Contents
Introduction 3
Why Molasses 4
Chemicals and Chemical Reactions 4
The Microbes 6
Case Study 1. 8
Case Study 2. 10
Case Study 3. 10
Conclusion 12
References 13
Appendix 15
Tables and Figures
Table 1. Species of bacteria for biodegradation
of chemicals 6
Figure 1. Pathways of microbial breakdown. 5
Figure 2. GC analyses of the remainder diesel
when the bacterial consortium was
grown in liquid culture. 7
Figure 3. PCE plume (mg/L) in the alluvial aquifer
at the maintenance shop area
1st quarter/March 2000 9
Figure 4. PCE plume (mg/L) in the alluvial aquifer
at the maintenance shop area
1st quarter/March 2001 9
Figure 5. Analytical Results for Well GM-7 at
Lycoming Superfund Site
(January 1997 to July 1998) 11
Introduction
Over the course of the last hundred years many chemicals have been introduced to the environment which have been found to be persistent in land and water. Several classes of chemicals pose a threat to human or ecological health and attempts have been made to remove these chemicals from both soil and groundwater. Depending on the toxicity of the chemicals involved, soil remediation techniques vary from high temperature incineration of soil to merely exposing it to the air. There are also a variety of techniques for decontaminating groundwater, mostly at great expense. Dealing with groundwater is difficult because of the depth and flow of the water. Often the extent of the contamination can be only estimated by inspection wells, which need to be drilled at regular intervals to allow a contamination plume to be mapped.
Decontamination can be achieved in situ, easier and less expensive than ex situ, which involves pumping and storage of groundwater while remediation is achieved, then recharging the groundwater with the treated water, or with fresh water. Methods vary with the particular contaminants as well as the geology of the area and the element of individual, or in most cases collective, choice. Decisions are based on risk assessment as well as technical, environmental, administrative and economic considerations (Bardos et al, 2000).
Regulations are also important in remediation. While each region, state, or country will have its own laws governing the clean up of sites and disposal (American Re, 1997), the principles are similar; don’t clean up one area by messing up another, and don’t cause a nuisance or hazard while cleaning up.
This paper examines a technology that has been found to be effective, economic and expedient, the use of molasses. The addition of molasses to enhance bioremediation is discussed in terms of chemical reactions, technical applications, cost and success.
Why molasses?
The object of adding molasses to a contaminated site is to facilitate biodegradation. Microbes feed on both the molasses and on the contaminant; the existence of the molasses allows rapid proliferation of the microbes, in turn causing more rapid breakdown of the contaminants.
There are some major benefits of using molasses in remediation. As a food grade product it will not leave any secondary contamination, which is important for both sustainability principles and in a regulatory context . If used in situ, by injection of the molasses into the plume or groundwater, there is minimal disruption to the local area; even if the groundwater has been pumped out to treat, the mechanism does not take up much space or need heating, aeration or other energy intensive processes. Stakeholders are normally content that molasses will not have any other impacts, which avoids lengthy repetitive discussions. Finally, the cost of molasses is relatively low, when compared with other methods of remediation.
Chemicals and Chemical Reactions
Biodegradation is the breaking down of complex compounds into smaller chemical units using bacteria or fungi. Mineralization is the breakdown to biomass and inorganic molecules, an ideal end product of the process.
The biodegradation technique involving molasses has been successfully applied to a variety of organic chemical compounds including:
· Trichloroethylene (TCE)
· 1,1 Dichloroethylene (DCE)
· Vinyl Chloride
· Carbon Tetrachloride (CT)
· (CF)
· Chlorinated Propanes
· Pentachlorophenol (PCP)
· Pesticides
And also metals:
· Hexavalent chromium
· Nickel
· Zinc
· Lead
· Cadmium
· Mercury
· Uranium
Processes involved in degradation of the material depend on certain environmental parameters (Liles et al, 2001). Limiting factors for biodegradation include aerobic (oxidizing) conditions, weak reducing conditions, organic carbon deficit, electron acceptor deficit, nutrient deficiency and a bacterial population that is compromised by stressors.
The most common limiting factor in the breakdown of organic chemicals is lack of organic carbon. For the most efficient breakdown to occur the ratio of organic carbon to chlorinated aliphatic hydrocarbons (CAHs) needs to be at least 100:1. Figure 1 shows the breakdown pathways of methane and TCE with co-metabolites.
For metal precipitation the mechanism is slightly different. Metals can be reduced in an aerobic environment, which is achieved by proliferation of bacteria, causing oxygen use within the soil matrix. Once an anaerobic state is achieved many metallic compounds precipitate out. Chromium hexavalent, for example, degrades to chromium trivalent, a less dangerous form of the chemical.
