1 INTERNATIONAL JOURNAL OF CHEMICAL REACTOR ENGINEERING V o l u m e Article A78 Bioenergy II: Bio-Ethanol from Municipal Solid W a s t e (MSW): The UK P o t e n t i a l and Implication f o r Sustainable Energy and W a s t e Management Aiduan Li Majeda Khraisheh University College London, Qatar University, ISSN
2 Bioenergy II: Bio-Ethanol from Municipal Solid Waste (MSW): The UK Potential and Implication for Sustainable Energy and Waste Management Aiduan Li and Majeda Khraisheh Abstract This paper investigates the feasibility of using municipal solid waste (MSW) as biomass substrates for bioethanol production. MSW are categorised into three types: paper and card, kitchen organics, and green organics. MSW data are collected from UK Department of Environment, food and rural affairs (DEFRA). Characterisations of cellulose, hemicellulose, and lignin contents have shown that MSW has high potential as biomass source. Experimental work on waste characterisation and MSW-to-ethanol conversion has been carried out in laboratory. The experimental results have shown that more than 85% of the cellulose from the waste can be converted to glucose which can be easily fermented to ethanol production. This MSW-to-ethanol process provides an alternative solution for both biomass resources for cost reduction and preventing organic fraction of municipal solid waste going into landfill. Projected estimates of waste to ethanol bases on London (UK) as a case study are made. Recommendation on both sustainable waste management and biofuel production are developed based on the result findings. KEYWORDS: municipal solid waste, ethanol, sustainability, UK The authors acknowledge the financial support from RCUK Dorothy Hodgkin Postgraduate Award and UCL Graduate School Fellowship.
3 Li and Khraisheh: Bio-Ethanol from Municipal Solid Waste 1 Introduction The drivers for the introduction of renewable transportation fuels (RTFs) into the UK are to: 1_ reduce transport sector dependency on non-renewable fuels; 2_ reduce Green House Gas (GHG) emissions from transportation fuel chains; 3_ reduce the impact of air quality and health for transportation use; 4_ improve energy security in the transport sector; and 5_ contribution to rural development through domestic production of biomass-based fuels (DTI, 2006). Moreover, the EU Biofuel Directive (2003/30/EC) requires from all EU Member States a minimum proportion of transport biofuels or other renewable fuels to be sold on their markets, with a target of 5.75% by 2010 and 10% by 2020 (DEFRA, 2007a). In 2008 the UK introduces the Renewable Transport Fuels Obligation (RTFO) that requires suppliers of road transport fuels to source increasing percentages of fuel from renewable sources. This starts at 2.5% in 2008 and will increase to 5% by volume in 2010 (DTI, 2006). The use of products such as corns to produce biofuel places considerable stress on the food market, and can no longer be regarded as a sustainable option. Instead, using non-food products to produce second generation biofuel should be preferred; lignocellulosic materials can for example be derived from forestry, such as plants, and residues, such as agricultural waste and municipal solid waste, without impacting on the global food production system. In particular, municipal solid waste (MSW), as a suitable lignocellulosic material, and the advantage of being available locally and in large quantity, has emerged as a promising biomass source: Previous research (Li, 2008) has shown that MSW is comparable with other lignocellulosic materials such as plants: it contains about 40-60% of cellulose, 20-40% of hemicellulose and 10-20% of lignin and other small contents; According to DEFRA, 36 million tonnes of MSW were collected in 2004/05 in the UK, among which 65% are biodegradable and could be used for fuel production. The practicality of extracting organic waste from MSW is already the focus of collection programmes such as those launched by Waste and Resources Action Programme (WRAP); Furthermore, diverting waste as resources for energy and fuel production would be financially attractive way to meet the target of the EU Landfill Directive (1999/31/EC). The directive imposes a reduction of the amount of biodegradable waste to landfill by 65% by 2016, compared to the level in 1995.
