Pre-treatments to enhance biogas yield and quality from anaerobic digestion of whiskey distillery and brewery wastes: A review
Introduction
Widespread usage of non-renewable fuels (in particular fossil fuels) for energy production has been implicated as the cause of many ecological and environmental concerns which impact on human migration and climate conditions. This is due primarily to the continuous emission of greenhouse gases such as CO2 [1] from such usage. In order to address this problem, The European Union aims to reduce the total greenhouse gas emissions in developed countries to 80%–95% of 1990 levels by 2050 [2]. Exploration of alternative energy sources has arisen as a result of increasing energy demand as well as economic and environmental reasons. Biogas (a methane rich gas produced by biological means) is considered to be one of the most environmental friendly fuels owing to its non-toxic characteristics and potential for ease of use as an alternative to traditional fossil fuels [3].
Whiskey and beer manufacturing processes generate large amounts of high strength co-products which contain high levels of chemical oxygen demand (COD), biological oxygen demand (BOD), phosphorus, ammonia, metal ions like copper and iron, as well as complex organic materials such as lignin, yeast cells, protein [4]. Due to the characteristics of these waste streams, the alcoholic beverage industry is a highly polluting industry [5]. Approximately 3.4 million tonnes of solid wastes, including spent yeast and spent grain, is produced per year in the EU, which is directed to animal feed ingredients. In addition, approximately 8–15 L aqueous waste generated per litre of malt whiskey and 3–10 L/L of beer [[4], [5], [6], [7], [8]]. Disposal of brewery and distillery wastes has been legislated for in most countries for more than 20 years [9]. In countries such as Ireland and the UK there has been a massive increase in the occurrence of small “craft” breweries and distilleries. These small or micro-breweries/distilleries, defined as such based on volume of production which is less than 1760 m3 annually, would in particular benefit from potential methods for reducing costs associated with waste treatment [10]. Fig. 1 for example provides the number of micro scale breweries in Ireland since 2012, with data referring to the number of breweries in production at approximately mid-year [11,12].
Furthermore according to Irish Whiskey Association data, the number of whiskey distilleries in operation increased from 4 to 18 between 2013 to August 2017, with 16 further planned [13].
AD is becoming more widely accepted as an efficient method to convert organic matter, in particular highly recalcitrant waste streams of distilleries/breweries, into biogas, which can significantly improve the energy balance and economics of the industry [14,15]. Anaerobic Digestion has proven to be more efficient than conventional methods as the existing waste management method for distilleries and breweries is mainly landfill applications and animal nutrition [16,17]. The establishment of anaerobic digestion plants for the treatment of high organic content wastes has undergone a major development amongst wastewater treatment facilities in Europe [15] and the application of AD in the treatment of distillery/brewery wastes is increasing.
Spent grain and yeast are the solid phase co-products of mashing and fermenting processes, which are initial and essential operations in distilleries and breweries [6], while pot ale is the main liquid phase by-product of the distillation process in whiskey production [18]. These waste streams are highly lignocellulosic, making them resistant to degradation by biological means [19]. Distillery/brewery wastes have a complex heterogeneous structure. Primarily due to the high lignin content, implementation of pre-treatments is necessary in order to obtain a higher biogas yield from AD [20]. Pre-treatments play a significant role in modifying the structure of the substrates to make them more easily degradable. Different types of pre-treatments are discussed in detail in Section 5.
The whiskey manufacturing process, outlined in Table 1 and Fig. 2, can be divided into six main steps: milling, malting, mashing, fermentation, distillation and maturation [18,21].
The manufacture of craft beer has many similarities with the initial stages of the whiskey production process. It also starts with malting and mashing steps of barley or other grains. Hops are also added to give the characteristic bitterness flavour of beer and avoid bacterial spoilage. The product of the fermentation step is then subjected to filtration and stabilization, maturated, and bottled/kegged [22].
