In our experience, Bioremediation is best accomplished with Bio-augmentation, the addition of large numbers of selected micro-organisms grown in the production laboratory. In addition to bio-augmentation, several physical and chemical parameters must be controlled in order to obtain optimal growth and maximum degradation of soil contaminants. The major factors that are controlled during Bioremediation are:
1. Microbial Population
ERE has developed a synergistic group of micro-organisms named Hydrocarbon Digestants. These micro-organisms digest short- and long-chain hydrocarbons, heavy tarry types of oil and grease, coal tars, phenolic compounds, chlorinated organic solvents and many other toxic chemicals.
The organisms in Hydrocarbon Digestant are non-pathogenic and use petroleum products, or other chemicals in the soil, as both a carbon source and an energy source. They convert toxic chemicals into cell mass and into carbon dioxide and water.
With Bio-augmentation, a sufficient amount of Hydrocarbon Digestant is applied to the soil. Our studies have shown that this concentration insures that a sufficient population of beneficial micro-organisms will be provided. It is not the intent to provide a ready-made population to immediately digest the contaminants, but rather to provide an inoculum that will insure rapid bacterial growth. As the project progresses, the population of perrophilic organisms will increase one thousand fold as they digest the contaminants.
Tightly compacted soil may require additives in order to increase its permeability and the soil should be treated with either a shredding device or a vibrating screen in order to produce the smallest size particles possible.
In comparing Bio-augmentation with natural degradation, the former greatly increases the rate of degradation. The micro-organisms naturally present that degrade petroleum products are usually present only in the upper few inches of the soil and in low concentrations. These organisms are greatly diluted when mixed with a large amount of excavated soil. The dilution factor can be so great that it will take an excessively long time, if ever, to establish a working population with the correct organisms.
2. Nutrient Concentrations
Nitrogen is required for amino acid, purine, and pyrimidine biosynthesis and can be obtained by the micro-organisms from either inorganic or organic sources. The most commonly used nitrogen sources in Bioremediation are ammonia and nitrate. Many of the bacteria in Hydrocarbon Digestion have two pathways for ammonia assimilation and which pathway functions depends upon the ammonia concentration. We purposely keep the concentration high in order to allow the bacteria to utilize the most simple process.
When nitrate is utilized as the nitrogen source it is reduced to ammonia through a stepwise process. Nitrate is first reduced to nitrite by a molybdenum-containing enzyme and then to ammonia. 50-100 mg/L is the average nitrogen level recommended and should be frequently monitored.
Phosphorus (in the form of phosphate) is utilized by micro-organisms primarily to synthesize phospholipids and nucleic acids (DNA and RNA). A minimum of 50-100 mg/L of phosphorus is recommended.
In addition to nitrogen and phosphorus, a variety of minerals is universally required, such as potassium, calcium, magnesium and iron. Many other elements are required only in trace amounts. These include zinc, copper, cobalt, manganese, and molybdenum. These metals function in enzymes or coenzymes.
3. Oxygen Supply
Utilization of aliphatic hybrocarbons by micro-organisms is strictly an aerobic process. The initial oxidation step of aliphatic hydrocarbons involves molecular oxygen as a reactant and one of the oxygen molecules is actually incorporated into oxidized product.
The aromatic group of hydrocarbons can be viewed as derivatives of benzene. The breakdown of aromatic hydrocarbons involves the action of either oxygenases or mixed function oxygenases. These two reaction sequences both form catechol, which can be degraded in a number of ways, leading to either acetyl CoA or TCA cycle intermediates. If the benzene ring contains added methyl groups or other constituents (as in toluene or xylene), these substituents may be attacked before or after the ring is oxidized.
It is important to note that the oxygen concentration is the rate-limiting factor in the biodegradation of petroleum-based products. Microbial activity is most frequently limited in terms of sufficient oxygen due to slow rates of diffusion into the interior of the soil layers or piles and into the centre of soil aggregates. Generally, the greater the mass of oxygen that can be distributed the more rapid and complete the cleanup; and effective application can result in greatly accelerated cleanups.
4. Temperature and Moisture Content
The bacteria in Hydrocarbon Digestion were all isolated from the soil and laboratory studies have shown that their optimum temperature for growth and contaminant degradation is 28°C. The temperature of the soil will be maintained as close to 28°C as possible. Similar studies have shown that soil moisture should be maintained at 15-20% moisture.
The bacteria in Hydrocarbon Digestion will achieve optimum results if the pH range is maintained between 6.5 and 8.0. Soil being treated using bioremediation should be monitored frequently and the pH should be manipulated to maintain the soil within the optimum range.