ReviewBiorefinery of spent coffee grounds waste: Viable pathway towards circular bioeconomy
Introduction
Global environmental awareness and strict legislation in recent years have focused research towards the sustainable biorefinery by utilizing waste as substrates. Wastes are identified as sources for high value-added compounds recovery in framing the circular bioeconomy (Kourmentza et al., 2018). The conceptions of bio and circular economy were familiarized in the European Union (EU) in relation with longstanding feasibility of the predominant resource having economy. The 2015 circular economy and 2012 bioeconomy approach of European Union possess interference areas such as biomass waste, food waste, and biologically originated products (Carus and Dammer, 2018). In addition, they possess ideas in general which includes chain strategy, sustainability, biorefinery and the complete waste utilization. Bioeconomy in waste management refers to economic activity involves in the conversion of the waste streams into value-added products, such as feed, biobased products and bioenergy (European Commission, 2012).
Coffee is one of the most popular beverages in world Jung et al. (2012). Consumption of coffee has been constantly increasing. The processing of coffee produces huge amounts of solid residues (wastes) in form of coffee pulp, coffee silver skin (CSS), spent coffee grounds (SCG) and coffee husk (CH), which have limited applications such as fertilizer, livestock feed, compost etc. Various processes such as fermentation, anaerobic digestion, transesterification and extraction, etc. have been adopted in SCG valorisation to produce biobased products. Implementing biorefinery methods with integrated approach can lead to the development of circular bioeconomy.
In this review, SCG waste biorefinery concept is being discussed including bioenergy production such as biodiesel, bioethanol, biogas, bio-oil etc. and value-added product recovery such as biochar, biopolymers, biosorbent, antioxidants etc. through various conversion techniques. Circular bioeconomy pathway through SCG valorisation, it’s technical challenges and economical and environmental aspects towards the pathway are also being discussed in this review.
Coffee, a renowned agricultural beverage product and world’s second most valuable trade commodity next to petroleum. Coffee production is about 16.34 billion pounds/year in the world level (Kondamudi et al., 2008). Currently, coffee consumption is over three billion cups per day at world level and commercially more than 50 countries are producing coffee (Coffee development report, 2019). Among them, Brazil is the leading coffee producer with an average yearly output of 53 million (M) 60 kg bags followed by Vietnam (28 M bags), Colombia (14 M bags), Indonesia (12 M bags) and Ethiopia (7 M bags). Fig. 1 represents the production and consumption of coffee in various countries. Coffea arabica (Arabica) and Coffea canephora (Robusta) are the two coffee species that are commercially produced. In global level, 70% of the people consume Arabica and remaining 30% of people consume Robusta (Fig. 1). Wet (washed) and dry (unwashed) are two processing methods used to convert the harvested raw cherry to green coffee bean. During wet process, pulp is removed mechanically and dried during dry process or entire coffee cherries are dried without removing the pulp. Processing of coffee bean from ‘cherry to cup’ involves many steps such as wet or dry process, milling, roasting, grinding and brewing, in which solid residues like CH, CSS and SCG are obtained (Karmee, 2018).
During dry process, CH is obtained from berries that contain cellulose (24.5%), hemicelluloses (29.7%), lignin (23.7%) and ash (6.2%) (Bekalo and Reinhardt, 2010). CSS is obtained as a by-product of the roasting process. It is a residue with high concentration of soluble dietary fiber, protein and sugars such as glucose, xylose, galactose, mannose, and arabinose (Mussatto et al., 2011). SCG, a residue or waste product obtained during brewing process. It contain huge amount of organic compounds such as fatty acids, lignin, cellulose, hemicellulose, and other polysaccharides that can be exploited as a source of value-added products. Presence of fiber content as 62.4, 60.5 and 31.9% in CSS, SCG and CH respectively showed the potential as fiber sources in the coffee waste (Janissen and Huynh, 2018).
SCG is an insoluble residue generated after milling and brewing of coffee bean. About 650 kg of SCG are generated from 1 ton of green coffee bean and about 2 kg of wet SCG are obtained from 1 kg of soluble coffee during preparation (Murthy and Naidu, 2012). Around 50% SCG is generated through soluble coffee preparation in industry and coffee shops, remaining accounts in domestic usage (Scully et al., 2016). SCG contains tannins, polyphenol and caffeine which makes it as toxic and if it dispose into the environment causes huge contamination (Cruz et al., 2012, Scully et al., 2016).
Biorefinery is a sustainable process of converting the biomass into fuels, power, and value-added chemicals through various conversion techniques and treatment methods. But implementation and development of biorefinery process is mainly depend on waste characteristics, availability and economic interest of produced product (Mata et al., 2018). In recent years, SCG gets more attention as a promising raw material for various processes and for its conversion into high value products through biorefinery process.
Biorefinery through a series of integrated conversion technologies, aim to maximize the end product value generated from the biomass. This integration system involves various processes such as extraction, transesterification, hydrolysis, fermentation and pyrolysis which convert the SCG into useful biofuels namely; biogas, bioethanol, bio-oil, biodiesel, hydrocarbon fuel, and fuel pellets and valuable added products namely; adsorbents, bioactive compounds, biochar, compost, glycerine carotenoids and polyhydroxyalkanoates (PHAs) and extraction of phenolic compound, caffeine, tannins and antioxidants etc with zero waste Ribeiro et al. (2013) reported that lipids extracted from SCG can be used for the production of cosmetics. The following sections will highlight all recycling potentials of SCG through integrated methods to both biofuels and value-added products. In this review, the SCG biorefinery route can be divided into three phases. First approach dealt with the biofuel and bioenergy production from SCG and it is covered in section 3. Second approach dealt with the value added product from SCG and it is covered in section 4. Third approach dealt with the technoeconomic analysis of SCG biorefinery and it is covered in section 8&9.