Due to the similarities of these two processes, solid waste fractions, spent barley and spent yeast, are not much different; however, distilleries also generate massive amounts of pot ale (8.5–11.5 L per litre of malt whiskey) as a co-product of the distillation steps. Spent wash from the fermenter is also a significant liquid waste (e.g. 16–21 L per litre of grain whiskey [23]).
In a typical whiskey distillery, liquid residues left in the wash and spirit still after the distillation steps comprise the majority of the waste stream, known as pot ale and spent lees, respectively. In terms of solid waste, spent grain (also called draff) arise from the mash tun and fermenter of both distilleries and breweries [18].
Pot ale is a highly turbid, concentrated, caramelised and cumbersome liquid effluent [21]; with large discharge volumes [24]. As such, disposing of this liquid waste is a major concern for distilleries; the characteristics of which are summarized in Table 2. Pot ale has high COD and BOD contents, and contains significant levels of phosphorus and ammonia [9,18,23,24]. As copper stills are typically used in the distillation step, copper, which is toxic to micro and macroorganisms, is commonly seen in pot ale due to mass transfer between refluxing liquid and hot stills [4,21]. Pot ale is harmful especially for aquatic life because of the high level of COD/BOD leading to decreases in the level of solubilised oxygen and eutrophication [25], and due to its dark coloured nature it can block the penetration of sunlight into the receiving water, reducing the level of dissolved oxygen by restricting photosynthesis [26]. Spent lees have lower COD/BOD and contain volatile organic acids such as formic, acetic, propionic, butyric and pentanoic, which are also the intermediate products of AD [18,27].
The polluting strength of these liquid waste streams is significantly high, due to the large amounts of biodegradable organic material (sugars, lignins, hemicelluloses, dextrins, resins and organic acids) and fertilizers such as potassium, phosphorus and nitrogen [9,23,28]. The by-products/waste streams produce undesirable odours as a result of the presence of skatole, indole and other sulphur compounds [9]. Furthermore, uncontrolled land discharge of distillery and brewery waste water causes high levels of acidification. It has been shown that land discharge of distillery liquid wastes can impair seed germination [29]; potentially due to a decline in soil pH, leading to inhibition of agricultural crops. It also potentially causes leaching of protein and carbohydrates from the seeds along with a decrease in the activity level of crucial enzymes for crops growth such as alkaline phosphatase and ATPase [30].
Due to the potential hazards of the land spreading applications, environmental regulations are forcing distilleries to enhance existing treatment technologies as well as adopt new and more efficient methods for waste management. Thus, recovery of organic waste streams has become a major focus of waste management policies, with biological processes, predominantly anaerobic digestion, being seen as the main solution for high organic content wastes [15].
Major solid waste streams of distilleries and breweries consist of spent yeast and spent grain, typically termed draff when combined. Yeast cells are covered by a thick cell wall, which is formed of a complex matrix of phosphomannans, glucans, chitin and protein, and as such are not readily biodegradable [23]. Spent grain, draff, such as spent barley, spent yeast and spent hops, in breweries only, is generated in relatively large amounts [6,33]. Spent grain basically consists of kernel husk, pericarp and seed coat, which have high levels of cellulose (16.8–25.4%), hemicellulose (mostly arabinoxylans) (21.8–28.4%), lignin (11.9–27.8%), proteins and fibres. Hence, it is considered a lignocellulosic material [34,35].
In the first step of AD (hydrolysis), cellulose and hemicellulose are broken down to their monomers, however lignin limits the degradation of lignocellulosic material due to its high level of recalcitrance [36]. Spent grain is often used for animal feed (mainly for cattle) due to its both highly nutritious content and low/no cost. It is used either in wet form or as dried conventionally [37]. However, when pot ale and spent grain mixture are used, this might lead to a high level of toxicity depending upon copper level (from the distillation in copper stills) as some animals (particularly sheep) cannot metabolise copper [21]. Although cattle are tolerant of high level of copper, digesting pot ale syrup is not suitable for their diet. Pot ale can only be used as a blending material to mix with hay, straw or molasses without exceeding 10% of the total amount, thus limiting the usage of pot ale in comparison to the discharge amount [38]. As such, treatment technologies focusing on treatment of pot ale are of high importance.
