Robert Rynk
BioCycle February 2004, Vol. 45, No. 2, p. 26
A bioremediation project in Emeryville, California is using cheese whey, the liquid by-product from cheese making, to treat groundwater that has been contaminated with chromium – hexavalent chromium to be specific. The goal of the project is to reduce the groundwater’s chromium concentrations to levels that meet drinking water standards.
The approach is to encourage microorganisms to convert hexavalent chromium into trivalent chromium. While hexavalent chromium is water-soluble and mobile, the trivalent form is poorly soluble and, at normal pH levels, precipitates from the water into the soil matrix. As the hexavalent compounds transform into the trivalent species, the trivalent compounds precipitate out of the groundwater.
The purpose of the cheese whey in this strategy is to inexpensively supply nutrients, especially carbon (C), to the microorganisms responsible for the chromium conversion. The C provided by the cheese whey supports a larger population of organisms and leads to faster and greater reductions of hexavalent chromium. The project is an example of the capacity and versatility of organic matter to foster healthy environmental processes.
THE SITUATION
The remediation project is located on an ordinary-looking parcel of commercial/industrial land in Emeryville, situated about a quarter-mile from San Francisco Bay on the outskirts of Berkeley. The property was once the site of a chrome-plating facility, which operated there from 1951 through 1967. Although there were apparently no major incidents of pollution, over the years of the plating facility’s operation, chromium was released to the surrounding soil, possibly from minor spills and routine drainage on the site. Chromium has since migrated to the groundwater beneath and near the site.
The property was recently purchased. A new commercial tenant already occupies part of the property (the less contaminated part), and tenants will be sought for the remaining part. While the threat is not imminent, the regional water quality authority has required that the soil and groundwater be remediated to protect human health and groundwater resources. There are two primary concerns at this site — future use of the groundwater and health risks to construction workers removing soil or working in trenches. To help finance the remediation, the new owner obtained a low interest loan from Emeryville under the city’s “Capital Incentives for Emeryville’s Redevelopment and Remediation” program. This program is based on federal funding and thus requires some oversight from the EPA.
As an initial step in the remediation, Jim Gribi’s company – Gribi Associates – conducted an Engineering Evaluation/Cost Analysis (EE/CA). Building on previous investigations at the site, Gribi found high levels of hexavalent chromium in surface soils in some areas, primarily near the southwest corner of the site, the apparent point of contamination. In addition, samples from monitoring wells found elevated levels of hexavalent chromium in the groundwater in a plume extending 300 ft. west of the site in the direction of groundwater flow. As measured by Gribi Associates, the concentrations of hexavalent chromium decreased rapidly from a maximum of 630 mg/l, at the southwest corner of the property to 63 mg/l at a point 50 ft. away and further decreased to between 1 to 5 mg 100 ft. away. Because of the relatively impermeable soils underlying the site, the chromium has not moved into the deeper soil profiles (below 20 ft.).
The EE/CA identified three remediation approach: no action, that is, relying on natural attenuation of hexavalent chromium; excavate the contaminated soil and pump-and-treat the groundwater; and in-situ treatment by injecting cheese whey into the soil and groundwater. Natural attenuation was estimated to be unacceptably slow (over several decades) and did not protect construction/trench workers. Although costly, the soil excavation/pump-and-treat alternative would involve removing and landfilling the soil with high levels of chromium plus pumping the plume of contaminated groundwater and treating it to remove the chromium (e.g. by ion exchange or reverse osmosis). In both cases, a contaminated waste stream would result.
Regarding the third alternative, Gribi Associates conducted a pilot test that involved installing injection wells into shallow groundwater, and injecting cheese whey into soil and groundwater. According to Gribi, during this pilot test, the hexavalent chromium concentrations measured decreased by more than 99 percent in several of the wells over a two-month period. Based on these results, plus the drawbacks of the other alternatives, in-situ remediation using whey was chosen as the most practical approach for the full-scale remediation.
THE PROJECT
The full-scale remediation of the site is scheduled to begin in early 2004 after EPA gives final approval of the project plan. The remediation will involve monthly injections of diluted cheese whey through a network of 22 injection wells. The project will require at least 15,000 gallons of cheese whey and the installation of 15 new injection wells and eight new monitoring wells. It is hoped that after six injections and one year of residence time, samples from the monitoring wells will show that the hexavalent chromium levels in the groundwater plume meet the 0.1 ppm requirement.
