http://www.jgpress.com/archives/_free/001206.html ENVIRONMENTAL REMEDIATION BY COMPOSTING
CONTAMINATION of soils with toxic and/or hazardous materials can be traced back to industrial, military, municipal and agricultural activities. The extent of this contamination is significant. In 2004, the U.S. Environmental Protection Agency estimated that 294,000 sites will need to be cleaned up over the next 30 years. This includes 77,000 known sites and an estimated 217,000 sites yet to be discovered. Total clean up costs are estimated to be $209 billion.
These estimates include the seven major clean-up programs: National Priorities List (NPL, or Superfund); Resource Conservation and Recovery Act (RCRA) Corrective Action; Underground Storage Tanks (UST); Department of Defense (DOD); Department of Energy (DOE); Other (Civilian) Federal Agencies; and States and Private Parties (including brownfields).
If the risk of human or ecological health damage is deemed high enough, remediation is required. Remediation is the process of taking action to reduce, isolate, or remove contamination from an environment with the goal of preventing exposure to people or animals. Remediation can be done by physical (air stripping), chemical (reagent addition), thermal (thermal desorption), or by biological means. Biological remediation techniques include composting, phytoremediation and landfarming, and lesser known methods such as bioaugmentation, biostimulation and bioslurping. Composting is considered to be an ex-situ (meaning “out of place”) treatment technology.ENVIRONMENTAL CONTAMINANTS
There are over 330 listed environmental contaminants with known human and/or ecological health affects. They are segregated into six groups of organic chemicals and two groups of inorganic chemicals:Organic Chemicals:
Nonhalogenated volatile organics (i.e. methanol, carbon disulfide); Halogenated volatile organics (i.e. carbon tetrachloride, perchloroethylene); Nonhalogenated semivolatile organics (i.e. malathion, dimethyl phthalate); Halogenated semivolatile organics (i.e. pentachlorophenol (PCPs), polychlorinated biphenyls (PCBs)); Fuels (i.e. gasoline, diesel, fuel oil); Explosives (i.e. TNT, RDX, nitroglycerine).
Inorganic Chemicals: Metals (i.e. arsenic, cadmium, lead, zinc); Radionuclides (i.e. cobalt-60, uranium, radium).Not all organic chemicals are amenable to biodegradation by composting.
Radionuclides and metals cannot be remediated (broken down) by composting; however, metals can be adsorbed into less bioavailable forms. The rate at which microorganisms degrade contaminants is influenced by the following factors: specific contaminants present; oxygen supply; moisture; nutrient supply; pH; temperature; availability of the contaminant to the microorganism (clay soils can adsorb contaminants making them unavailable to the microorganisms); concentration of the contaminants (high concentrations may be toxic to the microorganism); presence of substances toxic to the microorganism, e.g., mercury; or inhibitors to the metabolism of the contaminant.
The main advantage of ex situ treatment (removal of soils) is that it generally requires shorter time periods than in situ treatment (in the ground), and there is more certainty about the uniformity of treatment because of the ability to homogenize, screen, and continuously mix the soil. However, ex situ treatment requires excavation of soils, leading to increased costs and engineering for equipment, possible permitting, and material handling/worker exposure considerations.BIOLOGICAL MECHANISMS
Composting can change organic chemicals and bind metals through several different mechanisms:Biological degradation
is the process where microorganisms break down water-soluble chemicals with enzymes in solution to utilize them for metabolism. Two processes that can modify an organic chemicals structure to make it more water-soluble are hydrolysis (adding water to break chemical bonds) and oxidation.Extracellular decomposition
is the process where microorganisms secrete enzymes to break down large organic molecules into a smaller form for easier absorption into the microorganism. This is how cellulose, hemicellulose and lignin are degraded in composting. Fungi are the source of most extracellular enzymes.Intracellular decomposition
takes place once the chemical has been absorbed by the microorganism. Mineralization, the process of converting an organic material to carbon dioxide and water, is the predominant process at work inside the microorganism.Adsorption
is an electrochemical process where positively- or negatively-charged organic molecules bind with their charge-opposite counterparts in organic matter and clays. This is the mechanism by which metals can be bound and become less bioavailable. Volatilization
is a physical process that changes a material from one physical state to another (i.e. from liquid phase to gas phase). Mixing of contaminated soils is a major source of volatilization (up to 30 percent of an organic chemical can be lost this way). Volatilization of hazardous chemicals is both a public health and air quality concern (EPA regulates 188 hazardous air pollutants under the Clean Air Act). Volatilization is highly temperature-dependent (higher temperatures produce more volatilization). Moisture can either block volatilization by clogging air channels with water or can increase it by liberating weakly-adsorbed chemicals. By breaking weak adsorption bonds, liberated hazardous chemicals can volatilize due to the agitation of excavation.COMPOSTING CONSIDERATIONS
Traditional composting of nontoxic organic materials is based on proper recipe formulation, thorough mixing, aerobic composting and curing to produce a stable and mature compost for product markets in the shortest possible time at the least possible cost. Composting of contaminated soils has a different endpoint (the degradation of the contaminant), and product marketability is not an issue, so temperatures, time, C:N ratios, moisture content and porosity are somewhat less important considerations. Thermophilic temperatures have been shown to greatly accelerate the degradation of some contaminants (like polycyclic aromatic hydrocarbons (PAHs)), but at a greater risk of gaseous volatilization. (The Occupational Safety and Health Administration (OSHA) has set a limit of 0.2 milligrams of PAHs per cubic meter of air.)
In some cases, anaerobic conditions may be needed to degrade highly chlorinated compounds, a step which is then followed with aerobic treatment to degrade the partially dechlorinated compounds as well as the other constituents. Anaerobic microbial processes that are of significance in environmental remediation include denitrification, iron/manganese reduction, sulfidogenesis and methanogenesis. Sulfidogenic and iron-reducing microbial populations can dehalogenate and mineralize chlorinated and brominated aromatic compounds.
Moisture levels in the range of 20 to 80 percent are considered suitable for the bioremediation of soils, but the microbes in composting thrive best between 40 and 60 percent moisture. Nutrient process design is focused on C:N:P ratios, in which ratios of 120:10:2 are not uncommon. Each remediation project is different, due to variations in the nature of the contaminant, initial concentration of that contaminant, desired endpoint (often influenced by risk assessments), degradation rate (dependent on both the nature of the contaminant and the energy level of the compostable feedstocks), concerns over volatilization and leaching, and available space.
Composting can be done with aerated static piles, in-vessel systems, or with windrows. Windrow composting is considered to be the most cost-effective alternative, but it may also have the highest levels of fugitive emissions of Volatile Organic Compounds (VOCs). Composting with aerated static piles, also known as biopiles, is an effective means of remediating petroleum contamination.