**2. Bio-drying process**

solid waste (MSW) originates from biogenic matter both in developed and developing countries. However, the disposal of the organic fraction of waste in landfill has dire consequences on the environment including the generation of methane, which can pose a threat or contribute to the greenhouse effect. Some landfills have sought to collect methane, which may be used for fuel; nonetheless, the conversion to methane takes place on long time scales, wastes much of the internal energy of the waste, and is rather ineffective in recovering much of the

The search for sustainable solutions for biodegradable waste management represents a challenge not only for the waste management sector but also for the agricultural and industrial sectors. The enormity of this problem intertwined with the aforementioned issues associated with landfilling led to the introduction of the Landfill Directive of 1999 by the European Union (EU). According to the Landfill European Directive 1999/31/EC, member states are required to only landfill wastes that have been preliminary subjected to treatment or require a phased reduction in the amount of biodegradable waste disposed of to landfill [1]. Biodegradable waste refers to any waste that is capable of undergoing anaerobic or aerobic decomposition, such as food and garden waste, and paper and paperboard [2]. Similarly, the Energy Information Administration (EIA) of the Environmental Protection Agency (EPA) of the United States defines biodegradable/biogenic waste as any waste produced by biological processes of living organisms. Based on the definition by the EU and inter alia [3, 4], it is clearly that the concept of biodegradable waste is wide and regards not only the production of food waste at household level; however, it includes all agricultural waste. The UNEP estimates that the decay of organic proportion of municipal solid waste contributes about 5% of global Greenhouse Gas (GHG) emissions annually [5]. In curbing this menace, a number of technologies for waste treatment such as composting (organic fertilizer), landfilling, anaerobic digestion and thermal methods have been developed [6]. However, the implementation of some of these techniques has been hindered due to the high implementation costs and other related environmental concerns.

By virtue of these concerns and in line with the new European Union Landfill Directive (1999/31/EC), this has motivated research into the development of technologies to reduce the impact associated with landfilling of waste [7–9]. Consequently, composting has been identified as an alternative method for transforming the organic fraction of waste into a potentially safe, stable and sanitary product that can be used as a soil amendment or an organic fertilizer [10]. Nonetheless, high operational cost, low quality of final product and long residence time (30-50 days) associated with composting have hindered wider application of this technology

Energy from the biogenic part of waste is considered as one of a number of options that either have the greatest potential to help in a cost effective and sustainable way in waste management. Although, energy recovery may not be the first option according to the waste hierarchy, this option becomes paramount when the material is generated and considered as waste [13]. The EU directive categorized waste incineration either as a disposal or energy recovery technology depending on the energy efficiency of the incineration plant [14]. Thus, the operation and design of the aforementioned process highly require the knowledge of its thermal properties or the biogenic fraction of the waste. The carbon stored in waste originated

available energy content of the waste.

90 Agricultural Waste and Residues

as inept for waste treatment [11, 12].

Bio-drying, a concept similar to composting, aims at removing or reducing water from biodegradable waste with high water content and increasing the treatability and subsequent utilization value of the bio-dried material. In other words, it is the utilization of the heat released during the decomposition of biodegradable waste in order to reduce the moisture content and partially stabilize the waste. The removal or reduction of moisture contents in bio-drying process involves evaporation of liquid water through aerobic decomposition of the organic material or reduction of water vapor via aeration [17–21]. This mechanism is accomplished by relying on microorganisms, both bacteria and fungi to biologically degrade the organic component in order to reduce the moisture content while maintaining the energy content [22]. Therefore, metabolic heat production, air convection and molecular diffusion of oxygen and water vapor are the main mechanisms involved in water removal from wet wastes under bio-drying [23].

biodegradation of the organic component during bio-drying process. Despite the fact this technology is considered as a zero leachate approach, it is likely that a limited amount of free water may seep through the waste matrix and collected at the bottom of the bioreactor as leachate [11]. Bio-drying has mostly been studied for MSW (municipal solid waste) [24, 26–29], pulp and paper [23, 30, 31] and, garbage residues and sewage sludge [32–34] with 50–70% as the optimal initial moisture content range for bio-drying process [12, 28, 34]. The initial moisture content is important because microbial activity is impeded due to high initial moisture content favoring anaerobic conditions because water rather than air fills pore space limiting oxygen transport within the matrix, whereas low moisture content slows down the activity of the microorganisms resulting in reduce bio-drying performance. Conversely, if initial moisture content is low, microbial activity is slowed due to insufficient moisture which could results in reduced drying performance. It is suggested that, in order to improve the water content reduction and accelerated biodegradation of MSW with high water content, supplemented a hydrolytic stage prior to aerobic degradation and inoculated the biomass with the bio-drying products as leachate [29]. However, the concept of bio-drying has not been fully understood with regards to bio-drying of organic waste of high

Bio-Drying of Biodegradable Waste for Use as Solid Fuel: A Sustainable Approach for Green…

http://dx.doi.org/10.5772/intechopen.77957

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moisture content including food waste [11, 25], leaving a research gap to be filled.

bio-drying, affecting the temperature and improving the water removal [38].

