Sunita Baniya

ORCID Identifier(s)

0000-0002-6711-4702

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Melanie L Sattler

Abstract

Energy is a basic tool for social and economic development. However, approximately 90% of rural households in low-income countries still rely on unprocessed traditional biomass fuel as the major source of daily energy. These biomass fuels are typically burned indoors, which leads to exposures to high levels of indoor smoke and associated health risks. Sustainable management of household waste is another environmental problem for rural peoples of developing countries. Currently, 90-95% of total generated wastes in most of developing countries is disposed of in open areas and street curbs.Anaerobic decomposition of waste to produce methane is a cost-effective way of providing energy to rural peoples. Through anaerobic digestion, clean-burning fuel for cooking can be produced from degradation of household organic wastes. Despite being very popular in rural communities, biogas technology poses many challenges in high altitude areas of low-income countries. The methanogenic bacteria responsible for gas production are very sensitive to temperature. Colder temperatures inhibit bacterial activity. Since biogas plants in the mountainous areas do not have heating provisions, they become non-functional or out-of-order during winter months. The main objective of this study was thus to develop and design a cold-resistant anaerobic digester to improve the fermentation temperature and maintain an efficient biogas production rate. This can alleviate the problem in utilizing biogas technology in rural communities located in the mountainous areas of low-income countries. The second objective was to conduct a life-cycle environmental and economic analysis for the cold-resistant design, compared to a conventional digester design.To accomplish this, rice husk ash was examined as a locally available insulation material. Rice is one of the major agricultural crops at least in 75 countries of the world, leading to large volumes of rice husk as a readily available waste material. Rice husk ash can be integrated with reactor building materials such as bricks or blocks to enhance the digester performance in cold temperatures.For this study, rice husk was burned to make rice husk ash (RHA), and soil was obtained from City of Arlington’s landfill. The physical and chemical characteristics (pH, bulk density, moisture content and loss on ignition, CHON and S, surface metals analysis, sieve analysis) of soil and RHA were analyzed. Then rice husk ash was combined with soil in three different proportions (10% RHA + 90% soil, 20% RHA + 80% soil, and 30% RHA + 70% soil) to form bricks, which were fired at 500 °C and 700 °C, with burning duration of 4 and 6 hours. So, a total of 12 different types of bricks (3 RHA/soil proportions x 2 firing temperatures x 2 firing times) were tested for compressive strength and water absorption. It was found that the compressive strength of the brick decreases and water absorption increases with an increase in rice husk ash. The 8 bricks with the best compressive strength and water absorption values were then tested for resistance to heat transfer. Resistance to heat transfer increased with increased RHA percent. Leaching test results, determined with the LEAF procedure, showed that concentrations for all metals (primary standard) are lower than the maximum permissible limit on drinking water.Based on the test results obtained on the different types of brick, a best composition was selected for building reactors. The best composition of brick was 20% RHA and 80% soil with burning temperature of 700°C with 4 hours burning time. Two laboratory-scale brick masonry circular reactors with outside diameter of 2’6” were built, one with the bricks with RHA added and the other one with conventional bricks. Using cow manure (about 12 kilograms), the reactors were operated at a controlled outside temperature of 21°C initially, which was later lowered to 10 °C. The RHA reactor produced the gas continuously when the temperature dropped gradually from 21 °C to 12 °C and stopped after 10 °C; however, the conventional reactor stopped producing methane after 14 °C. Compared to the conventional reactor, the reactor with insulation had an average inside temperature about 3.5 °C degrees higher, started gas formation earlier, peaked earlier and had a higher rate of methane generation. The cumulative volume of methane in RHA reactor was 33% greater than the conventional reactor after 102 days of reactor operation. The likely reason is that the higher temperature helped microorganisms grow faster and degrade the organic matter quickly. So, the RHA seems to be promising insulating material for the use in building the biogas reactor in the temperate areas. Also, microbial analysis of both reactor sludge was done to find out different microbial communities in each reactor during anaerobic treatment of cow dung. An economic analysis was done to compare the cost and benefit of the cold resistant reactor with the conventional reactor. The cost of both reactor types was calculated based on a data available in a developing country, Nepal, using the present worth method. RHA reactor has a higher benefit-cost ratio (4.3 vs. 3.8) and internal rate of return (80% vs. 71%) for 20 years lifetime. Based on the energy balance results obtained from environmental impact analysis, the net annual energy production of cold resistant reactors is 684 MJ higher than conventional reactors.

Keywords

Biogas digester, Cold resistant, Household waste, Rural community, Fuelwood, Clean burning fuel-methane, Mountainous areas, Developing countries, Insulating materials, Rice husk ash, Resistance to heat transfer, RHA mixed brick, Economic analysis, Environmental analysis

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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