4 2 International Journal of Chemical Reactor Engineering Vol. 7 , Article A78 In order to follow the European Union Directives for energy demand and waste reduction, it is required new strategies for the waste management. A potential strategy is the bioconversion of lignocellulosic municipal solid waste to bio-ethanol. Although there are a number of studies regarding the bioconversion of lignocellulosic biomass, the studies for municipal solid waste is limited. London produces 18 million tonnes of waste every year and municipal solid waste is a large amount of 4.2 million tonnes. Municipal solid waste is collected by London Boroughs (UK) for the year (DEFRA, 2007a). Municipal solid waste can include recyclable paper, garden paper, other plastics, compostable food waste, unclassified fines, card, paper, packaging, noncompostable organics, textiles and shoes, glass bottles/jars, nappies, steel cans, other metals, plastic bottles, wood, aluminium, and other glass (DEFRA, 2007b). Table 1 shows the total amount of municipal solid waste from the period to and the sub-categories of each source. Table 1: Total amount of municipal solid waste in London from to , the numbers are in thousands (Source: DEFRA, 2007c) London Household waste from: 2000/ / / / / /06 Regular household collection 2,231 2,262 2,216 2,201 2,081 2,112 Other household sources Civic amenity sites Household recycling Total household 3,390 3,408 3,379 3,331 3,297 3,326 Non household sources (excl. recycling) 1, , Non household recycling Total municipal waste 4,438 4,438 4,446 4,342 4,370 4,213 Most research on characterization of this waste focus on the primary biomass such as agricultural crops or paper sludge. However, little information can be obtained on waste biomass especially on organic waste. The main objective of this study is to consider the suitability of the MSW produced in the UK, especially London, for the use as a source of biofuel production, in specific the production of ethanol. In this particular paper, analysis of the waste streams, the characterisation of waste as biomass sources and chemical compositions of the selected samples were carried out. This study shows the potential of using BMSW as an important biomass source for biofuel production: a choice motivated by the availability and sustainability of this biomass, but also by its potential of simultaneously solving waste management problems.
5 Li and Khraisheh: Bio-Ethanol from Municipal Solid Waste 3 Materials and methods Chemical composition analysis Cellulose, hemicellulose and lignin make up a major portion of selected waste biomass samples. These constituents must be measured as part of a comprehensive biomass analysis; Carbohydrates can be structural or nonstructural. Structural carbohydrates are bound in the matrix of the biomass, while non-structural carbohydrates can be removed using extraction or washing steps. Lignin is a complex phenolic polymer. The determination of cellulose content, hemicellulose content and lignin content are followed the procedures of determination of structural carbohydrates and lignin in biomass provided by National Renewable Energy Laboratory (Sluiter et al., 2008). Waste-to-ethanol conversion process A typical BMSW-to-ethanol process is shown in Figure 1. It includes biomass, pre-treatment, hydrolysis, fermentation, ethanol recovery ad waste treatment. The BMSW used in this study was selected from typical biodegradable waste, such as kitchen waste, garden waste and paper waste. The sample consists of 20% carrot peelings, 20% potato peelings, 20% grass, 20% newspaper, and 20% scrap paper. All selected waste was milled to small particles with size of 0.2mm-1.2mm, and then followed by prehydrolysis with sulphuric acid and steam treatment as reported in previous paper (Li et al., 2007). MSW biomass Pretreatment Hydrolysis Fermentation Waste treatment Ethanol recovery Ethanol production Figure 1 Bio-ethanol conversion processes
6 4 International Journal of Chemical Reactor Engineering Vol. 7 , Article A78 Calculation In order to estimate ethanol production from MSW, it is necessary to assess both the availability and quality of waste biomass. Sufficient quantity of feedstock is required to enable bioethanol production at an economically viable scale. Availability is influenced by current and future waste management options, the waste management system, and the national and local political policy, and external factors such as the climate or cultural influences on waste generation. The quality of MSW is represented by the chemical composition of feedstock (i.e. cellulose, hemicelluloses and lignin) which shows the potential of ethanol production. MSW proportion estimation Current best MSW data is from DEFRA (2007 b&c), who have consolidated estimates from a number of studies (ranging from 2002 to 2005) and made a number of assumptions regarding less well surveyed sub-streams to form an overall estimate for the BMSW fractions. A project supported by the Greater London Authority (GLA) investigated how factors affecting waste composition may influence the waste composition in London and developed a model which enables an approximation of household waste composition by ethnicity and type of property (GLA, 2007). This model was used to estimate the BMSW composition of London. Theoretical ethanol yield estimation As shown in Figure 1, waste biomass is converted to ethanol via enzymatic hydrolysis and fermentation. Hence, the estimation is based on both processes theoretical yield. Glucose yield as percentage of the theoretical yield [percentage digestibility, obtained from the equation which involves the transfer of cellulose to sugar ((C 6 H 10 O 5 )n + nh 2 O = (C 6 H 12 O 6 )n) was computed by using the formula given by the National Renewable Energy Laboratory (Standard Biomass Analytical Procedures) (Sluiter et al., 2008). Ethanol yield is calculated based on the equation reported by DOE (2007).
