Biodegradable Waste Processing
The composting method is one of the most feasible and economical among the various options available. The techniques available for waste management are minimization, recycling, sanitary landfilling, incineration, and composting (Mohammed et ah, 2018). Composting is most suitable for biodegradable waste, which is waste capable of undergoing anaerobic or aerobic degradation, such as biodegradable food waste, garden waste, paper including paper board, etc. (Council Directive 1999/31/ EC, 1999). The Energy Information Administration (EIA) of the U.S. defines biogenic waste as the waste produced by the biological process of living organisms. The UNEP data shows that outdoor decay of biogenic organic matter of MSW contributes to about 5% of global greenhouse gas (GHG) emissions annually (UNEP, 2011). Thus, to combat this global issue, a number of technological developments for waste treatment have taken place. Mainly the composting process is to make organic fertilizer, anaerobic digestion to generate biogas, and thermal treatment to generate energy have all been developed (Laner et ah, 2012). The major drawback of the implementation of some of these techniques at field levels is due to the high implementation costs and environmental concerns.
Organic waste could be utilized in composting and the anaerobic digestion process, which is used as an agricultural fertilizer. The produced natural compost is rich in micronutrients and macro-nutrients (like phosphorous, nitrogen, potassium). The biodegradable waste has the potential to produce more than 50% of the energy content of MSW in both developed and developing countries (Beneduci et ah, 2012). Another process to convert organic waste into compost is anaerobic composting. The next section is discussed about both the aerobic and anaerobic processes.
Aerobic Composting
Biological decomposition of MSW degradable materials carried out under the predominantly aerobic conditions. The produced compost is sufficiently stable for nuisance free handling, storage, and satisfactorily matured to increase the agricultural productivity (UNEP, 2005). The microbes play a significant role in the biological decomposition process that involves the oxidation of organic carbon in the organic waste. The microbe-prepared compost is a good soil amendment and can be used as fertilizer. Care must be taken for pollutants such as heavy metals in the organic waste. During the composting process, energy is released which is responsible for the rise in the temperature during the composting. Energy loss and aerobic composting converted into the anaerobic composting in the hierarchy of the waste management system. Waste handling in environmentally friendly manner materials recovery is considered for extracting important raw materials and new products manufacturing by helping the life cycle assessment. The mixture of SW is used to form compost by aerobic composting technique and it can be contaminated by mainly heavy metals. Thus, MSW compost has the chance of heavy metal contamination and can cause environmental degradation as well as harm to public health. The major environmental concern is the restriction to use in agricultural practices as manure (Lasat, 2000; Pinamonti et al., 1997). Thus, mixed SW composting might not be a preferred option for the composting; thereby, it is not considered a sustainable waste management option. However, mixed waste composting is widely used (Ravi, 2011) in various countries including India where more than 91% of MSW is landfilled and there are no other options.
Micro- and macro-flora along witli the pH, temperature, aeration, and moisture content, as well as the chemical and physical availability of the nutrients, are key factors in controlling the composting processing (Beneduci et al., 2012). It is very important to consider the public health issue and environmental impacts without the unsanitary landfilling that are more firmly formed by research. Thus, it can reduce the heavy metal contamination to groundwater than the open dumping of wastes. Previously, industrial waste generators disposed of their waste into the environment without any treatment and now regulation is very strict to treat and manage their waste as per regulation (Musee et al., 2008; Thandavamoorthy, 2016). One investigation (Ball et al., 2017) shows the effect composting to bio-stabilize the biodegradable fraction of MSW from an advanced waste treatment system in Australia. The aerobic system process observed a reduction in oxygen consumption of 30% in immature compost and 45% in mature compost when compared with the input material. The aerobic composting technique is the most used of MSW technology in India. The previous estimation has shown that 6% of the total MSW collected in composts in various membrane bioreactor facilities (DEA, Ministry of Finance, Government of India, 2009).
Anaerobic Digestion
Anaerobic digestion of MSW on a large scale has a challenge in terms of source segregation of organic waste stream. However, for smaller scales e.g. hotels or restaurants, for vegetables and meat markets and at household levels anaerobic digestion is successful. The anaerobic process recovers both energy and compost and is positioned at the higher levels in the hierarchy of waste management, as compared to other options i.e. energy recovery. The compost quality further depends upon the quality of the input of MSW. The composting of MSW results in a low-quality of compost that is less productive to crops and has the potential to introduce toxic heavy metals into the food chain. Pellera et al. (2016) evaluated the aerobic and anaerobic conditions for the combined treatment of an organic fraction of SW in the laboratory-scale landfill bioreactors. The initial operating system under anaerobic conditions could allow energetic exploitation of the substrates, while the implementation of a leachate treatment system ultimately aimed at the nutrient recovery. The organic fraction of MSW for the composting process takes around 37 days followed by a curing phase of 79 days. Various process parameters were tested and their optimum range (oxygen>140 mL H, moisture content > 500 g kg-1, temperature < 65°C w.w.). The processing of the sample is of utmost importance to increase biogas production from the production of organic acids, which, further reduces the pH and interfere with the biogas generation process (Adani et al., 2000). Another advantage is the considerable reduction of greenhouse gas emissions by diverting the biodegradable SW from the landfill (Ball et al., 2017). One interesting example from Biotech Company, Kerala (India), who installed twenty thousand household biogas units to divert about 2.5% of biodegradable waste from landfill that ultimately saves up to 4.5 million USD every year in terms of transportation cost.
Further, biogas production from these units avoids the generation of 7,000 tons of carbon dioxide equivalent to (TC02) emissions annually. The mixed waste composting is a better option as compared to landfilling or open burning due to environmental constraints (Ravi, 2011).
