Biological Cleanup of Biogas

The most valuable component of biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases. However, biogas also contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. Hydrogen sulfide is oxidized into sulfur dioxide which dissolves as sulfuric acid. Sulphuric acid, even in trace amounts, can make a solution extremely acidic. Extremely acidic electrolytes dissolve metals rapidly and speed up the corrosion process.

The corrosive nature of H2S has the potential to destroy expensive biogas processing equipment. Even if there is no oxygen present, biogas can corrode metal. Hydrogen sulphide can become its own electrolyte and absorb directly onto the metal to form corrosion. If the hydrogen sulphide concentration is very low, the corrosion will be slow but will still occur due to the presence of carbon dioxide.

The obvious solution is the use of a biogas cleanup process whereby contaminants in the raw biogas stream are absorbed or scrubbed. Desulphurization of biogas can be performed by biological as well as chemical methods. Biological treatment of hydrogen sulphide typically involves passing the biogas through biologically active media. These treatments may include open bed soil filters, biofilters, fixed film bioscrubbers, suspended growth bioscrubbers and fluidized bed bioreactors.

Biological Desulphurization

The simplest method of desulphurization is the addition of oxygen or air directly into the digester or in a storage tank serving at the same time as gas holder. Thiobacilli are ubiquitous and thus systems do not require inoculation. They grow on the surface of the digestate, which offers the necessary micro-aerophilic surface and at the same time the necessary nutrients. They form yellow clusters of sulphur. Depending on the temperature, the reaction time, the amount and place of the air added the hydrogen sulphide concentration can be reduced by 95 % to less than 50 ppm.

Most of the sulphide oxidising micro-organisms belong to the family of Thiobacillus. For the microbiological oxidation of sulphide it is essential to add stoichiometric amounts of oxygen to the biogas. Depending on the concentration of hydrogen sulphide this corresponds to 2 to 6 % air in biogas. Measures of safety have to be taken to avoid overdosing of air in case of pump failures.

Biofiltration

Biofiltration is one of the most promising clean technologies for reducing emissions of malodorous gases and other pollutants into the atmosphere. In a biofiltration system, the gas stream is passed through a packed bed on which pollutant-degrading microbes are immobilized as biofilm. A biological filter combines water scrubbing and biological desulfurization. Biogas and the separated digestate meet in a counter-current flow in a filter bed. The biogas is mixed with 4% to 6% air before entry into the filter bed. The filter media offer the required surface area for scrubbing, as well as for the attachment of the desulphurizing microorganisms. Microorganisms in the biofilm convert the absorbed H2S into elemental sulphur by metabolic activity. Oxygen is the key parameter that controls the level of oxidation.

The capital costs for biological treatment of biogas are moderate and operational costs are low. This technology is widely available worldwide. However, it may be noted that the biological system is capable to remove even very high amounts of hydrogen sulphide from the biogas but its adaptability to fluctuating hydrogen sulphide contents is not yet proven.

Different Strategies in Composting

Composting can be categorized into different categories depending on the nature of decomposition process. The three major segments of composting are anaerobic composting, aerobic composting, and vermicomposting. In anaerobic composting, the organic matter is decomposed in the absence of air. Organic matter may be collected in pits and covered with a thick layer of soil and left undisturbed six to eight months. Anaerobic microorganisms dominate and develop intermediate compounds including methane, organic acids, hydrogen sulphide and other substances. The process is low-temperature, slow and the compost formed may not be completely converted and may include aggregated masses and phytotoxic compounds.

Aerobic Composting

Aerobic composting is the process by which organic wastes are converted into compost or manure in presence of air. In this process, aerobic microorganisms break down organic matter and produce carbon dioxide, ammonia, water, heat and humus, the relatively stable organic end-product. Although aerobic composting may produce intermediate compounds such as organic acids, aerobic microorganisms decompose them further. The resultant compost, with its relatively unstable form of organic matter, has little risk of phytotoxicity. The heat generated accelerates the breakdown of proteins, fats and complex carbohydrates such as cellulose and hemicellulose. Hence, the processing time is shorter. Moreover, this process destroys many micro-organisms that are human or plant pathogens, as well as weed seeds, provided it undergoes sufficiently high temperature. Although more nutrients are lost from the materials by aerobic composting, it is considered more efficient and useful than anaerobic composting for agricultural production.

There are a variety of methods for aerobic composting, the most common being the Heap Method, where organic matter needs to be divided into three different types and to be placed in a heap one over the other, covered by a thin layer of soil or dry leaves. This heap needs to be mixed every week, and it takes about three weeks for conversion to take place. The process is same in the Pit Method, but carried out in specially constructed pits. Mixing has to be done every 15 days, and there is no fixed time in which the compost may be ready. Berkley Method uses a labor-intensive technique and has precise requirements of the material to be composted. Easily biodegradable materials, such as grass, vegetable matter, etc., are mixed with animal matter in the ratio of 2:1. Compost is usually ready in 15 days.

Vermicomposting

Vermicomposting is a type of composting in which certain species of earthworms are used to enhance the process of organic waste conversion and produce a better end-product. It is a mesophilic process utilizing microorganisms and earthworms. Earthworms feeds the organic waste materials and passes it through their digestive system and gives out in a granular form (cocoons) which is known as vermicompost. Earthworms consume organic wastes and reduce the volume by 40–60 percent. Each earthworm weighs about 0.5 to 0.6 gram, eats waste equivalent to its body weight and produces cast equivalent to about 50 percent of the waste it consumes in a day. The moisture content of castings ranges between 32 and 66 percent and the pH is around 7.

The level of nutrients in compost depends upon the source of the raw material and the species of earthworm. Apart from other nutrients, a fine worm cast is rich in NPK which are in readily available form and are released within a month of application. Vermicompost enhances plant growth, suppresses disease in plants, increases porosity and microbial activity in soil, and improves water retention and aeration.