The Microbes
Many of the microbes that are useful for remediation are also the subjects of research into genetic manipulation. Their usefulness can be extended to other fields.
Table 1 gives some examples of bacteria species that could be usefully employed in remediation.
Table 1. Species of bacteria for biodegradation of chemicals
Species / UseSphingomonas aromaticivurans / May help in degrading toxic compounds in soil, including aromatic hydrocarbons such as toluene.
Ferroplasma acidarmanus / May help study remediation of damage caused by metal ore mining.
Ralstonia eutropha / May be involved in bioremediation related to heavy metals.
Rhodobacter sphaeroides / Has possible applications in renewable energy production and bioremediation.
Pseudomonas fluorescens / Useful in bioremediation of organic compounds.
Desulfitobacterium hafniense / Involved in remediation of solvents containing chlorine.
Burkholderia cepacia / LB400 may prove useful in bioremediation of PCBs.
Original data from: Amber, D., (2000)
Some bacteria can use the contaminants as their sole carbon source and have been found effective against fuels in soil, but have been improved by the addition of some minerals. In an experiment on microbial consortiums carried out in Mexico, diesel fuel in soil was consumed by the bacteria at an impressive rate (Marquez-Rocha et al, 2001). After 13 days the diesel had decreased by almost 90%, compared to 5% in the control, with the bacterial population increasing at the same time. Figure 2 shows a gas chromatograph analysis for the soil at the start of the experiment (A) and after 13 days (B). While consuming the carbon in this case the bacteria also degraded many other chemicals in the fuel. Molasses acts as the carbon source for compounds that do not contain large amounts of hydrocarbons. In other cases molasses feed the bacteria that cause anaerobic conditions.
Illustrations of some bacteria that are useful in biodegradation.
Azoarcus tolulyticus, an anaerobic toluene degrader that was isolated from a Michigan gasoline contaminated aquifer (Zhou et al, 1995).
utrophus
Alcaligenes eutrophus, one of a family of metal tolerant bacteria used in membrane bioreactors (Van Roy et al, 1997).
Desulfitobacterium hafniense. Members of this genus dechlorinate both aromatic and alkyl chlorinated compounds including some of the most problematic pollutants, e.g. chlorinated phenols, chlorinated ethenes (widely used solvents) and there is suggestive evidence that they may also dechlorinate polychlorinated biphenyls (Christiansen and Ahring, 1996).
Case Studies
1. RCRA Cleanup Reforms: Region 4 Success Story - Successful Pilot Test: Enhanced In-Situ Anaerobic Dechlorination of PCE and Reductive Precipitation of Uranium. (EPA, 2001)
RCRA is the EPA Resource Conservation and Recovery Act and deals with sites that are still being used, whereas the Superfund deals with sites that are no longer being polluted but just need remediating. This case study is about a Nuclear Fuel Services Inc. plant in Erwin, Tennessee. The problem is groundwater contamination at this uranium fuel production facility and the contaminants concerned are dissolved uranium and perchloroethylene (PCE). There are several requirements to be met from:
NRC, Nuclear Regulatory Commission
EPA, Environmental Protection Agency
TDEC, Tennessee Department of Environment and Conservation
The facility covers 64 acres of the mountainous region and is within Erwin city limits. Geologically the site is in the alluvial valley of the Nolichucky River. Up to 30 feet of unconsolidated alluvium underlies the plant. This consists of cobble, gravel, sands, clays and silt. As the alluvial bed becomes deeper it is coarser with a cobble, boulder layer over fractured bedrock of shale, sometimes steeply dipping and interbedded with dolomite and siltstone.
The plant has been in operation since the late 1950’s and in 2000 the PCE plume that exceeded drinking water standards was found to be 19 acres, with concentrations ranging from 0.005 to 14 mg/L. The uranium plume was much smaller and covered about 0.7 acres with concentrations of 30 to 1100 pCi/L (picoCuries per litre), where the EPA MCL is 30 pCi/L. Although a risk assessment had indicated that no further action was warranted the decision was made to remediate the groundwater to ‘as low as reasonably achievable’ (ALARA).
The selected technology was enhanced anaerobic bioremediation and reductive precipitation, patented by ARCADIS Geraghty and Miller. This was the first time this technology was tried on dissolved uranium and involved molasses expediting oxygen depletion and thereby enhancing reductive dechlorination of the PCE and immobilisation of the uranium as an insoluble precipitate.
