Chapter 15
Consequences of Biogeochemical Cycles Gone Wild
David C. Herma and Raina M. Maier
1.
Think about the activities involved in the carbon, nitrogen, and sulfur cycles and
develop your own idea for applying one of these activities in the commercial sector.
This is a discussion question and there are many possible answers, your imagination is the limit.
A. For the carbon cycle one could consider creating soil amendments for disturbed
environments using carbon cycling activities (e.g., composting)
B. For the nitrogen cycle, one could consider developing a bioremediation system for
nitrate-contaminated groundwater based using a bioreactor run under nitrate reducing
conditions using a carbon-based waste material from a local processing facility as the
carbon source.
C. For the sulfur cycle, one could consider developing (and engineering) a cost-effective
process that removes reduced sulfur contaminants from fossil fuels such as petroleum
and coal based on bacterial sulfur oxidation to make them cleaner burning materials.
2.
As city mayor, you are informed that the concrete sewer system is corroded and will
need to be replaced within the next 3 years. You are running for reelection and the
cost of replacing the sewer system means that taxes will have to be raised. How
would you explain, in lay terms, to your constituents why the concrete pipes have
corroded and justify raising taxes to replace the system.
I have called this press conference today to inform you of a problem with the sewer system in
this great city of ours. This problem must be addressed immediately or there could be
public health consequences, specifically, our sewer pipes may start leaking and release
sewage containing disease-causing organisms. Since our drinking water pipes are near
the sewer pipes these leaks could possibly contaminate our drinking water supply with
raw sewage. The reason for the leaks is that our sewer pipes are made of concrete, which
was material of choice at the time they were installed. Unfortunately, the sewage flowing
through these pipes has created an environment that drives the corrosion of the concrete
material. Sewage is rich in organic matter (as we all know) which creates anoxic
conditions that result in the microbial production of sulfide. Sulfide is a gas with a smell
that you would all recognize as a rotten egg smell. The gas migrates into the headspace of
the sewer pipes which is rich in oxygen and where microbial activity converts the sulfide
into sulfate producing sulfuric acid. This acid has corroded the concrete pipes. The
solution to this problem is to immediately replace the pipes with other materials that are
not subject to this corrosion process. In fact, PVC pipes can be used safely. Therefore, I
propose to raise the city sales tax by 1 cent per dollar for the next two years to provide
sufficient funds to take care of this problem. While I know this will be a sacrifice, I hope
that you will all support this request that will keep our great city safe and functioning.
3.
A mine that supports much of the local economy is releasing acid leachates into a
receiving stream that eventually feeds a fishery important to local tourism. The
concern is that a significant decrease in pH of the fishery waters will affect fish
survival. What suggestions would you make to the local mine to 1) prevent acid
leachate formation, 2) treat acid leachates before their release so that this does not
become an insurmountable problem.
1) Acid leachate formation is the result of buried ores containing reduced forms of sulfur, such
as pyrite, being exposed to the air (oxygen). As a result, acid tolerant sulfur oxidizers
oxidize the sulfur to form sulfuric acid which is a primary component of acid leachates.
The sulfur oxidizers are obligately aerobic bacteria. Therefore, if the mine carefully
covers up any ores it is not currently processing to avoid exposure to oxygen, the
formation of acid leachates will be reduced. Further, for ores that are being actively
processed, the processing should take place on pads that allow for collection and
treatment of acid leachates that are generated.
2) One way to treat acid leachates before their release is to simply add lime (calcium carbonate)
to neutralize the acidity. This also helps precipitate any contaminating metals that are in
the acid leachate. This might require large amounts of lime (and $) depending on the
acid potential of the leachates. An alternative is to use constructed wetlands which
consist of water-saturated soils and sediments that have vegetation which is adapted to
anaerobic conditions in the rhizosphere. As acid leachates move through this system, the
pH is neutralized and contaminant metals are reduced and precipitated in the rhizosphere.
Finally, a combination of these two approaches can be used. The acid leachate can be
released to percolate through a limestone bed and then into a constructed wetland.
4.
Methylation of metals increases their volatility. Is it a good idea to use microbial
methylation as a basis for removal of metals such as mercury and selenium from
contaminated soils? Support your answer.
Methylation is a good approach for removal of some but not all metals. For example, when
mercury is methylated, its toxicity increases. So methylation is not a desired outcome for
mercury contaminated sites. One the other hand, when selenium is methylated it is less
toxic. In fact, selenium methylation has been used as a form of bioremediation in the
Kesterson Reservoir in the San Joaquin Valley, CA. Agricultural irrigation in the San
Joaquin Valley over a number of years resulted in the build of selenium in the soils and a
mass die-off or birds, fish, and plants in the Kesterson Reservoir (Case Study 21.1).
Carbon amendments were added to the soils in Kesterson to drive the methylation of
selenium and remove it from the soils.
5.
Municipal sludge or biosolids left after wastewater treatment can be landfilled,
combusted, or composted. Discuss the advantages and disadvantages of each
approach.
Landfilling of biosolids is a rapid but increasingly expensive solution as landfill space becomes
more scarce.
Combustion of biosolids is also a rapid solution which results in a large reduction in the biosolids
volume and weight, destroys all pathogens, and can often be done on site at a wastewater
treatment facility. Disadvantages include the generation of waste ash that must be
disposed of as well as the need for air pollution control technologies for the incineration
process.
Composting of biosolids is a slower process but results in the formation of a commercial organic
soil amendment (compost) that provides beneficial organic matter and nutrients (such as
nitrogen and potassium) to the soil. There are no residual waste products leftover after
composting. Some disadvantages of composting include the possibility for odor
production at the composting site, the survival pathogens during the composting process,
and a lack of consistency in product quality in terms of stability and metal (or other toxic)
content.
6.
Groundwater under cattle feedlot operations is often found to have nitrate
contamination. Explain the microbial basis for nitrate contamination of
groundwater in these areas.
Cattle feedlots are concentrated in wastes (urine and feces) from these animals. These wastes
have high levels of ammonium which is released into the environment as the waste undergoes
natural decomposition processes (ammonium mineralization). Chemoautotrophic ammonium
oxidizing bacteria (the nitrifiers) utilize the released ammonium as an energy substrate and it is
oxidized to nitrite and then nitrate. Nitrate, being negatively charged, is more mobile than
ammonium and is leached into groundwater.