7 Li and Khraisheh: Bio-Ethanol from Municipal Solid Waste 5 Results and Discussion Waste biomass availability The biodegradable fraction is itself a composition of many materials which can be classified into three main fractions. This paper categorises different type of BMSW into three groups: kitchen organic waste (KOW), green organic waste (GOW) and paper & card waste (PCW). The following is a detailed definition of these: KOW, is the entire mixed composition of kitchen organic waste. GOW, is the entire mixed composition of green organic waste. PCW, is the entire mixed composition of paper & card waste. BMSW: is assumed to be a mixed composition of PCW, KOW, and GOW. These waste mentioned above are heterogeneous feedstock, both in physical and biochemical composition. A number of studies have been undertaken to determine the composition of MSW in the UK, but few have addressed the entire MSW stream (Burnley, 2007). The following brings together this limited research to provide the best available information on the potential for bioethanol production and considerations for conversion systems. To have a good reflection on the availability of this waste, the following paragraphs are a discussion on the available information of this waste in the UK as a whole and in London as a case study. The BMSW accounts of an estimated 54.5% in England and 57.4% in London of the total wet weight of all MSW (Table 2). GOW are estimated to be significantly lower in London compared to the rest of the UK. This can be concluded that this is due to a smaller number of people per household and a higher proportion of properties with little or no garden. Other, smaller fractions which will not be evaluated in this study are wood, fines and textiles. Composition data for England is considered of limited and poor quality and a comprehensive government survey is expected to improve this situation in the near term (Defra, 2007d). KOW accounts for approximately 26% of all MSW arising in London. However, KOW waste is very difficult to analyse as its individual materials are difficult to separate after it has been collected. The analysis of this work is based on the few studies available. Potato peelings and carrot peelings are representatives of uncooked fruit and vegetables which constitute the majority of KOW waste. With the lack of information on the waste stream composition a study of food consumption in the London region may provide an insight to the expected subsequent waste composition.
8 6 International Journal of Chemical Reactor Engineering Vol. 7 , Article A78 Table 2 Estimated proportions of MSW fractional arisings (% non-dry weight). Waste Fraction England Arisings Proportion of MSW steam London 2003/04 MSW arisings London Waste Streams Household waste All Recycling Civic amenity sites Other waste streams PCW KOW GOW BMSW Waste biomass quality Waste biomass consists of fibers composed of three major structural components: cellulose and hemicellulose (polysaccharides), and lignin (see table 5). The cellulose and hemicellulose content determines the maximum theoretical ethanol yield and their physical and chemical relationship has a large influence on the conversion process. There are also typically much smaller amounts of ash, soluble phenolics and fatty acids termed extractives, and other minor components. The chemical composition analyses were based on 10 grams dry weight waste biomass. These included cellulose, hemicellulose, lignin (which includes acid insoluble lignin (AIL) and acid soluble lignin (ASL)) and ash content. The results are shown in Table 3. Table 3 shows the characterisation results obtained from laboratory following methods reported (Li et al., 2007). It is obvious that highest cellulose content is found in PCW 53.99%, followed by BMSW 36.21%, KOW 32.13% and GOW 22.50%. However, the largest amount of hemicellulose is found in GOW with39.59%% and the smallest in PSW with 15.60%. The hemicellulose content indicates how much by-product (xylose, galactose, mannose and arabionose) will be produced after hydrolysis process. Highest lignin content are found in KOW (AIL 20.48% plus ASL 13.01%), which means the biomass of KOW will encounter the most difficulty to be broken down during the pretreatment process within the selected samples as the main purpose of pretreatment is to remove lignin in biomass. PSW with the lowest lignin (AIL 7.12% plus ASL 12.80%) is expected to provide easier access to the cellulose content in the material. ASL, acid soluble lignin is found higher than AIL (acid insoluboe
