Optimisation of anaerobic digestion of pot ale after thermochemical pre-treatment through Response Surface Methodology
Introduction
Increased need for expanding the renewable energy use in a sustainable manner as boost the second generation biofuels research as the Renewable Energy Directive II has set the overall EU renewable energy target to 32% with a 7% cap on the first generation biofuels by 2030 [1]. The major drawback of the first generation biofuels is known to be their indirect land use and its potential negative impact on the food industry due to increased demand on animal based diet [[2], [3], [4]]. Furthermore, sustainability issues such as water usage and greenhouse gas emission risk in land preparation step for crop growth arose from first generation biofuel production [3]. Therefore, second generation biofuel production technologies are attracting more and more interest. Among the second generation biofuels, biogas (produced by anaerobic digestion) is considered as one of the most environmental friendly fuel due to its non-toxic and easy and varying use [5].
Whiskey is one the most consumed alcoholic beverage globally correspondingly distilling industry has recently grown in countries as such Ireland and the UK [6,7]. Due to the high organic content of the liquid waste stream, known as pot ale originating from the bottom of the copper distilling stills, as well as large discharge volumes (approximately 8–15 L aqueous waste per litre of malt whiskey [8]), alcoholic beverage industry is highly polluting [9]. Uncontrolled pot ale release to the water bodies severely threatens the aquatic life in different ways i.e. decreasing the solubilised oxygen due to its high chemical and biological oxygen demand, potentially causing eutrophication and restricting photosynthesis by blocking the sunlight due to its dark colour. Excessive land and landfill applications on the other hand, are commonly associated with inhibition of seed germination and agricultural crops as a result of its acidic nature [7].
Not only the high organic content but also the yearlong supply of pot ale with no seasonal effects renders it a promising feedstock for second generation biofuel in particular biogas production through anaerobic digestion (AD). Volatilizing pot ale as feedstock of AD is also considered to be a sustainable waste management method for the distilling industry where produced biogas can be used in the manufacturing. Furthermore, the co-product of the anaerobic digestion, called digestate, is considered as a safe, eco-friendly biofertiliser due its rich inorganic content – a potential superior replacement to conventional chemical fertilisers [10]. In the process of AD biochemical reactions take place at parallel steps knowns as hydrolysis, acidogenesis, acetogenesis and methanogenesis [11]. For a well-balanced reactor operation, identical reaction rates are desired to prevent accumulation of intermediate products such as volatile fatty acids (VFAs) [12,13]. Accumulation of VFAs might be inhibitory for methane production due to the pH sensitivity of the methanogenic bacteria [[14], [15], [16]]. Hydrolysis is the rate limiting step for AD of lignocellulosic material due to rigid, impermeable nature of lignin as well as its resistance to microbial attack and oxidative stress [17]. A pre-treatment step is therefore commonly required prior to AD of pot ale in order to eliminate structural resistance arising from the presence of lignin. Furthermore, presence of sulphate in the substrate results in formation of H2S as a result of thermodynamically more favourable side reaction known as sulphidogenesis than methanogenesis [18]. The current state of art technology for AD of whiskey distillery waste streams has recently been reported in detail along with applied pre-treatments [7].
Chemical pre-treatments are well accepted and commonly applied on various lignocellulosic substrates prior to AD. For AD of pot ale alkaline pre-treatments are considered to be more suitable due to the necessity of neutralisation of acidic nature of pot ale [8,19]. Application of the alkaline pre-treatment reduces degree of polymerisation and crystalline structure of cellulose in addition to providing partial hydrolysis and solubilisation of hemicellulose and lignin [7,[20], [21], [22]].
Thermal pre-treatments are also widely applied on lignocellulosic materials. Microwave pre-treatment in particular, increases intracellular compounds by means of cell wall lysis and gel structure degradation [23,24]. It also creates continuous alignment and realignment of macromolecules in polar liquid and result in breakage of hydrogen bonds, generation of frictional heat, modification of the hydration zone, increased solubilisation of sludge, improvements of volatile solids destruction and enhancement of biogas production [[23], [24], [25]]. The success of microwave pre-treatment depends on parameters such as the microwave power and intensity (penetration depth), the contact time, and the temperature [23,26].
In this study, combined alkaline and microwave pre-treatment on pot ale is introduced as a hybrid thermochemical pre-treatment method prior to AD of pot ale. Moreover, a detailed pot ale characterisation along with the lignocellulose fractions as well as the impacts of applied thermochemical pre-treatment on the lignocellulosic fractions has been assessed. To the best of authors’ knowledge, these approaches have never been performed on pot ale prior to anaerobic digestion In addition, micro and macro nutrient content of pot ale based digestate is introduce to literature for evaluation of its potential agricultural use. Finally, AD of thermochemically pre-treated pot ale is modelled and optimised to achieve maximum CH4 yield and minimum CO2 generation at the same time by using Design Expert Software.
Section snippets
Feedstock and inoculum
Pot ale was obtained from a small-scale whiskey distillery in Dublin, Ireland. Activated sludge with a 6% dry matter was used as inoculum source and it was collected from a full scale biogas plant processing industrial food waste operating within mesophilic range, along with an industrial digestate sample which is used as biofertiliser based in Ireland.
Analytical methods
Solid content of pot ale sample as received was determined in terms of total solids (TS), volatile solids (VS) and moisture content (MC)
Pot ale characterisation and modifications in lignocellulose fractionations
Characteristics of non-treated pot ale sample are given in Table 3.
The lignocellulosic composition of non-treated, alkaline (1 M NaOH) and combined alkaline and microwave (at 10, 30 and 50% power) pre-treated pot ale are given in Fig. 1. Pre-treated samples had a significant increase (p < 0.05) in the hemicellulose fraction as well as a significant delignification (p < 0.05) indicating an enhanced biodegradability in comparison to the non-treated pot ale (Control). The maximum lignin removal
Industrial implementation
Annual volume of pot ale discharge is directly linked to the production capacity of whiskey distilleries. However, mass balance of whiskey manufacturing process does not vary remarkably for individual plants depending on manufacturing scale, raw material selection etc [49,70]. Therefore, a typical mass balance for malt whiskey distillery producing 1 million litres of alcohol per annum (Fig. 6) was used to estimate annual biogas production and predict the economical reliability of the technology.
Conclusions
Implementation of thermochemical pre-treatment has amended the lignocellulosic structure of pot ale which resulted in increased reaction kinetics and correspondingly enhanced biogas quality and quantity. Combining 1 M NaOH pre-treatment with 30% microwave pre-treatment has led a 3-fold increase in the CH4 yield reaching a maximum of 1614 ± 168 ml gVS−1 (std no 4 in Table 4) in comparison to control. In addition to increased methane yields, applied pre-treatment has significantly increased the
Acknowledgements
The authors gratefully acknowledge Alltech Ireland Ltd. for the financial support for this research, Project ID 15-S-ST-10568. The authors are most grateful to Brain Fay and Indre Sinkunaite for their help in analysis to produce relevant data.
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