Aerobic composting, or aerobic digestion, is a bio-oxidative process. During this process, a large portion of the degradable organic carbon is converted into carbon dioxide and water. During the composting process, methane can be generated in composting piles due to the partial anaerobic conditions; when the moisture is high, the ventilation is not enough. Heat is produced during composting, which elevates the temperature of the pile to more than 60 °C. This helps reduce the concentration of pathogens (microorganisms that causes disease) inside the composter (Hochman et al., 2015; Zafar, 2015). As the substrate becomes the only source of food to the microorganisms in composting, the nature of substrates is the most dominant factor in any composting process (Gajalakshmi and Abbasi 2008). For this reason, the organic waste characteristics paramount for ensuring good composting. There are two ways of composting, according to the Database of Waste Management Technologies http://www.epem.gr/waste-c-control/database/html/Composting- 03.htm
a. Windrow Composting
Windrow composting is widely employed for the treatment of plant matter from gardens, parks and amenity areas. A windrow is a long pile of shredded organic waste with a triangular cross- section. The shape of the windrow allows passive airflow as hotter gases exit from the top of the
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windrow, allowing the flow of air to the sides. Windrows are typically turned at frequencies ranging from a few days to weeks. Turning promotes pathogen destruction by moving the material from the cool outside to the hot core, thereby restoring permeability. Turning is undertaken by a number of methods; self-propelled windrow turners either lift the material up and drop it back down behind the machine or raise it onto an elevator that drops the material to one side.
Following treatment, the composted material is typically screened to achieve an even product size and then recycled to land, being used as a soil conditioner, mulch and, in some cases, employed to produce soils. Importantly, a windrow composting system only requires an area of concrete and some mobile plant to allow the success of an operation. As the composting process requires a minimum level of moisture, maintaining the required moisture content can be problematic in arid countries. Windrow composting process is summarised in figure 2.2:
Figure 2.2: Open Windrow Composting Process. Source: Kakosimos, (2015)
b. In-vessel Composting
In-vessel composting (IVC) is widely used for the treatment of organic waste which entails biosecurity or odour issues impacting their treatment. In practice, IVC embraces a variety of
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techniques wherein the organic waste is composted in an enclosed vessel or tunnel. Enclosing the process requires the employment of aeration and process control systems, which renders the process more expensive than windrow composting. The IVC is more controlled than open windrow composting and can be designed to achieve specified temperatures in order to facilitate pathogen destruction. It also minimizes the risk of vermin and birds gaining access to organic wastes, which may pose the risk of animal diseases, such as those contained in uncooked foods and other animal products or wastes.
IVC has a global application for the treatment of source segregated organic waste; its use is growing with the increasing need for reducing organic waste from landfill increases. For IVC to operate successfully, structural material such as green waste or wood chip is needed. The quality of the output of the IVC is predicated on the input material and therefore, good quality compost is only produced from source of segregated organic waste.
This method is particularly recommended for source segregated organic waste. It can also be potentially used for organic waste that is separated from mixed waste streams if there are markets for the composted product.
Figure 2.3 shows the in-vessel composting (IVC) process
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Source: CTCN, https://www.ctc-n.org/technology-library/waste-management/solid- waste/landfill-composting
Meanwhile Figure 2.4 summarizes the concept of composting
Figure 2.4: The Concept of Composting
Source: CTCN, https://www.ctc-n.org/technology-library/waste-management/solid- waste/landfill-composting
According to Frederick and Keener (2016), the most important composting process parameters are the following: temperature, moisture content, aeration and oxygen• pH and C:N ratio.
Therefore, the parameters and characteristics of organic waste that are essential for the composting technology are listed below to denote the optimum ranges for the technology:
Carbon: Nitrogen (C:N) ratio: The relative proportion of carbon and nitrogen is a major controlling factor in the composting process (Hansen et al., 2002; Ekinci et al., 2000; Agnew and Leonard, 2003). Carbon primarily serves as an energy source for the microorganisms, while a small fraction of the carbon is incorporated into the microbial cells. Nitrogen is paramount for microbial population growth. If nitrogen is limited, microbial populations will remain small and decomposition rates for available carbon will be lower. Excessive nitrogen is lost from the system
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as ammonia gas. According to Golueke (1973), rapid and entire humectation of substrates by the microorganisms primarily depends on it, initially having a C:N ratio between 25 and 35. Importantly, C:N ratio between 25:1 and 31:1, with the 30:1 ratio is considered optimal because the active bacteria digest carbon twenty-five to thirty times faster than nitrogen. Leaves, straws and woody materials serve as a major source of carbon, whereas grass and food scraps serve as the major source of nitrogen. For this reason, it is important to provide carbon and nitrogen in appropriate proportions. With C: N ratios below 20:1, the available carbon is fully used without stabilizing the entire quantum of nitrogen. The excess nitrogen may be lost to the atmosphere as ammonia or nitrous oxide, and odour can also pose a challenge.