This review paper presents a comprehensive investigation of whiskey and beer manufacturing processes, anaerobic digestion technology including the thermodynamics of the biochemical reactions, as well as presenting the details of the pre-treatment strategies applied to whiskey distillery and brewery wastes. The challenges to full-scale implementation of pre-treatments prior to AD as a more efficient waste management method are discussed. Research gaps and areas for future research are highlighted. Future prospects are also outlined to highlight the importance of pre-treatments in an anaerobic digestion context. This review is timely due to the proliferation in small-scale breweries and distilleries at European level currently, who could benefit greatly from improvement in facility energy management from the application of appropriate pre-treatment prior to anaerobic digestion of their wastes. The anaerobic digestion of distillery and brewery wastes has not been dealt with comprehensively in the literature to date.
Section snippets
General process description of anaerobic digestion
AD is considered as a widely accepted and well-studied technology for the treatment of organic wastes [39], appropriate for stabilizing high organic content wastes with limited environmental impact and high energy recovery potential [40,41]. It can be possible to convert a significant amount of COD (>50%) to biogas which might be used as an in-plant fuel (self-energy efficient distilleries/breweries) depending on the further purification due to the existence of impurities such as hydrogen
Reactor configurations applied to distillery/brewery waste streams
A variety of reactor configurations have been used for anaerobic digestion of whiskey distillery/brewery wastes at industrial scale (Table 4). Reactors can be categorized based on the design (vertical, horizontal, inclined), feedstock (single, co-digestion), mode of operation (batch, continuous, semi-continuous) and operating temperature (psychrophilic, mesophilic, thermophilic). Batch reactors provide better process control than continuous mode reactors [46,76]. Reactors for AD can also be run
Current application of AD technology at industrial scale for distillery wastes
Although anaerobic digestion technology is commonplace e.g. for municipal wastewater, industrial scale implementation for whiskey distilleries or breweries has not been widely utilised. Scotland is the most progressive, with several companies applying anaerobic digestion as a waste and energy management method. The Scottish Whiskey Association targets to deliver 20% of the primary energy requirements from sustainable energy sources by 2020, with a further aim of 80% by 2050 [118].
Diageo is the
Co-digestion strategy and pre-treatmens for AD of distillery/brewery waste
Co-digestion (simultaneous digestion of two or more organic matters simultaneously) has some advantages over mono digestion such as mitigation of the inhibitory effects by dilution, enhancement of the C:N ratio and the balance of nutrients and improvement of methane production kinetics, operating at a higher OLR [126]. It has been recently reported that the brewery wastes, in particular, are considered as an attractive feedstock for anaerobic co-digestion [127]. Composition and properties of
Research gaps, future prospects and practical implications
Although anaerobic digestion is a promising sustainable technology for whiskey distillery and brewery waste management, there are several obstacles that challenge scaling up to industrial level. The major challenge is considered to be the predominance of empirical methodologies in the fundamental studies of AD of whiskey distillery wastes. Moreover, the pre-treatment aspect of the process has received a very scant attention in literature to the date. The link between applicability of the
Conclusions
As outlined in this review, whiskey and beer manufacturing processes generate large amounts of organic waste. AD appears to be a sustainable treatment alternative method over conventional aerobic wastewater treatment processes and land spreading by means of converting the organic matter into methane. This can further be converted to an energy source and eventually be utilised in alcohol production. Different reactor configurations from lab scale to full scale which have been utilised for AD of
Acknowledgements
The authors gratefully acknowledge Alltech Ireland Ltd. for the financial support for this research, Project ID 15-S-ST-10568.
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