: Microbial Degradation of Wastes for Environmental Protection
MANOBENDRO SARKER'-2 and MD. MAKSUDUR RAHMAN2
'Department of Food Engineering and Technology, State University of Bangladesh, Dhanmondi, Dhaka-1205, Bangladesh
2Biomass Energy Engineering Research Center, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai-200240, PR China
At present, meeting the growing demand of food for the rapidly increasing population is a maj or challenge around the world. According to Alexandratos (2011), crop production would need to increase by 60% in 2050 to meet the gr owing demand of food. However, higher production of agricultural commodities is generating a huge amount of agricultural residues, which is one of the major causes of environmental pollution when discarded as waste. In addition, postharvest loss of agricultural commodities is almost 49 to 80% in tropical countries (Gustavo et al., 2003) and one-third of world food production (approximately 1.3 billion metric tons) is being wasted every year that causing environmental pollution (Gustavsson et al., 2011). On the other hand, a large volume of industrial effluents with organic, highly toxic compounds and synthetic chemicals is being disposed directly and indirectly into the environment, which has become one of the major global concents. Environmental scientists have studied on several processes to remove or detoxify organic and hazardous pollutants, like lead, copper, and cadmium, while byconversion has aroused as an effective and environmental-friendly technique (Singh and Tripathi, 2007; Wasi et al., 2008). In bioconversion, chemical reactions are catalyzed biologically to reduce pollution in terms of yielding biofuels and other high-value products (Srirangan et al., 2012). At present, bioethanol and biogas are the most fermented products, while intrinsic and extrinsic factors exert a significant effect on the microbial growth and the yield of renewable energy from the biodegradable wastes of different sources.
In general, biodegradable waste is known as biomass, which refers to the organic material and can be converted into high-value products by microorganisms or their enzymes (Table 6.1). Long et al. (2013) stated that biomass is one of the leading renewable energy sources that contribute 9-13% to the total global energy demand. In Europe, the production of biomass energy is almost double in the last decade, and 6% of the total amount of primary energy (Gabrielle et al., 2014). Although biogas is not used as gaseous vehicle fuel, it can be easily used to produce heat and electricity (Oleskowicz-Popiel et al., 2012).
TABLE 6.1 Organic Wastes from Different Sources and Possible Use
According to Weiland (2010), anaerobic digestion is the most efficient and sustainable process for biomass energy production, refers to the biological degradation of organic compounds by mesophilic or thermophilic anaerobic or facultative bacteria and results in biogas. The anaerobic digestion favors several advantages, like reducing odors and pathogens as well as decreasing greenhouse gas and other undesirable air emissions. Therefore, various biodegradable materials can be treated by using anaerobic digestion (Nallathambi, 1997; Ward et al., 2014). Zheng et al. (2014) reported that the agricultural residue is lignocellulosic and easily biodegraded, which primarily consists of three major structural elements, namely cellulose, hemicelluloses, and lignin. Meanwhile, biogas has obtained a very good response due to its economical and eco-friendly usage of agricultural residues.
Several researches have also motivated to enhance the biogas production through co-digestion, which refers to a combination of different wastes as substrates in the fermentation process. Carucci et al. (2005) noted that synergistic effect in the co-digestion process enhances the biogas production. Interestingly, co-digestions of slaughterhouse wastes with municipal solid wastes (Cuetos et al., 2008), and fruit and vegetables wastes with slaughterhouse wastes (Alvarez and Liden, 2008) exert significant effect in biogas production. In addition, effluent of anaerobic digestion can be further used as substrate for ethanol production, and solid byproducts can be used as fertilizer because of incompletely degraded organic and inorganic matter content (Alburquerque et al., 2012).
In the context of diminishing fossil fuel reserves, bioethanol has emerged as an attractive and promising renewable energy. On the other hand, abundant amount of lignocellulosic biomass, like rice straw, wheat straw, com stover, and sugarcane bagasse has motivated researchers to the production of bioethanol (Gruno et al., 2004). Usually, bioethanol production from lignocellulosic biomass takes place in three steps. The first step is delignification in which cellulose and hemicellulose are liberated from lignin, the second step is depolymerization of cellulose in which hemicellulose converts into free sugar (hexose and pentose), and the third step includes the fermentation of free sugar to produce ethanol. Several microorganisms have been found to use in bioethanol production from different wastes, like Saccharomyces cerevisiae for sugarcane bagasse (Irfan et al., 2014), for sorghum stalk juice (Shen et al., 2011), for algae, fish waste and pineapple (Hossain et al., 2008), Aspergillus ellipticus
and A. fiimigatus for banana pseudostem (Ingale et al., 2014). Therefore, Saccliaromyces cerevisiae is used for bioethanol production on an industrial scale from sugar and starch-based materials (Hahn-Hagerdal et al., 2007; Tian et al., 2009).
Organic waste refers to biodegradable materials like agricultural wastes, industrial wastes, household wastes, fruits, and vegetable wastes, human, and animal wastes, while biodegradability largely depends on the composition and microbial strain associated with degradation, hi this sense, organic wastes show potentiality in the production of various valuable products through solid-state fermentation (SSF) (Pandey et al., 2000a, b, c).