9 Li and Khraisheh: Bio-Ethanol from Municipal Solid Waste 7 lignin) in each type of biomass. This indicates that acid pre-treatment method will be able to remove lignin for more than 50% at maximum extent, which means acid treatment can not break down the lignin structure completely. The last column in Table 3 shows the sum of components contents of each biomass. Table 3: Raw material composition on dry weight basis (105 C) Biomass Cellulose AIL ASL Hemicellulose Ash Total (%) (%) (%) (%) (%) content (%) KOW ±2.54 GOW ±2.51 PSW BMSW ± ±2.12 As a biomass sources, kitchen waste provides about 86% biodegradable matters. As kitchen waste accounts for about a quarter of total waste, it provides a very good biodegradable source. However, the high portion of lignin existing in the biomass requires some particular pretreatment methods in order to obtain good ethanol conversion rate. This is because lignin together with hemicelluloses creats a protective sheath around the cellulose making the cellulose difficult for the enzymes to access. The higher lignin content of GOW implies the need for greater pretreatment to separate the cellulose and hemicellulose components. Most PCW materials are chemically processed to remove lignin from mixtures of hardwoods and softwoods. There are exceptions to this, most notably newsprint that is primarily mechanically processed spruce and pine, and therefore has the same composition on a dry-weight basis as native wood (Wyman, 1996). The pulping process alters the biomass structure of paper and card and a number of sources indicate that this could improve the conversion process and reduce the need for pretreatment (Rivers and Emert, 1988; Clanet et al., 1998). Enzymatic hydrolysis of separated paper has been shown to convert quickly and nearly completely to bioethanol and the lower levels of five carbon sugars reduce the dependence on higher hemicellulose hydrolysis efficiency (Dale and Musgrove, 2007). Different pretreatment methods have been shown to improve the conversion process for different PCW materials and an optimal process would have to be developed based on site-specific samples (Rivers and Emert, 1988).
10 8 International Journal of Chemical Reactor Engineering Vol. 7 , Article A78 Projected estimates of waste-to-ethanol MSW, as one of the promising biomass sources has the potential benefits of replacing primary biomass sources. By analysing the available national MSW data, about 60% of MSW generated is biodegradable and readily available in London. Moreover, the requirement of EU landfill directives and current national waste management strategy requires the alternative solution of BMSW disposal. Hence, using BMSW as biomass source for ethanol production, it will not only bring the economic benefits of fuel production but also prevent the pollution from waste to the environment. Based on the case study in London, the analysis of BMSW biochemical quality shows that KOW (accounting for 26% of total MSW) has a theoretical ethanol yield of 363 L/dry tonne; GOW (accounting for 8% of all MSW theoretically) can yield 420 L/dry tonne; and PCW (accounting for 23.6% of MSW generated) has a theoretical yield of 505 L/dry tonne. Waste management varies from developing nations to developed countries. Under different economic and social environment, different types of waste management methods from reuse, recycle, and energy/material recovery to disposal etc are used. With this analysis, it is obvious that the large amount of BMSW has the potential of becoming bioethanol sources. With the both potential economic viables and environmental benefits, further investigation is necessary. If the entire projected potential of around 1.6 million tonnes of BMSW for new recovery processes was made available for ethanol production, an estimate 346 million liters could be produced by Approximately 2,378 million liters of petrol were consumed by cars in London in Therefore, bioethanol production could replace a maximum of 14.6% of the London 2004 petrol car needs by This could either be provided in a neat form or a more favourable entry strategy could be to blend all petrol sold in London at 5-10% and avoid the need for new vehicles, vehicle modification, or infrastructure changes. This evaluation should however be considered rough, and further consideration should to be made to a wider range of performance criteria and system requirements. Recommendation for sustainable energy and waste management With this analysis, it is obvious that the large amount of BMSW has the potential of becoming bioethanol sources. With the both potential economic viables and environmental benefits, further investigation is necessary if bioethanol production from BMSW to be successful. Increased source segregation and utilisation of materials on the basis of Best Practicable Environmental Option will be critical in
