Organic waste is defined as “any waste that is capable of undergoing anaerobic or aerobic decomposition through a biological treatment process” (DEHLG 2006). This includes animal manures, crop residues, garden waste (green waste), food processing wastes, municipal biosolids, and wastes from wood industries (Westerman and Bicudo 2005; Ministry for the Environment New Zealand 2007a). They are usually called
“wastes” because they are not the primary product of a specific production process (Westerman and Bicudo 2005); indeed they can represent an inefficient use of valuable resources (Ministry for the Environment New Zealand 2007b).
Management of wastes has become a major environmental challenge (Odlare et al. 2011) as a consequence of the rapid population growth, urbanization and increasing rate of consumption of natural resources (Ministry for the Environment New Zealand 2007b; Narayana 2009; Odlare et al. 2011). It was estimated that around 8.7 million tons of municipal solid waste were generated in New Zealand in 2006 (Ministry for the Environment New Zealand 2007c). The Solid Waste Analysis Protocol of Waste to Landfill indicated that in 2004 organic waste comprises 23% of the waste disposed to landfill (excluding paper, cardboard and timber and other biodegradable wastes) (Ministry for the Environment New Zealand 2007a). This large volume of waste necessitates an efficient system of disposal (Narayana 2009) and management with low negative impacts (Moberg et al. 2005), as it can otherwise pose a risk to human health and the environment (Ministry for the Environment New Zealand 2007b). In an ideal world all organic waste would be converted to useful products by such processes as recycling of the nutrients, replenishment of soil organic matter, or generation of useful energy (Sims 1996). A waste management infrastructure should be based on a hierarchy
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of the following principles: (i) waste prevention; (ii) recycling/re-use; (iii) the use of waste as a source of energy; and (iv) controlled final disposal. At present, the most common methods used to treat waste are the following: incineration, landfilling, composting and anaerobic digestion, and land application, with land filling being the least recommended.
2.1.1.1 Incineration
Incineration turns waste into gas (including SOx, HCl, NOx, CO, and organic
compounds such as polycyclic aromatic hydrocarbons (PAHs) and halogenated aromatic compounds) (Heger et al. 1998) and an ash residue (Narayana 2009), greatly reducing the waste volume and generating some energy (Bogner et al. 2008). However, this process can also represent a significant local source of air pollution in developing countries where the incinerators do not usually have post-combustion air pollution control systems (Diaz et al. 2005), constituting a health risk for nearby communities. Some products (such as polychlorinated dioxins/furans (PCDD/PCDF)) formed during incineration are far more difficult to deal with than the original waste (Narayana 2009). A few regions and nations, e.g. Ontario (Canada), the Philippines and Argentina, have banned or restricted waste incineration.
2.1.1.2 Landfilling
A landfill is an area of land where waste is deposited onto, or, into the soil, aiming to avoid any contact between the waste and the surrounding environment, particularly the groundwater (Narayana 2009). Disposing of organic waste via landfill does not allow either the recycling of nutrients or the mitigation of GHG emission, and may generate toxic leachates and occupy a large landfill space; therefore it is considered an unsustainable option (Carey et al. 2008). Among all the waste treatment methods,
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landfilling has the lowest priority (Moberg et al. 2005). However, landfilling still plays a major role in waste treatment all over the world. For example, in New Zealand, about 6.3 million tonnes out of total 8.7 million tonnes of solid waste are sent to landfill and cleanfill sites each year (Ministry for the Environment New Zealand 2007a). Diverting organic wastes from landfills is critical to reduce landfill emissions and their contribution to global warming and climate change (Carey et al. 2008) and, obviously, to minimize the risk of groundwater pollution.
2.1.1.3 Composting and anaerobic digestion
Composting and anaerobic digestion are biological methods that effectively reduce the amount of organic wastes (DEHLG 2006). Composting is the aerobically- controlled decomposition of organic waste through biological processes, resulting in
products as CO2, water, and an organic matter fraction (Bogner et al. 2008; Narayana
2009). It can reduce the bulk volume (Westerman and Bicudo 2005) and odour, kill pathogens and produce a stabilized product for transport. Compostable materials in
developing countries accounted for 80–85% of that of organic wastes (Narayana 2009).
However, composting is not a suitable option for sequestering C and recovering some nutrients such as N (Macías and Camps Arbestain 2010). Additionally, the quality of the end products strongly depends on the raw material and the operating conditions (Narayana 2009). Under poor management conditions, in which suboxic, or, anaerobic
conditions are generated, CH4 and N2O form during composting (Bogner et al. 2008).
Anaerobic digestion produces biogas (CO2 and CH4) – used for energy
generation – and biosolids (Bogner et al. 2008). It is strongly recommended by many
governments as a method to reduce the C footprint of waste treatments (Odlare et al. 2011). However, the resulting biosolids still need to be disposed somewhere and these
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can contain high amounts of heavy metals , organic contaminants and pathogens, and have a high risk of nitrate leaching (Cooke et al. 2001; Gove et al. 2002; Magesan and Wang 2003; Egiarte et al. 2006; Egiarte et al. 2009; Jalali and Arfania 2010).
2.1.1.4 Land application
For organic waste, biological treatments (composting and anaerobic digestion) are obviously preferable options compared with landfilling and incineration (Bogner et al. 2008). However, both composting and anaerobic digestion results in substantial amounts of leftover material, namely biosolids and composts (Odlare et al. 2011). A sustainable, economical and safe application of these products (Odlare et al. 2011) is needed to avoid the generation of new waste. Land application of organic wastes offers a promising approach. High quality organic waste can be used as fertiliser, contribute to the pool of soil organic carbon (Odlare et al. 2011), and improve soil physical, chemical and even biochemical properties and thus enhance crop growth (Westerman and Bicudo 2005; Diacono and Montemurro 2010). However, recommended application rates should be considered to avoid both excessive leaching of nitrate to groundwater, and excessive loading of heavy metals, organic pollutants and undesirable microorganisms in soils (Egiarte et al. 2005).