Exploitation of Energy Potential through Utilization of Municipal Solid Waste

Abstract

The epic challenge of the present era is filling the gap between energy demand and supply with clean, reliable, and affordable energy. Energy from fossil fuels has remained the choice of Pakistan like other countries. The huge quantity of municipal solid waste (MSW) generation and over consumption of fossil fuels because of growing population have created many environmental as well as socio-economic problems in Pakistan. Converting waste to renewable energy source has two-fold benefits. One, it reduces environmental degradation and two, fulfills the energy needs necessary for economic growth. The present study was formulated in order to overcome dual problems of the country that are open dumping and/or burning of solid waste and energy shortage. Core objective of present study is to develop an integrated MSW management model that would focus on the use of waste to energy (WtE) as the key component of sustainable MSW management. Hyderabad city of Sindh, Pakistan was selected as a case study for quantification, composition, characterization and energy potential of MSW.

In the first phase of the study, samples of MSW were collected and characterized according to the sampling methodology. About eight scenarios were developed according to the nature of components of MSW and were compared. From comparison it has been found that scenario three and five are the best options for energy potential of biological treatment (1.13MW/100tons/day) and thermal treatment (11.86-22.40MW/100tons/day) respectively. The contribution of energy from solid waste has been estimated that is 0.07% through bio-chemical and 0.34% through thermo-chemical in the total primary energy supply of the country. Moreover, results of study revealed that about 70% of imported energy can be reduced by bio-chemical and completely can be replaced by thermochemical process of solid waste. Not only this but also burden on energy from other primary sources of the country would be reduced up to 1.86% cumulatively by adopting thermo-chemical process of waste. In the second phase of the study, fresh samples of food waste (FW) and yard waste (YW) were collected and characterized for biochemical methane potential (BMP) of substrates. Three inoculums namely buffalo dung (BD), sewage sludge (SS) and effluent from continuous stirrer tank reactor (CSTRE) at three inoculum to substrate ratios (ISRs) were optimized by BMP test system. The highest methane yield of FW and YW was achieved as 428 Nml g-1 VS added and 304 Nml g-1 VS added respectively by using BD as inoculum at ISR-5. Also, the first order decay model was used, which gave best fit for methane potential of substrates with BD inoculum at ISR-5. In the third phase of the study, further optimized inoculum and ISR were used to optimize methane potential of fruit, vegetable and yard waste (FrVYW) of summer and winter seasons by mixing at different ratios. From overall results, maximum methane was obtained from tri-substrates at mixing ratio of (1:1:1) that is in the range of 530-621Nml/gmVS, maximum stands for winter season wastes whereas minimum is from summer season wastes. The anaerobic digestion (AD) of optimized mixing ratio of FrVYW was carried out for optimization of hydraulic retention time (HRT) and organic loading rate (OLR) by continues stirrer tank reactor (CSTR). The maximum methane production, VSR and methane content of selected substrates were obtained at HRT of 20days with OLR of 5%TS (2.5kgVS/m3/d) that are 0.530 L/gVS, 86% and 76% respectively. Therefore, 20days and 5%TS (2.5kgVS/m3/d) are optimum HRT and OLR respectively for biomethane yield of FrVYW at mixing ratio of (1:1:1).

In the fourth and last stage of the study, on the basis of above results and findings, four scenario options were developed by mixing eight scenarios and tested by different treatment options. The 3E (energy, economical and environmental) assessment of WtE technologies including composting, AD, LFGR, Incineration, Gasification, MRF of each scenario performed to develop integrated MSW model. Study concluded that AD for S-2 (putrescible components of MSW), Gasification for S-3 (non-putrescible components of MSW) and MRF for S-6a (recyclable components of MSW) are best options as a premise an integrated MSW management model for Hyderabad city, Pakistan. The findings of present study would be useful for decision makers to decide that which treatment option would be best for exploitation of energy through utilization of MSW in Pakistan.