11 Li and Khraisheh: Bio-Ethanol from Municipal Solid Waste 9 establishing a viable market share of the waste resources as the number of competing waste management options increases. Moreover, to realise the potential for bioethanol production requires performance across the lifecycle of the fuel chain against other fuels to be competitive within the end-product (biofuel and wider transportation fuel) market and the feedstock (waste resource) market. Net energy balance, greenhouse gas balance, and production costs are fundamental indicators for sustainable performance. There is a lack of published research examining this in regards to MSW-bioethanol by biochemical processes. Biofuel production from UK domestic biomass could make a significant contribution to UK road transport fuel needs and requirements under the EU Biofuels Directive. Waste biomass is a significant resource which is currently available. MSW presents a good opportunity, being a low-cost and available in significant quantities. London s MSW accounted for around 13% (4.2Mt) of the total MSW waste produced in England in 2005/06 and represents one of the greatest potential resources for bioethanol production in England. This BMSW-toethanol process provides an alternative solution for both biomass resources for cost reduction and preventing organic fraction of municipal solid waste going into landfill. However, for bioethanol production to be successful it will requires integration with a well developed waste management system. Increased source segregation and utilisation of materials on the basis of Best Practicable Environmental Option will be critical in establishing a viable market share of the waste resources as the number of competing waste management options increases. References Burnley, S.J. Ellis, J.C.; Flowerdew, R.; Poll, A.J.; Prosser, H Assessing the composition of municipal solid waste in Wales. Resources, Conservation and Recycling 49(3): Clanet, M.; Durand, H.; Tiraby, G Enzymatic Saccharification of Municipal Wastes. Biotechnology and Bioengineering 32: Dale C. M.; Musgrove D Continuous Conversion of MSW-derived Waste Paper to Bio-Ethanol Using a 100L 6-stage Continuous Stirred Reactor Separator. AIChE Fall : Food, Pharmaceutical & Bioengineering Division Department for Environment, Food and Rural Affairs (DEFRA). 2007a. UK Biomass Strategy
12 10 International Journal of Chemical Reactor Engineering Vol. 7 , Article A78 Department for Environment, Food and Rural Affairs (Defra). 2007b. Municipal Waste Management Statistics Department for Environment, Food and Rural Affairs (Defra). 2007c. Waste Composition Analysis, Guidance for Local Authorities Department for Environment, Food and Rural Affairs (Defra). 2007d. Waste Strategy for England 2007 Department of Energy (DOE) Energy Efficiency and Renewable Energy. Information Resources, Theoretical Ethanol Yield Calculator. Department of Trade and Industry (DTI). (2006) The Energy Challenge, Energy Review Report 2006, Chapter 6: Transport. European Commission (EC). Directive 2003/30/EC of the European Parliament and of the Council of 8 May 2003 on the promotion of the use of biofuels or other renewable Greater London Authority. Waste Composition Model Li, A., Antizar-Ladislao, B. and Khraisheh, M Bioconversion of municipal solid waste to glucose for bio-ethanol production. Bioprocess and Biosystems Engineering 30 (3): Li, A Bioconversion of biodegradable municipal solid waste to glucose for bioethanol production. PhD thesis, University of London Rivers, D.B.; Emert, G.H Factors Affecting the Enzymatic Hydrolysis of Municipal-Solid- Waste Components. Biotechnology and Bioengineering 31: Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., and Crocker, D. (2008) Determination of Structural Carbohydrates and Lignin in Biomass. National Renewable Energy Laboratory (NREL) Laboratory Analytical Procedure (LAP). Wyman C Handbook On Bioethanol: Production And Utilization: Production & Utilization. London: Taylor & Francis