BioCycle July 2010, Vol. 51, No. 7, p. 60
BP used to stand for British Petro-leum. Then BP got a new advertising agency and BP became Beyond Petroleum. I would like to suggest an alternative: Bioremediate Petroleum.
As I write this, potentially the worst “point source” environmental disaster ever has been going on for almost two months. Chances are unfortunately pretty good that it still will be occurring when you read this. Now this column is supposed to focus on how organics can impact the largest pending “nonpoint source” environmental disaster: climate change. However, it just so happens that in this case, the point source disaster and the nonpoint source disaster have something in common -fossil carbon. I’m talking about the BP spill in the Gulf of Mexico.
How you might ask is the BP spill remotely related to management of organic residuals? Well the first connection that one might make and that I have spent many columns ranting about is that we need alternative fuels and green energy. This is often used as justification for burning wet stuff. Burning wet stuff (read food scraps, yard trimmings and municipal biosolids) makes just about as much sense from a climate change perspective as saying that BP stands for Beyond Petroleum. I am sure that this argument will be made. Hopefully what I’m about to talk about will provide yet another justification for putting these residuals to a higher and better use.
Instead of burning these materials, how about we make them into compost and use the compost to bioremediate the petroleum-contaminated wetlands and soils? There has been a fair amount of research that says composting or compost itself can be a means to both reduce total hydrocarbon concentrations as well as reduce the portion of that total that can cause harm to an ecosystem.
COMPLEXITY OF CRUDE OIL
An oil spill is a complicated mess. Crude oil consists of not just one kind of carbon compound but of a wide array of organic chemicals. These different compounds will vary in their ability to degrade based on their different chemical structures and how soluble they are in water. For example, oil contamination often includes a range of PAHs (short for polyaromatic hydrocarbons) – carbon compounds with different numbers of ring structures. The ring structures are just carbon atoms sharing electrons and circling around each other. The more rings there are, the more likely that the PAH will be slow to degrade and be insoluble in water. If compounds are insoluble in water, they are likely to stick to other organic compounds.
The fact that the hydrocarbons provide fuel for your car, also means that they are potentially good eating for microorganisms. It is a delicate balance here; they are certainly energy rich compounds. Breaking the bonds that hold the carbon atoms together can get you from 0 to 60 in just a few seconds. However, the bonds holding them together can be tight enough so that they are very tough to break. For some of these compounds, they are also toxic so that anything that tries to eat them may be killed in the attempt.
These compounds are also very rich in carbon with minimal concentrations of nutrients like nitrogen and phosphorus. Not to push food metaphors too heavily, but think of someone with diabetes eating peanut brittle without the peanuts. You can break your teeth and make yourself really sick if you eat too much.
ENTER THE COMPOST WARRIOR
So how can composts or composting help degrade these PAH compounds? Biological degradation of petroleum hydrocarbons is carried out by soil microorganisms. Plants can also play a part as the root zone or rhizosphere of plants will have much higher concentrations of microorganisms than the bulk soils. In many cases, however, highly contaminated soils will be toxic to plants and microbes.
Studies on remediation of these soils using composts have focused on two basic approaches: actively composting a portion of the contaminated soil with other feedstocks or adding composts or biosolids to contaminated soils. Both approaches operate on the same general principles. Composting is a microbially mediated process and composts and biosolids are both rich in organics. Use of these materials or of the composting process is a means to provide enhanced microbial activity to the contaminated soil, and can potentially result in faster decomposition of the petroleum hydrocarbons. Going back to the food metaphor, it is like diluting the brittle by not only adding the nuts back in but providing a glass of milk to go alongside it.
For those hydrocarbons that are very resistant to decomposition, the organic matter in the composts or biosolids may provide a type of glue or fly paper – a highly adhesive surface that will bind these compounds and make them less available to do harm. So even though you can still extract them from soils with very strong solvents, the soils will be able to grow plants and cycle nutrients. The contaminants are there, but locked up and unable to do any harm.
Different studies have generally shown that use of these organics is a viable means to restore contaminated soils. Not necessarily a silver bullet but a much better option than burning dirt (currently used technology). Here are a few examples.
A range of studies have monitored the degradation of hydrocarbons during composting. These were reviewed in a paper by Antizar-Ladislao et al. in 2004. Removal seems to be fastest in piles that are a little bit cooler than the thermophilic range as there are a wider variety of microorganisms at cooler temperatures. Some studies looked at 2 ring compounds, some 3 and 5 ring compounds, some with even more rings. Although while reading the review you might find yourself inadvertently starting to juggle (all of those rings), the message is clear that composting is a way to degrade a wide range of hydrocarbons. For the BP spill, however, that might mean a windrow that stretches up and down the length of Louisiana.
Another approach might be to mix finished compost in with the contaminated soil. Puglisi et al. (2006) added compost to soils contaminated by phenanthrene (a 3 ring hydrocarbon compound). The scientists measured how tightly the compost absorbed the phenanthrene by sterilizing some of the samples. This differentiated any changes that occurred as a result of microbial activity from changes that occurred from adsorption of the compound onto the compost organic matter. They noticed a significant decrease in the bioavailability of phenanthrene in the compost-amended soils. The compost glue was at work!
They also tested the ability of microbes to degrade the hydrocarbon in compost and control soils that had not been sterilized. The compost-amended soils had higher numbers of phenanthrene degrading bacteria, and a higher portion of the compound was degraded in these soils in comparison to the unamended soils. So the portion of the phenanthrene that wasn’t stuck to the organic matter was able to degrade faster in the compost-amended soils because of a more active microbial population.
In another study, Dickenson and Rutherford (2006) added municipal biosolids to a soil contaminated with diesel fuel. Some of the soils were planted with brome grass. The soils with the biosolids degraded the petroleum hydrocarbons more quickly than soils without biosolids, although the lighter weight fraction eventually degraded in both. The grass only grew in soils with biosolids. So if you add biosolids you get faster degradation and a healthy plant cover. Or you could just burn the soil and burn the biosolids. Then you get lots of smoke and no soil.
Maybe this column won’t be enough to convince British Petroleum and the Coast Guard to stop burning or landfilling the soils that have been contaminated by the spill. This is a very political site and it may take a while before those in charge are willing to try “alternative remedial strategies.” But hopefully this column has convinced you of the potential for BP to mean Bioremediate Petroleum and provided more evidence of the value of organics.
Sally Brown – Research Associate Professor at the University of Washington in Seattle – authors this regular column. E-mail Dr. Brown at email@example.com.