Before the test was initiated field parameters of water-level elevation, pH, conductivity, temperature, total dissolved solids, dissolved oxygen, oxidation-reduction potential, alkalinity, and ferrous iron were measured. Approximately 2300 gallons of diluted molasses was injected over six months and the groundwater was tested four times during the six months through test and monitoring wells. The water was analyzed for: PCE, trichloroethylene, trans- 1,2-dichloroethylene, cis-1,2-dichloroethylene, and vinyl chloride, tributyl phosphate, total and dissolved iron and manganese, phosphate, sulfate, nitrate/nitrite, total organic carbon, dissolved organic carbon, chloride, biological oxygen demand, chemical oxygen demand, ammonia nitrogen, total and dissolved uranium, ethene, ethane, carbon dioxide and methane.
The PCE was reduced by 83% and the uranium by 60% overall with a greater reduction below the injection well for both contaminants. Figures 3 and 4 show the plumes before and after the treatment.
The report on this site states that while the technology may produce similar results at other sites the rock formation is a controlling factor and a lower hydraulic conductivity will slow down both the injection and the dispersion of the reagent. The following case study looks at an instance where the molasses treatment was not as successful as other reagents.
2. Bioremediation of Trichloroethylene-Contaminated Sediments Augmented with a Dehalococcoides Consortia. (McKinsey et al, 2003)
The site is the Department of Energy’s site at Savannah River in Aiken, South Carolina, where a number of chlorinated ethenes are to be found in sediments. The main contaminant is trichloroethylene (TCE) which degrades to vinyl chloride (VC) and cis-dichloroethylene (cDCE) among others. Initially the sediments were tested in the laboratory with the addition of a variety of nutrients: acetate, lactate, molasses, soybean oil, methanol, sulfate, yeast extract, Regenesis HRC®, and MEAL (methanol, ethanol, acetate, lactate mixture). After 9 months there was no significant degradation of TCE.
Fresh TCE was put into the sediments and a mixed culture of bacteria, including Dehalococcoides ethenogenes, was added. Within two weeks some of the degradation products, VC and cDCE, were detected, but there were mixed results for each of the bioaugmentation nutrients.
Lactate and sulfate augmentation showed the highest rate of degradation, with complete transformation of the TCE to ethenes after 40 days. Enhanced biodegradation was also found with soybean oil, acetate, yeast extract, and methanol. Molasses and Regenesis HRC showed only limited transformation, although after 210 days there was noticable degradation.
The conclusion to this study states that in field tests, even with nutrient additions, the indigenous microorganisms only impacted the contaminants minimally. Does this present a case for introducing microorganisms as well as nutrients? Had other types of contamination at the site killed the bacteria that otherwise would have been able to provide anaerobic conditions to degrade TCE?
3. Molasses Injection at the Avco Lycoming Superfund Site, Williamsport, Pennsylvania. (EPA 2000).
This 28 acre site has had a variety of industrial uses since 1929. Bicycles, sewing machines, sandpaper, silk and tools have been produced, for a number of different companies. Currently aircraft engines are manufactured there. Disposal methods have historically been of the dump and hide type, involving both wet and dry wells and a slurry storage lagoon.
Groundwater contamination was officially identified in the mid 1980’s, with volatile organic compounds (VOCs) in the well field of the local municipality 3,000 feet south of the site. These included trichloroethylene (TCE), dichloroethylene, (DCE) and vinyl chloride (VC). As well as the VOCs there are at least two heavy meatals present in the groundwater, cadmium and hexavalent chromium. In 1990 the site was placed on the National Priority List and in 1991 a Record of Decision was issued requiring pump and treat remediation for the shallow groundwater. This was never implemented due to failure to secure a permit to discharge the treated water into a local stream!
In 1995 it was decided to try in situ bioremediation using molasses, and air sparging for vapour extraction. The two technologies were pilot tested from October 1995 to June 1996. Due to high water levels the air sparging has not been impleted but the molasses treatment began in 1997. The system consisted of 20 four inch injection wells ranging from 19 to 30 feet in depth. The molasses is added from a building connected to each injection well with the rate and concentration depending on system monitoring results; the whole system is regulated by a programmable logic controller.
By July 1998 concentrations of TCE, DCE and hexavalent chromium were reduced to below their cleanup goals in many of the monitoring wells. The chromium was reduced by 90% from 1,950 mg/L to 10 mg/L. TCE was reduced by 90% and DCE increased initially due to TCE breakdown but then decreased, as did VC. The concentrations of these through time, from one monitoring well, are shown in fig. 5.