Encyclopedia of Renewable Energy. James G. Speight

      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.

Biopiles are a hybrid of land farming and composting. Biopiles provide a favorable environment for indigenous aerobic and anaerobic microorganisms. Soil biopiles (also called biocells or bio-cells) are a biodegradation technique used for the remediation of excavated soil contaminated with crude oil and/or crude oil products. This technology involves the accumulation of contaminated soil into piles and the stimulation of microbial activity either aerobically or by the addition of nutrients, minerals or moisture. A typical height of biopiles is between 3 and 10 ft. Biopiles are in a way similar to land farms due to the fact that this technology also uses oxygen as a way to stimulate bacterial growth. However, while tilling or plowing aerates land farms, biopiles are aerated by forcing air to move by injection through perforated piping placed throughout the pile . The soil is usually mixed with a bulking agent (such as straw) to improve aeration and therefore enhance the growth of the microbial population.

      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.