The strategy is to place the injection wells in a line that creates a reaction zone near the point of contamination. Previous work has shown that the cheese whey injected in this reaction zone will promote the conversion of hexavalent chromium to trivalent as the groundwater moves down gradient. Additional wells will be dispersed at a few down gradient locations. At this site, placement of the wells is limited as much of the chromium contaminated groundwater plume lies under buildings. Thus, the network of injection wells will be located on the property, on an adjacent railroad parcel (no longer used by trains), and on public streets near and down gradient from the spill property.
During each monthly injection event, the whey will be diluted with water, and approximately 300 gallons of diluted whey solution will be injected into each of the 22 injection wells. Whey dilution and injection is conducted using a portable injection truck. The truck has a whey tank and a dilution tank. Whey is combined with water in the dilution tank, to make about 300 gallons of whey solution, and the solution is then pumped through a manifold into three injection wells simultaneously. This is repeated twice for the three wells, so that a total of about 900 gallons of whey solution is injected into the three wells (300 gallons per well). Each monthly injection event will take about eight days to complete.
Because the bioremediation process can result in methane production, the concentration of the cheese whey solution will be lowest in wells next to buildings. In addition, the volume of cheese whey actually injected and the dilution will be adjusted depending of varying soil characteristics and on the results of the monitoring program. Gribi notes, “since this is a relatively new technology, we will add to our knowledge base as we go, thereby allowing us to modify the injection and monitoring program to optimize results.”
To assess remediation effectiveness, groundwater will be periodically monitored for total and hexavalent chromium, as well as for chemical indicators such as pH, oxygen-reduction potential, methane and dissolved oxygen. The monitoring wells will be dispersed at key points around the plume. A few wells are situated outside of the plume boundaries to determine whether the injections force the plume to expand outward.
The cheese whey for the project will be obtained from a small specialty cheese producer in Benicia, California. This source also provided the whey for the pilot tests so Gribi is confident that it will be effective. The technique is novel enough such that the key characteristics of the whey (e.g. organic matter concentration) have not been fully established yet. “We hope to better define the important parameters during the remediation project, since we will have more time and resources,” says Gribi. “We do know that both temperature and pH are key factors. We prefer to have a low pH, in the 4.5 to 5.5 range, because acidic conditions favor reduction of the hexavalent chromium, but this range is not so low as to kill the microorganism.” According to Gribi, the pH of the undiluted cheese whey has consistently been between 3 and 4. The Benicia cheese manufacturer produces 5,000 to 6,000 gallons of whey per week so there is not likely to be a supply problem. Just in case, Gribi has lined up other cheese producers to supply whey if needed. A storage tank will be installed at the remediation site to hold about 2,000 gallons of whey and thus allow a flexible delivery schedule over the injection period.
The Emeryville project is only one example of the use of organic by-products, such as cheese whey, to help reverse contamination of soils and water. Whey injections also enhance reductive dechlorination of common chlorinated solvents (such as the dry cleaning solvent tetrachloroethene, also known as PCE or “perc”). The whey injections promote anaerobic degradation of PCE to various “daughter” products, and eventually to less harmful methane and carbon dioxide gases. This degradation process can typically occur in a short time period (several months). Gribi Associates is currently conducting active cheese whey remediation of a PCE-contaminated groundwater plume on a former industrial site in Emeryville, California. Whey injections were conducted over a three-month period, and initial results have shown up to 90 percent reduction in PCE, with concurrent increases in breakdown products.
THE SCIENCE OF CHROMIUM REMEDIATION
At high enough concentrations, chromium is considered a health risk. Short-term exposure to high concentrations in water (e.g., 1 ppm) may cause skin irritations or ulcers. Long-term exposure potentially leads to damage to the liver, kidney, nerve tissue and skin. It is generally not considered a carcinogen when ingested (e.g. with drinking water), though it may be a cancer risk when inhaled. The U.S. Environmental Protection Agency (EPA) has established a drinking water standard for chromium at a Maximum Contaminant Level (MCL) of 0.1 parts per million (ppm). This MCL is for total chromium, although it is largely the hexavalent chromium compounds that are present in water.
A valence, also called an oxidation state, refers to the number of electrons that an element shares when it combines with other elements. Chromium can share electrons in different combinations with different elements and so it has more than one valence. In the environment, chromium primarily exists in two valances, hexavalent (Cr+6 or Cr(VI)) and trivalent (Cr+3 or Cr(III)).As noted above, chemical compounds that include hexavalent chromium are water soluble, mobile and comprise the more toxic form of chromium. Compounds that develop from trivalent chromium are poorly soluble in water and remain immobile in soils, except under extremely acid or alkaline conditions. Compared to the hexavalent varieties, trivalent chromium compounds are more stable and much less toxic.
