Feasibility Study of Biogas Generation from Municipal Solid Waste and Sewerage within the Colombo City Limits
Colombo City is the commercial capital of Sri Lanka with an estimated resident population of over 750,000 spread over 3741 hectares (ha) and has a population density over 1188 per ha. It is located in the western coast of Sri Lanka and is in wet zone. The topography is of flat terrain with a mix of land and water. Considering the population and the limited undeveloped land available, the disposal of Municipal Solid Waste (MSW) and Sewer is a major environmental problem in Colombo. The major object of this thesis is to identify and evaluate a productive waste management system that is not only environmental friendly but also sustainable and cost effective. In this context, as a sustainable technology, applicability of anaerobic digestion is investigated and methane generation potential of the waste is evaluated.
In order to identify a sustainable waste management system, the quantity of waste generated within the city of Colombo is identified. The current practices of disposal of these wastes are then reviewed to identify any issues regarding its sustainability. The majority of the MSW is currently disposed as open landfill that is causing pollution of waterways, with its leachate, as well as the polluting the atmosphere around it with its bad odour. The sewer is discharged to sea or disposed via a self-contained soakage pit. Except for few small-scale anaerobic digestion plants that use solid waste for generation of biogas for localized use, there is no large-scale waste to energy projects in operation in Sri Lanka. The sewer is not used productively at all.
Having identified the quantity of waste and the disposal methods practiced, the priority is to identify sustainable and productive methods of disposal of wastes that suits best the local conditions. With this in view research hitherto carried out are studied and available literature is reviewed. The objective is to ascertain the processes that productively harness the energy potential of MSW and Sewer, individually or in combination.
There are many physical and chemical methods for treatment of wastes. However bioconversion of waste provides the best options for tapping the energy of the wastes. Of the two main bioconversion methods aanaerobic processes exhibit many advantages over aerobic digestion with its ability of handling high organic loading rates and low sludge production. However, the reason for the increase in applications of anaerobic processes, is, its potential for production of energy using the biogas generated. The methane so produced can replace fossil fuel and therefore has a direct positive effect on greenhouse gas reduction. Therefore, compared with other bioconversion technologies for treatment of MSW and sewer/wastewater, the energy and environment benefits make anaerobic digestion an attractive option.
Anaerobic treatment of waste in an engineered landfill bioreactor is found to be the best option for treatment of MSW. Whilst providing a decrease in long term environmental risks and low operational and closure costs it provides with valuable energy source in generation of methane. As for sewer generation of methane in anaerobic processes can be enhanced with co-digestion of different types of waste suitably selected. In this regard co-digestion of sewer and wastewater with food waste is found to be productive and is applied in this study.
Literature review is carried out to determine suitable models to predict the methane generation potential. The "First Order Decay Model" is identified as the appropriate model for prediction of methane from MSW in landfills. The "Anaerobic Digestion Model No.1" is applied for prediction of methane from sewer and waste water. Chemical composition of MSW is the primary parameter which affects the methane generation. The chemical composition is computed applying the ultimate analysis and using a stoichiometric based approach. For sewer and waste water the basic parameters of BOD and COD concentrations are available from data gathered. Apart from these two parameters the other parameters relevant to local conditions are not available. Therefore values that closely fit the local conditions are taken from the literature. The study determines the methane generation potential of MSW approximately 2.1 x 106 m3 per annum and anaerobic co-digestion of sewer and food substrates generate 9.1 x 106 m3 per annum. If parameters could be determined for the wastes generated locally the accuracy of the methane generation potential could be further enhanced.
In this context, further studies, should be directed from the conventional landfills to "Anaerobic Bioreactor Controlled" landfill, where circulation of liquids including leachate is carried out to increase the biogas yield. For sewer the focus should be on the most economical foot print of parallel banks of number of continuous-flow stirred-tank reactors (CSTR) operating in series to accommodate the total flow rate of sewer.
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