Optimizing the performance of biofilter absorbing hydrogen sulfide from landfill gas

Master thesis in the field of civil and environmental engineering

Abstract

The main problem in using landfill biogas is the presence of pollutants such as hydrogen sulfide. Hydrogen sulfide gas is colorless, toxic, flammable, and has an unpleasant odor. It is highly toxic and produces SO2 when biogas is burned. In addition, hydrogen sulfide has a corrosive effect. It costs a lot to make devices that are resistant to corrosion. Most of the commercial technologies that exist to remove hydrogen sulfide from gas streams are chemical and physical processes that are expensive in terms of operation. Gas washing processes, surface adsorption on carbon, and thermal and chemical oxidation are among them. Among the advantages of microbial processes, the direct conversion of hydrogen sulfide to elemental sulfur, the need for low input energy, the absence of the production of contaminated side products, low costs and the production of biomass or biomass. Another advantage of biological methods is the low cost of installing machines and low strategic costs. In this study, in order to investigate the efficiency of biofilter in removing hydrogen sulfide pollutant from landfill gas, a Plexiglas biofilter column with a diameter of 15 cm and a height of 2 meters was used. This column was used with a natural substrate containing vermicompost produced in the cemetery and Gosh Mahi. The efficiency of this biofilter was 97%. In this research, measures were taken to optimize the performance of the biofilter removing hydrogen sulfide from biogas. Including the use of cheap and available bacteria and culture medium, installing a water pump to sprinkle water inside the column and create sufficient humidity for the growth and survival of bacteria inside the biofilter beds. In addition, installing a tap to regulate the amount of water, easily and successfully injecting bacteria into the biofilter, adding bacteria through a bucket containing water and a pump, installing a biofilter column in the landfill and connecting it to the natural biogas produced by the well and creating stable conditions for the biofilter's performance, investigating the effect of biogas flow rate and hydrogen sulfide concentration on the biofilter's performance are among other things. Also, checking the amount of hydrogen sulfide absorption by each biofilter bed separately, using a compressor between the biogas well and the biofilter column to provide more appropriate biogas and simulating with the landfill power plant, checking the changes in the concentration of hydrogen sulfide in the biogas well and the input of the power plant, and checking the possible problems of the main biofilter of the landfill and providing suitable suggestions are among other things. The present study showed that the biofilter column with the optimizations can be used in the main scale to remove hydrogen sulfide from landfill biogas.  

Key words: landfill gas, hydrogen sulfide, vermicompost, Thiobacillus bacteria, biofilter

Chapter 1

Introduction

The most important goal of forming urban landfills and collecting their produced biogas is to prevent the emission of greenhouse gases such as methane and also to use renewable energy in It is biogas. Today, in most countries of the world, landfilling is preferred over other existing methods such as burning garbage or turning it into fertilizer, etc. due to its cheapness. But in the past, there were no specific regulations on the place of garbage disposal, and landfills were smelly and uncovered places that created many environmental problems. With the growing awareness of the negative impact of non-engineered landfills on the environment and the establishment of special laws and regulations, burying in uncovered pits has been abandoned and engineering landfills have been established in compliance with environmental laws and regulations. Landfill is the most important method for urban solid waste disposal, which is used for more than 80% of the total amount of waste in China. Unpleasant odors in landfills are mainly caused by gaseous compounds emitted from landfills that are created during chemical and physical activities to decompose waste materials, such as hydrogen sulfide H2S, methyl mercaptans, and methyl sulfide, and it is one of the main complaints by people living around landfills. More than 100 compounds have been identified as the main sources of unpleasant odors in landfills. H2S is the main factor in creating unpleasant odors in landfills in concentrations less than 1%.  Hydrogen sulfide not only offends people, but also causes death in concentrations of 100-200 ppm. Various technologies have been developed to reduce H2S output, including adsorption by activated carbon, oxidation by ozone, biofilters and activated sludge (1).

1-1- The importance of the subject and the necessity of conducting the study

The energy problem is one of the basic problems of all countries in the world, especially developing countries. Providing fuel to remote villages is very difficult and expensive, even in a country like Iran that has rich energy resources. The use of renewable and local energy is one of the solutions proposed today. Biogas is one of these renewable energies that, in addition to energy production, creates agricultural fertilizers and increases the public health level of society and disease control, and is a suitable solution for solid waste disposal. Sewage and solid waste materials produced by industries and communities cause severe environmental pollution, which can be greatly reduced by extracting biogas and by using organic fertilizers. Biogas extraction can be done from anaerobic wastewater treatment processes as well as from landfills and compensate part of the consumption costs. For example, one of the problems that livestock farms are dealing with is the control of animal waste to reduce the amount of odors and products that cause environmental problems. Biogas can help us in facing these problems. The environmental benefits of biogas systems go beyond conventional treatment systems that have been used so far (such as storage tanks, ponds, and lagoons). These environmental benefits include odor control, improving the quality of water and air, improving the nutritional value of the produced fertilizer, reducing the amount of greenhouse gas emissions and obtaining biogas as an energy source (2 and 3).

Biogas systems use a process called anaerobic digestion. During the process of anaerobic digestion, bacteria break down manure in an oxygen-free environment. One of the natural products of anaerobic digestion is the production of biogas, which usually contains 60-70% methane gas and 30-40% carbon dioxide gas. Some other gases such as hydrogen sulfide have been detected in biogas. The resulting biogas can be used to produce heat, hot water, and electricity (at a cheaper price) than other fuels such as natural gas, propane, and black oil, even if biogas energy recovery is not practical, this system is highly effective in odor control. Burning or using biogas can reduce the effects of traditional fossil fuels. Methane production from anaerobic digesters creates rural electricity production cooperatives whose energy source is nature-friendly. These companies sell their produced electricity to people who request electricity produced with renewable sources. Biogas can also be useful as a rural energy source to assist in electricity production and distribution by other electricity generation methods. Due to the reduction of nitrogen loss in the anaerobic decomposition of manure produced in the biogas process, it has a high value in terms of nitrogen for growing plants. In the fertilizer produced from biogas, due to the lack of nitrification process, which is performed only in the presence of sufficient oxygen, there is nitrogen in the form of ammonium ions in the fertilizer, which is easier for plants to absorb. Before the industrial revolution, waste mainly consisted of ashes, wood, bones, animal carcasses and vegetable scraps. These materials were buried in the soil and acted as compost [1] and helped to strengthen the soil. In the past, everything that could be reused was used, the human population was small and people lived in small concentrated groups, so the production of waste was not considered an important issue, but with the evolution of human life from a period of camping to an agricultural life, leaving waste in the environment of human life became a growing problem. Therefore, with the increase in the population of the cities, the space for waste disposal decreased and the communities started thinking about the development of waste disposal systems. In this way, the cemetery [2], which is a structure with a precise design, underground or on the ground, to separate garbage and waste from the surrounding environment, was built (4).