*2.1.2. Air-flow rate*

used (120 m3

Most organic wastes like dewatered sewage sludge, food waste and garden waste contain abundant water with a typical moisture content around 80% or higher, and this excessive moisture affects particle aggregation, causes packing and reduces void space, which all prevents efficient air movement throughout the matrix and limits aerobic decomposition [35, 36].

According to literature, it has been established that air-flow rate is the main operational parameter used both in laboratory and commercial applications for process control in biodrying process. The air-flow rate has a direct influence on the matrix temperature and drying efficiency. The effect of air-flow rate on bio-drying has recently been studied extensively by several researchers. On the one hand, a higher air flow rate leads to higher heat loss, resulting in a decrease in the matrix temperature, which is unfavorable for water evaporation. On the other hand, an increase in the airflow rate will also increase the amount of water carried, improving the water loss. Adani et al. [26] and Roy [37] established that high air-flow rate contributes to effective and fast drying, and high calorific value. In another study, the simultaneous effect of initial moisture content and airflow rate on bio-drying of sewage sludge was investigated, and the results revealed that initial moisture content has a stronger effect on

Skourides et al. [39] investigated the agitated bio-drying of the organic fraction of municipal solid and the results showed maximum drying rate achieved for the highest aeration rates

w/w) with a short retention time of less than 7 days. In a similar study to investigate the effect of air-flow on the bio-drying of gardening wastes, it was found that higher air-flow rate corresponds to greater weight loss (40–57% weight loss) and leachate production at low air-flow. Even though higher air-flow rate causes higher water removal, it was further stressed that it is imperative to identify the optimal air-flow rate for bio-drying, since excessively high air-flow

/h), leading to lower final moisture content levels (20% w/w from an initial 40%

The importance of bio-drying process of waste includes:


Compared to traditional composting process, the essential distinguish feature of bio-drying is the application of a higher ventilation rate to reduce moisture content by using the heat generated during the aerobic degradation process as well as forced aeration [24]. Also, the output from composting is stabilized organic material whereas that of bio-drying is partially stabilized. Bio-drying also has added advantage of pre-treating the waste at the lowest possible retention time to produce a high quality solid fuel. Furthermore, bio-drying process tends to increase the energy content of the bio-dried material by reducing the moisture content in the waste matrix and preserving most of the calorific value or energy content of the organic matter present through minimal biodegradation [25]. Besides these benefits, bio-drying process also renders the output material more suitable for short-term storage and lessens the transportation cost by reducing its weight via moisture loss and partially biostabilizing it. In contrast, composting is used to stabilize the biodegradable organic material of waste prior to landfill disposal, minimizing leachate and landfill gas generation. It is also used to produce humus-like compost that can beneficially and safely apply to land. The difference between composting and bio-drying also depends on the control parameters including temperature, oxygen content, air flow rate, and moisture content. In order to ensure high degradation performance for the former, the temperature, oxygen concentration, and moisture content should be kept within an optimal range whereas for the latter, the process should be managed to accelerate drying and to reduce organic matter degradation.

#### **2.1. Factors affecting bio-drying process**

#### *2.1.1. Moisture content*

Moisture content of the waste is considered as a single critical parameter for evaluating the efficiency of bio-drying process. The moisture content influences microbial activity and biodegradation of the organic component during bio-drying process. Despite the fact this technology is considered as a zero leachate approach, it is likely that a limited amount of free water may seep through the waste matrix and collected at the bottom of the bioreactor as leachate [11]. Bio-drying has mostly been studied for MSW (municipal solid waste) [24, 26–29], pulp and paper [23, 30, 31] and, garbage residues and sewage sludge [32–34] with 50–70% as the optimal initial moisture content range for bio-drying process [12, 28, 34]. The initial moisture content is important because microbial activity is impeded due to high initial moisture content favoring anaerobic conditions because water rather than air fills pore space limiting oxygen transport within the matrix, whereas low moisture content slows down the activity of the microorganisms resulting in reduce bio-drying performance. Conversely, if initial moisture content is low, microbial activity is slowed due to insufficient moisture which could results in reduced drying performance. It is suggested that, in order to improve the water content reduction and accelerated biodegradation of MSW with high water content, supplemented a hydrolytic stage prior to aerobic degradation and inoculated the biomass with the bio-drying products as leachate [29]. However, the concept of bio-drying has not been fully understood with regards to bio-drying of organic waste of high moisture content including food waste [11, 25], leaving a research gap to be filled.

