Encyclopedia of Renewable Energy. James G. Speight

Чтение книги онлайн.

Читать онлайн книгу Encyclopedia of Renewable Energy - James G. Speight страница 108

Encyclopedia of Renewable Energy - James G. Speight

Скачать книгу

the processing of contaminated solid material such as soil, sediment, sludge, or water through an engineered containment system.

      A slurry bioreactor is a containment vessel and apparatus used to create a three- phase system, such as solid, liquid, and gas, mixing condition to increase the biodegradation rate of soil-bound pollutants and water-soluble pollutants as a water slurry of the contaminated soil and biomass capable of degrading target contaminants.

      In general, the rate and extent of biodegradation are greater in a bioreactor system than in situ or in solid-phase systems because the contained environment is more manageable and hence more controllable and predictable. However, the contaminated soil may require pretreatment or, alternatively, the contaminant can be stripped from the soil via soil washing or physical extraction before being placed in a bioreactor.

      See also: Biodegradation, Biodegradation In Situ, Biodegradation Processes, Biodegradation – Solid Phase.

      Biodegradation – Solid Phase

      Solid-phase biodegradation, often referred to as land farming, treats wastes using conventional soil management practices to enhance the microbial degradation of the wastes. Land farming is a relatively simple technique in which contaminated soil is excavated and spread over a prepared bed and periodically tilled until pollutants are degraded. The goal is to stimulate indigenous biodegradative microorganisms and facilitate their aerobic degradation of contaminants. In general, the practice is limited to the treatment of superficial 4 to 8 in. of soil. Since land farming has the potential to reduce monitoring and maintenance costs, as well as cleanup liabilities, it has received much attention as a disposal alternative. Nutrients and minerals are also added to promote the growth of the indigenous species.

      Typically, the process requires excavation of contaminated soil or pumping of groundwater to facilitate microbial degradation. Ex situ biodegradation techniques involve the excavation or removal of contaminated soil from the ground. Depending on the state of the contaminant to be removed, ex situ biodegradation is classified as (i) a solid-phase system, which includes land treatment and soil piles or (ii) a slurry-phase system, which includes solid-liquid suspensions in bioreactors. A particular advantage of the ex-situ biodegradation process is that the process requires less time than the in situ process. Another advantage is the certainty of the control treatment due to the ability to uniformly screen, homogenize, and mix the soil. Ex-situ treatment technology is further divided into solid-phase biodegradation and slurry-phase biodegradation.

      Solid-phase biodegradation is an ex-situ technology in which the contaminated soil is excavated and placed into piles. Bacterial growth is stimulated through a network of pipes that are distributed throughout the piles. By pulling air through the pipes, the necessary ventilation is provided for microbial respiration. Moisture is introduced by spraying the soil with water. Solid-phase systems require a large amount of space, and cleanups require more time to complete than with slurry-phase processes.

      Nutrients and microorganisms are normally added to the wastes which are routinely tilled during the treatment process. This tilling improves aeration and the contact of the organisms with the wastes. While treatment may occur throughout the upper 3 to 5 ft of the soil, most occurs within the top foot, called the zone of incorporation.

      Composting of chemical wastes is the biodegradation of solid or solidified materials in a medium other than soil. Bulking material, such as plant residue, paper, municipal refuse, or sawdust, may be added to retain water and enable air to penetrate to the waste material. Successful composting of chemical waste depends upon a number of factors, such as the selection of the appropriate microorganism or inoculum. Once a successful composting operation is underway, a good inoculum is maintained by recirculating spent compost to each new batch.

      Other parameters that must be controlled include oxygen supply, moisture content (which should be maintained at a minimum of approximately 40% w/w), pH (usually around neutral), and temperature. The composting process generates heat, so, if the mass of the compost pile is sufficiently high, it can be self-heating under most conditions. Some wastes are deficient in nutrients, such as nitrogen, which must be supplied from commercial sources or from other wastes.

      Soil heaping is piling wastes in heaps of several feet high on an asphalt or concrete pad. Nutrients, microorganisms, and air are provided through perforated piping placed throughout the pile. The pile is covered to contain volatile organic compounds, to stabilize the environment of the microorganisms, and to control soil erosion. The volatile organic compounds can be further controlled by applying a vacuum to the pile and treating the exhaust.

      In this process, the wastes are normally mixed with a structurally firm bulking material such as chopped hay and wood chips. As with the other biodegradation technologies, nutrients, air, and microorganism must be added. The three major types of composting are open windrow, static windrow, and reactor systems. The differences among the three relate to how aeration is accomplished. In the open window system, the compost piles are open to the air, whereas in the static windrow system, the air is mechanically forced into the compost piles. When reactors are used, the compost is mechanically mixed to ensure aeration.

      See also: Biodegradation, Biodegradation In Situ, Biodegradation Processes, Biodegradation – Slurry Phase.

      BioDeNOx Process

      In addition, the BioDeNOx process is a biological process removes nitrogen oxides from flue gases. An iron chelate selectively absorbs the nitrogen oxides which are reduced to nitrogen with ethanol in the presence of microorganisms. Then, a wet gas scrubber is used to contact the circulation liquid with flue gas feed and absorb the nitrogen oxides (NOx).

      In the sump underneath the scrubber, the absorbed nitrogen oxides are biologically reduced to nitrogen and ethanol is also consumed and, thus, the iron chelate solution is regenerated. The presence of oxygen and acid compounds in the flue gas, such as hydrogen chloride and hydrogen fluoride, oxidizes a part of iron chelate to the ferric (Fe3+) state. Therefore, a purge and makeup of iron chelate is necessary to eliminate this oxidized ferric material. To minimize iron chelate consumption, a nano-filtration can be installed and bleed from the unit is passed through the filter and the chelate is recovered.

      See also: THIOPAQ DeSOx Process.

      Biodesulfurization

      Biodesulfurization is a technology to remove sulfur from the feedstock. However, several factors may limit

Скачать книгу