Fortunately hexavalent chromium compounds can be chemically “reduced” to the trivalent forms, that is, the hexavalent chromium takes on electrons from other chemical compounds that “donate” the electrons. Readily-available electron donors in the soil include iron, reduced sulfur compounds and soil organic matter. Given time and available electron donors, hexavalent chromium tends to form trivalent chromium. Under “oxidizing” conditions, the reverse could happen; trivalent chromium can be converted to hexavalent. However, natural environmental conditions favor the former reaction, so the tendency is for hexavalent chromium to change to trivalent (until the two forms reach a balance that depends on their relative concentrations, the availability of other reducing and oxidizing compounds and environmental conditions such as pH and moisture).
Biology is another important factor in the chromium picture. Several bacteria, and at least one type of fungus, can reduce hexavalent chromium. Although microorganisms can do this under aerobic or anaerobic conditions, anaerobic environments are more conducive to chemical reduction. Because of their ability to synthesize enzymes, microorganisms usually accelerate the rate of chemical reactions. Jim Gribi, a geologic consultant overseeing the Emeryville project, explains: “These aerobic and anaerobic microbes, as part of their normal physiology, reduce hexavalent chromium to trivalent chromium, which in turn, precipitates out of the groundwater, primarily as chromium hydroxide. This reduction process can occur relatively quickly, over several months, making cost-effective in-situ remediation possible.”
It is well-recognized that for in-situ groundwater remediation, an easily available organic food source increases the growth of microorganisms and thus enhances treatment,. The food source provides the microorganisms with energy, carbon and other nutrients. In addition, the organic matter depletes oxygen and shifts the environment toward anaerobic and reducing conditions. The organic compounds can also serve as an electron donor for the chemical reduction process. Molasses is the typical organic material used for these applications. In the present project, cheese whey serves the same purpose. The advantage of whey is that it is frequently available in abundance and at little expense.
THE REMEDIAL QUALITY OF ORGANICS
Organic materials possess versatile powers to remediate environments where chemical compounds have, for one reason or another, accumulated in excess to create serious environmental hazards. The chemical may be synthetic and toxic at even low concentrations, as in the case with many pesticides. Application of organics often changes the situation enough to degrade, convert or otherwise render the toxin immobile and harmless. Sometimes, the organic amendment serves as a food source for plants or microorganisms that do the work. Sometimes it is the physical and chemical qualities of the organic substrate that makes the difference in achieving remediation.
BIOREMEDIATION OF A PCB-CONTAMINATED SOIL VIA COMPOSTING
(Based on studies at Ohio State University and Michigan State University) PCBs (polychlorinated biphenyls) are a group of hazardous environmental pollutants classified as carcinogens by the U.S. EPA. Due to their widespread use and the lack of regulations governing their disposal in the past, PCBs now contaminate many industrial and natural areas. They are of great concern because of their environmental persistence in both terrestrial and aquatic environments. Although now banned in most areas, PCBs were used in the past as heat transfer fluids, dielectric fluids, flame retardants, solvent extenders, and ink carriers.
In this study — performed by Fred Michel, John Quensen and C.A. Reddy — a PCB-contaminated soil from a former paper mill was mixed with a yard trimmings amendment and composted in field-scale piles to determine the effect of soil to amendment ratio on PCB degradation. Temperature, oxygen concentrations and a number of other environmental parameters that influence microbial activity during composting were monitored. The PCBs in contaminated soil had an average of four chlorines per biphenyl. The soil was composted with five levels of yard trimmings amendment (14 percent to 82 percent by weight) in pilot scale compost piles (25 m3) turned once per month. Results showed up to a 40 percent loss of PCBs with amendment levels of 60 percent and 82 percent. Bench-scale studies indicated that less than one percent of the PCBs in the contaminated soil were volatilized from composts during incubation with forced aeration at 55°C. In conclusion, PCB loss observed during the composting of the PCB-contaminated soil appeared to be largely due to biodegradation and not volatilization. Effective bioremediation of aged PCB-contaminated soils may require coupling of composting with additional remediation technologies to reduce levels of PCB congeners with greater than four chlorines.