Moisture: Moisture is one of the composting variables that affect microbial activities to a considerable extent. It provides a medium for the transport of dissolved nutrients necessitated for the metabolic and physiological activities of microorganisms. The microbial decomposition process augments the interdependence and mutual control between two of the main composting parameters: oxygen levels and temperature.
Bobeck (2010) argued that the optimum moisture content for composting must be of 50-60 percent, while Frederick and Keener (2016) mentioned that the optimum moisture for composting is between 34-65 percent. Moreover, water content is important because the microorganisms can only dissolve nutrients from the liquid phase. Oxygen level needs to be sufficient enough to ensure aerobic decomposition. Importantly, the temperature should reach up to 60°C from the microbial activity.
pH: The composting process is relatively insensitive to pH within the range commonly found in mixtures of organic materials, primarily due to the broad spectrum of microorganisms involved. The preferred pH level is in the range of 6.5-8.0; pH level should be between 5.5 and 8 (Bobeck, 2010). pH becomes a consideration with raw materials containing a high percentage of nitrogen. A high pH, above 8.5 encourages the conversion of nitrogen compounds to ammonia. (Parker, 2017)
As is the case with the AD, composting also needs low heavy metals content since high heavy metal concentrations inhibit the microorganisms’ enzymes and in effect, stymie the entire process. (Bobeck 2010; Khan et al., 2016)
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Brinton (2000) compared the compost heavy metal content in MSW between source-separated composting in relation to American standards. This comparison gives an indication of the heavy metals content of the waste so it can be compared to the heavy metals content of Muharraq governorate’s OHW, which will be presented in chapter 5. Table 2.1 outlines the heavy metals content in MSW of America:
Table 2.1: Heavy Metal Content in MSW vs. Source-Separated Compost in Relation to Standards in America Element Mixed MSW Compost (Avg 4 regions) mg/kg Bio-Waste Compost (Avg 4 regions) mg/kg German Standard mg/kg Pb 420 83 150 Cu 222 41 150 Zn 919 224 500 Cr 107 61 150 Ni 84 26 50 Cd 2.8 0.4 3 Hg 1.9 <0.2 3
Abdel-Shafy et al., (2014) argued that the general advantages of anaerobic technology in comparison to the aerobic processes are: lower energy input, lower waste sludge production, yield of biogas with a calorific value of about 5000–6000 kcal m3 (6–7 kW/m3) as a valuable energy source, particularly for gas power station with heat recovery and no odour nuisance due to a closed reactor system. Previous studies reported that certain heavy metal ions can inactivate enzymes, thus inhibiting the growth of bacteria such as Cu, Pb, Cr VI and Zn, consequently inhibiting the anaerobic digester.
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According to Götze et al. (2016), data of chemical waste characterization is available from China, Europe, and North America, whereas very little or no data is available from other regions.
According to Asian Development Bank, 2011, MSW in South Asia contains 70 percent organic waste, which is why composting and the AD is considered highly suitable. Both need source segregation in order to improve the quality of the product and the biogas productivity. Composting and AD need low heavy metals content given the fact that high heavy metal concentrations inhibit the microorganisms’ enzymes, thereby impacting its process.
According to Asian Development Bank, (2011), moisture in the South Asian organic waste was found to be 70- 80 percent, thus hinting that both composting and AD are suitable options. Zafar (2017) believed that there is no alternative to the AD and composting for management of organic fraction of MSW. Since AD and composting necessitates a high C: N that may reach 25- 30, low C: N ratio can be increased and moisture can be decreased to acceptable levels (for the AD and composting) through the addition of dry leaves, grass clippings, sawdust, paper and wood chips. High levels of moisture can also be reduced by solar drying of raw MSW for a period of 24- 48 hours prior to its composting or anaerobic digestion. These pre-processing steps will not impose a financial burden.