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Integrating Waste to Green Energy

Every economic activities and living of the people produce waste which management is challenging for the urbanizing world. The large amount of organic fraction of municipal solid waste (OFMSW) and agricultural waste, especially cattle manure from the emerging trend of large scale livestock handling in growing cities and the country requires sustainable management practice. The majority of municipal solid waste is currently disposed of in landfills from where it emits landfill gas (LFG) that contains methane (CH4). It also produces leachate, a liquid containing toxic compounds that disperses through the soil in and around the landfill areas. The majority of cattle manure is used to produce compost, the aerobic biological stabilization of organic wastes. The composting process also emits methane (CH4) and nitrous oxide (N2O), a greenhouse gases responsible for the global warming. Both landfilling and composting process are the lost opportunity of recovering energy.

About 80% of the total energy in Nepal is consumed in the residential sector including 70% for cooking. This energy was mainly supplied from traditional biomass (mainly fuel wood). There is huge scope for using clean energy to meet the residential demands. This would mitigate health problems, such as eye, acute respiratory infections (ARI) and pneumonia resulting from using wood fuel. Replacing fuel wood would overcome environmental problems, such as deforestation and soil erosion.

Anaerobic digestion of  waste (OFMSW and cattle manure) produce biogas that contains 50-70% CH4, a reliable source of clean energy suitable for cooking and other thermal applications and slurry, suitable for fertilizing agricultural land and feeding fish or pig. A large size modern biogas plant would be an adequate solution for integrating waste management to green energy. There are different sizes, types and designs of large biogas plants are available namely continuous stirred tank reactor (CSTR), biogas induced mixing arrangements (BIMA), mechanical-biological waste treatment (MBT), upflow anaerobic sludge blanket reactor (UASB) etc. These biogas plants are available from few tons to hundreds of ton per day digestion capacity. Digestion of cattle manure can use CSTR, BIMA system while unsegregated municipal solid waste usually applied MBT system to segregate inorganic fraction of waste that might be recyclable or produce refused derived fuel (RDF).

 

Biogas production from waste largely depends upon organic matters contained in the waste measured in terms of organic fraction of dry matters (ODM) besides digester technology and environmental factors such as temperature, loading rate, retention time, pH, toxicity etc.

To get an overall feeling of commercial viability of large biogas plant a simple estimate may be sufficient. In an anaerobic (absence of oxygen) digestion of waste at temperature about 37 oC, cattle manure produces 0.2 – 0.35 m3 biogas per kilogram ODM while OFMSW produces 0.3 – 0.6 m3 biogas per kilogram ODM. On Average per ton cattle manure produce 35-50 Nm3 and OFMSW produce 30-80 Nm3 biogas (1.1 – 2.2 cylinder LPG equivalents) per day. The installation cost of biogas plant that is usually 30-50% of total investment varies with manufacturer and technological options, however, 4 to 8 million rupees per ton waste digestion is what one can estimate at current market price excluding land cost. The running cost also varies from place to place and technological choice, nevertheless, 1-3% of total investment could be sufficient for it. When taking an optimistic biogas production of 2 LPG cylinder equivalents, one can earn about 1 million rupees per annum. The earning from bio-slurry could add up some more on the total revenue. The modest estimate clearly indicates that the payback period would be 8 to 10 years, perhaps not attractive to the investors.

 

Biogas plant is not plug and go device, one need to feed it carefully, nurse it when it is not well and most importantly, one must know what to do when something goes wrong. A chemical laboratory is necessary to continuously monitor important parameters such as dry matter, ODM, VFA, pH, C:N ratio etc. Continuous monitoring of few important parameters using supervisory control and data acquisition (SCADA) system could facilitates immediate feedback and adjustment of the plant operation.

To commercialize large biogas system and make an attractive investment, government needs to facilitate with conducive policy. Integrating waste management to green energy not only provides energy that replaces fuel wood and LPG consumption but also provides tremendous environmental benefits.

 

By: Dr. Sunil Prasad Lohani

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