Most organic wastes like dewatered sewage sludge, food waste and garden waste contain abundant water with a typical moisture content around 80% or higher, and this excessive moisture affects particle aggregation, causes packing and reduces void space, which all prevents efficient air movement throughout the matrix and limits aerobic decomposition [35, 36].

### *2.1.2. Air-flow rate*

involves evaporation of liquid water through aerobic decomposition of the organic material or reduction of water vapor via aeration [17–21]. This mechanism is accomplished by relying on microorganisms, both bacteria and fungi to biologically degrade the organic component in order to reduce the moisture content while maintaining the energy content [22]. Therefore, metabolic heat production, air convection and molecular diffusion of oxygen and water vapor are the main

Compared to traditional composting process, the essential distinguish feature of bio-drying is the application of a higher ventilation rate to reduce moisture content by using the heat generated during the aerobic degradation process as well as forced aeration [24]. Also, the output from composting is stabilized organic material whereas that of bio-drying is partially stabilized. Bio-drying also has added advantage of pre-treating the waste at the lowest possible retention time to produce a high quality solid fuel. Furthermore, bio-drying process tends to increase the energy content of the bio-dried material by reducing the moisture content in the waste matrix and preserving most of the calorific value or energy content of the organic matter present through minimal biodegradation [25]. Besides these benefits, bio-drying process also renders the output material more suitable for short-term storage and lessens the transportation cost by reducing its weight via moisture loss and partially biostabilizing it. In contrast, composting is used to stabilize the biodegradable organic material of waste prior to landfill disposal, minimizing leachate and landfill gas generation. It is also used to produce humus-like compost that can beneficially and safely apply to land. The difference between composting and bio-drying also depends on the control parameters including temperature, oxygen content, air flow rate, and moisture content. In order to ensure high degradation performance for the former, the temperature, oxygen concentration, and moisture content should be kept within an optimal range whereas for the latter, the process should be managed to accelerate drying and to reduce organic matter degradation.

Moisture content of the waste is considered as a single critical parameter for evaluating the efficiency of bio-drying process. The moisture content influences microbial activity and

mechanisms involved in water removal from wet wastes under bio-drying [23].

The importance of bio-drying process of waste includes:

• Pre-treatment

• Short residence time • Partial biostabilization

92 Agricultural Waste and Residues

• Increasing energy content

• High quality solid fuel production

• Reduce green house emissions

• Reduce volume of waste to be landfilled

**2.1. Factors affecting bio-drying process**

*2.1.1. Moisture content*

According to literature, it has been established that air-flow rate is the main operational parameter used both in laboratory and commercial applications for process control in biodrying process. The air-flow rate has a direct influence on the matrix temperature and drying efficiency. The effect of air-flow rate on bio-drying has recently been studied extensively by several researchers. On the one hand, a higher air flow rate leads to higher heat loss, resulting in a decrease in the matrix temperature, which is unfavorable for water evaporation. On the other hand, an increase in the airflow rate will also increase the amount of water carried, improving the water loss. Adani et al. [26] and Roy [37] established that high air-flow rate contributes to effective and fast drying, and high calorific value. In another study, the simultaneous effect of initial moisture content and airflow rate on bio-drying of sewage sludge was investigated, and the results revealed that initial moisture content has a stronger effect on bio-drying, affecting the temperature and improving the water removal [38].

Skourides et al. [39] investigated the agitated bio-drying of the organic fraction of municipal solid and the results showed maximum drying rate achieved for the highest aeration rates used (120 m3 /h), leading to lower final moisture content levels (20% w/w from an initial 40% w/w) with a short retention time of less than 7 days. In a similar study to investigate the effect of air-flow on the bio-drying of gardening wastes, it was found that higher air-flow rate corresponds to greater weight loss (40–57% weight loss) and leachate production at low air-flow. Even though higher air-flow rate causes higher water removal, it was further stressed that it is imperative to identify the optimal air-flow rate for bio-drying, since excessively high air-flow rate may induces physical drying [40]. It is shown that forced aeration during sewage sludge bio-drying controlled the matrix temperature and improved evaporation, establishing it as a vital parameter influencing water loss [18]. In effect, an increase in the air-flow rate increases the amount of water carried, improving the water loss and an output with high calorific value. Likewise, low air-flow rates result in decomposition without significant moisture removal.