Section snippets
Composition & characterization of SCG
The characteristics of SCG plays a key role in SCG biorefinery process. Composition of SCG are as follows: (i) Elemental compositions are carbon – 56.1%, hydrogen – 7.2%, nitrogen – 2.4%, sulphur – 0.14% and calcium – 0.20% (Vardon et al., 2013). (ii) Chemical compositions are protein – 13.7 (mg/kg), cellulose – 8.6 – 15.3 (mg/kg), hemicellulose – 36.7 (mg/kg), total sugar – 8.5 (mg/kg), lipids – 15 (mg/kg), ash – 3%, C/N ratio – 17:1 and lignin – 33.6% (Caetano et al., 2012). (iii) Mineral
SCG biorefinery for biofuel and bioenergy production
SCG contain high quantity of lignocellulose and lipids. Therefore, it is a potential feedstock for biofuel such as bioethanol and biodiesel. Fig. 2 represents the various integrated biorefinery routes of biofuel production. Waste or residue generated during biofuel production can be used as a feedstock for generation of fuel pellets and compost. Bio-oil is extracted during biodiesel production and biochar is derived from the oil extracted waste of SCG through pyrolysis. Due to its high
SCG biorefinery for value added products recovery
The organic compounds in SCG can be used as a major source for the production of value-added products. Fig. 3 represents the various integrated biorefinery routes of value-added products recovery. The value added products such as biopolymer, caffeine, biosorbents, biochar, carbon material, antioxidants were produces due to its capability of exploiting the organic fractions of SCG in different pathways. Table 2 represents the different approaches for deriving value-added products from SCG.
SCG biorefinery approach – its design, scheme & structure
About 2 billion tons of SCG and coffee skin were produced as by product from coffee industry. The biorefinery approaches mainly aim at the proper designing of system to solve the issues related to management and the valorization of waste (SCG) in circular economical way. The techno-economic analysis is the critical parameter for the designing of process scale and to analyse the sustainability base on the cost. Before designing the SCG biorefinery, the physico-chemical characteristics of the
Integrated biorefineries of SCG
Integrated biorefinery is the concept of complete utilization of substrate to produce bio-based product such as biochemical, biofuel and bioenergy. Integrated biorefinery of SCG, remarkably reduce economic and environmental impact. Recently research towards complete utilization of SCG biomass has increased globally. The fresh SCG contains excess amount of fat in its composition. It is essential to remove the fat from SCG for effective production of various bio-based products. SCG oil with high
Challenges in integrated biorefinery of SCG
SCG is the leading waste generated throughout the world. Still most of the countries practices conventional landfill for SCG management, which is unprofitable. The landfilling of SCG leads to emission of methane gas, which causes negative effect on environment. SCG mainly consist of sugar compounds, which required proper system for disposal. SCG management by individual approaches result in recovery of valued-added by-products and it cause certain negative impacts on environment. For example
SCG biorefinery strategy- circular bioeconomy
The biorefinery concepts have been the emerging technology for the transformation of different waste by multiple process (Moncada et al., 2016). The bioeconomy is a strategy where the resources have been recycled to get multiple products. The scalability is one of the major issue due to incompleteness of micro-economy of coffee since the collection of SCG needs space for storage and the additional facilities for production and many other logistic things. The circular bioeconomy is applied to
Energy, economic and environmental assessment
For a successful implementation of biorefinery at commercial level, energy, economic and environmental assessment of the process is essential. Energy assessment includes the amount of energy spent and gained to recover various bioenergy from waste biomass (Banu et al., 2018a, Kannah et al., 2017a, Kavitha et al., 2019a). The net energy (Enet) is referred as difference between total energy spent (input) and total energy gained (output) (Banu et al., 2018b, Kannah et al., 2019a, Kavitha et al.,
Business models and market outlooks
It is clear, there is a need of design protocol for selection of economically viable integrated biorefinery approach which increase the global market demand and overcome operational and technical issues. A proper methodology is required for design, optimization and execution of integrated biorefinery. While planning methodology for cost –effective integrated biorefinery the following aspects to be considered such as i) feedstock availability and production cost, ii) technologies implementation,
Conclusion
This review mainly pinpointed the SCG biorefinery which is a main food waste worldwide. It is a nutrient rich excellent resource so that it can be exploited for biofuel and multiple value-added products recovery. SCG based integrated biorefinery is found to be economically feasible approach as they increase the biomass use and biofuel- multiple products recovery. In addition, since SCG based biorefinery contributes more to sustainable and circular bioeconomy, it has increased possibilities
CRediT authorship contribution statement
J. Rajesh Banu: Supervision, Methodology. S. Kavitha: Conceptualization, Writing - review & editing, Data curation. R. Yukesh Kannah: Writing - original draft. M. Dinesh Kumar: Writing - original draft. Preethi: Writing - original draft. A.E. Atabani: Resources. Gopalakrishnan Kumar: Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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