Basics of Compost Remediation

[Justin Boland]

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.

[Justin Boland]

Source:
http://www.intervalecompost.net/art/TestingCompost.htm

We asked Will Brinton, Ph.D., and the staff members of the Woods End Research Laboratory to test bags of compost from all regions of the United States. Products included composts made from cow, chicken, horse, and sheep manure; used mushroom "soil" and food wastes. The lab measured five key compost characteristics, and as you'll see, many of the brands flunked several of the tests. (We didn't include any products that contained sewage sludge, often labeled as "biosolids," because we believe most sludge-based composts should not be used in home gardens due to probable contamination with toxic wastes and heavy metals.)

What We Tested: Organic Matter Content

Why it matters:
If the organic-matter level is over 60%, then the compost isn't yet mature, and it could temporarily inhibit plant growth when mixed into the soil (although it could still be used safely as a surface mulch). If the level of organic matter is too low, then the compost simply won't improve the soil as well as a better-quality product would. "Organic matter is the essence of compost," Dr. Brinton explains. "It's the energy source that feeds soil microorganisms and regulates the steady release of plant nutrients. It also creates the 'glue' that improves soil texture, and it increases the soil's ability to hold moisture." Should be 30 to 60%.

What the tests showed:
16 out of our 30 composts were too old or had been diluted with soil, resulting in an organic-matter content less than 30%. Although using them wouldn't harm your soil, they were definitely not good buys. Only a third of our samples fell within the preferred range of 30 to 60% organic matter. Four products contained levels over 60%, indicating that they were probably not yet fully composted.

What We Tested: Content
May vary from 0.5 to 2% or more; should be indicated on the label if above 1%.

Why it matters:
Nitrogen is the nutrient that demands the most attention because it's the most likely to be in short supply in your garden and because it's also the nutrient most likely to cause pollution problems if it's overapplied. The nitrogen content should be a key factor in determining appropriate application rates of compost.

What the tests showed:
Only one-third of the bagged composts listed the nitrogen content on their labels. But Dr. Brinton's lab tests revealed that the producers were not using nitrogen content to set their recommended application rates. The rates provided on the bags were often too high. No labels made a distinction between annual rates and higher one-time rates for new beds.

What We Tested: pH
Should be in the neutral range, between 6.5 and 7.5.

Why it matters:
pH is a measure of acidity or alkalinity, represented by a number on a scale in which 1 is very acidic, 7 is neutral, and 14 is extremely alkaline. For optimum plant growth, you want to maintain a nearly neutral soil pH in your garden. Regular applications of good-quality compost help maintain neutral soil pH, but you should avoid using overly acidic composts on soils that are already naturally acidic, and avoid high-pH products on already alkaline soils. (If you don't know your soil's pH, have it tested.)

What the tests showed:
About half of our samples fell within the 6.5-7.5 pH range, nine were too high (as alkaline as 8.3), and four were too low (as acidic as 4.5). If you think a difference in pH of just one point or so probably doesn't matter much, think again: the pH scale is logarithmic, which means that for each one-point change, the alkalinity (or acidity) increases or decreases by 10 times.

What We Tested: Carbonate
Levels Should be indicated when high.

Why it matters:
Using an overly acidic compost won't usually do any long-term damage to your soil, but using one that's too alkaline might. High-pH composts often contain carbonates, usually in the form of lime (calcium carbonate.) If you have naturally alkaline soil (most common in drier regions) or if your soil is acidic and you already apply lime to reduce the acidity, Dr. Brinton warns that you should avoid using a high-pH compost. "Once a soil contains too much carbonate, other nutrients, such as phosphorus and zinc, will become unavailable," he says. "And there is no easy way to bring the soil back into balance." What the tests showed:
Dr. Brinton found that 30% of the composts had high carbonate levels, making them poor choices for use on alkaline or recently limed soils. And not a single compost producer had included information about carbonate levels on its label.

What We Tested: Salinity
Should not exceed 5 mhos/centimeter. (An mho is a unit used to measure salt conductivity.)

Why it matters:
As organic matter decomposes, minerals are slowly converted to salts that dissolve in water and become available for plant roots to absorb. If compost production is not managed properly, or if a large amount of chicken manure is used, salts can sometimes accumulate to a level high enough to injure plants-especially seedlings. Low salinity is particularly important in dry regions, where soils are already naturally high in salts because there isn't enough rainfall to leach the salts down into the subsoil. You should also choose a low-salt compost for heavy applications before direct seeding and for container mixes.

What the tests showed:
25% of our samples exceeded the 5 mho/cm standard. And none of the compost products we evaluated listed the salinity level on the label.

What our inspections revealed
Here's what we found when we examined our bagged samples:

* Texture: One out of every four of the 30 brands we inspected contained compost that was so sticky and clumpy that it would have been impossible to spread in the garden. (I made a beautiful "clay" pot from one brand, while another dried into serviceable rock-hard bricks!) Several others were obviously too woody and not fully composted.
* Color: The color of all the brands was similar while the composts were moist, but when we dried them out for a few days, three of the brands were too light in color to be good-quality composts.
* Moisture: All of the the sticky composts-one out of every four-were too wet. Most others were appropriately moist. Only one product had a very low moisture content, and it was labeled as "Chicken Manure Fertilizer-dry, composted, will not burn." We suspected a problem because of the product's strong odor, and, sure enough, Dr. Brinton's tests confirmed that the chicken manure was not fully composted.
* Smell: We found several brands that smelled sour or reeked of ammonia, indicating poor-quality immature products. Only one bag had that desirable earthy, woodsy smell when we first opened it, but several more developed the earthy odor after they had been exposed to the air for a few days.

[Justin Boland]

Source:
http://compost.css.cornell.edu/science.html

Cornell has a thriving compost research center who run a richly informative website.

This is absolutely Holy Grail material.

Composting can be pursued at many different levels, from the gardener who likes to produce "black gold" to the operator of a multi-acre commercial composting facility. Gardeners who compost their own landscaping and food scraps can follow a few simple rules of thumb and needn't worry about complex formulas, chemical equations, or studies of microorganisms. These are, however, important considerations for municipal and commercial composting operations because of the need to ensure that the composting proceeds rapidly, doesn't cause odor or pest problems, and achieves temperatures high enough to kill pathogens.

Some of the topics in the Science and Engineering section may be far too technical to be relevant to casual composters. On the other hand, some may be intriguing. You might, for example, wish to learn more about the invertebrates or the microorganisms that create compost. You might be curious about the temperature curve produced by compost as it goes through its cycle of heating and cooling. Or you might like to learn how to measure the pH or moisture content of your compost. You might even want to try calculating desirable proportions for the materials you wish to compost.

We invite you to explore these pages to whatever level your curiosity takes you, realizing that compost is a rich topic for scientific research and discovery as well as a practical method of recycling organic matter and reducing solid waste.