9]. So, balancing this pH drop is essential for an efficient fermentation.

Co-digestion is also a common way to utilize different kinds of wastes. Co-digestion can improve the methane yield by increasing the synergistic effect between microorganisms, balancing the macro/micro nutrients, moisture and dilutes the inhibitory compounds. Also, co-digestion can provide a better economic feasibility of the process. Limited studies on co-digestion of vinasse are available in the literature [5], [12], [13], [14], [15]. Moraes et al. [14] stated that the addition of co-substrates enhanced the anaerobic digestion of sugar beet vinasse. BMP studies with vinasse illustrated methane production potantial as 267.4 ± 4.5 L CH₄ kg⁻¹ VS. It was indicated that the addition of cow manure and lime fertilizer improved the trace element content and straw enhanced the C/N ratio of the material as co-substrates, even if BMP values of co-digested materials were very close to BMP of vinasse. Furthermore, stable methane yields and low VFA accumulation were achieved. López González et al. [5] investigated the synergistic effect of co-digestion of vinasse and sugarcane press mud where vinasse was used to supply essential nutrients for bioconversion process such as P₂O₅, K, Ca and Mg. Methane yield as 365 L CH₄ kg⁻¹ VS was obtained which was 64% higher in comparison to mono-digestion.

Kinetic models can supply crucial information about the feasibility of planned plants that if the biogas potential of substrates enough to meet the capital and operating costs of a real scale plant. Data on optimal mixing ratios of different kinds of substrates for co-digestion, specific methane yields, and kinetics along with dynamics of the degradation process are essential information for the development of cost-effective technologies. Different kinds of kinetic modeling were developed for anaerobic digestion of various organic materials by many researchers. There are lots of papers that have been addressed the structural kinetic modeling [12], [16], [17], Anaerobic Digestion Model 1 - ADM1 [18], [19], [20] and simple methane potential prediction model [5], [13], [14], [21], [22], [23], [24], [25] of anaerobic digestion of vinasse generated from ethanol production and from wine distillery in literature. On the other hand, only a few studies dealt with the simple methane potential prediction modeling of anaerobic co-digestion of pomace from the grape in the literature [22], [23], [24]. In contrast to anaerobic digestion modeling of vinasse and pomace, to the best of our knowledge, no previous study has modeled wastes from raki production including vinasse, pomace, and aniseed as a lignocellulosic source for anaerobic digestion.

In this study, it was aimed to evaluate the effects of co-digestion of alcohol distillery wastes from raki production in different mass ratios on the biogas production. According to our knowledge, it is the first investigation on anaerobic co-digestion of vinasse, pomace, and aniseed as alcohol distillery wastes from raki production. Vinasse was chosen as primary waste based on the waste production rate so, vinasse was used in all BMP test. Due to the low C/N ratio of vinasse, biogas production should be enhanced by the addition of co-substrates having high nitrogen content to ensure 400/7–700/7 of C/N ratios in digestion [25], as it is also important in activated sludge and nitrification–denitrification processes [26]. However, in this study, it was focused to determine the waste mixture ratios in anaerobic digestion of alcohol distillery wastes from raki production without considering the adjustment of the C/N ratio. Aniseed and pomace were added in different ratios as co-substrates to the BMP test. The original contribution of this work is the application of Modified Gompertz, First Order, Cone, and Reaction Curve models to predict the biogas production and kinetics of anaerobic digestion along with the evaluation of synergistic and antagonistic effects.