Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
  • B09C—RECLAMATION OF CONTAMINATED SOIL
  • B09C1/00—Reclamation of contaminated soil
  • B09C1/10—Reclamation of contaminated soil microbiologically, biologically or by using enzymes

Definitions

• This invention relates to a controlled microorganism degradation process for decontaminating soil or sediments contaminated with hexachlorobenzene (HCB).
• Hexachlorobenzene is produced as a reaction side-product in the production of chlorinated solvents, mainly carbon tetrachloride, perchlorethylene, trichlorethylene and dichlorethylene.
• HCB is also used in the manufacture of seed treatment agents and a variety of industrial applications. Numerous land sites exist that are contaminated, and have been contaminated for decades, with the highly toxic HCB. Many of these sites are unusable and will remain unusable until some economical process becomes available for large scale decontamination of the soil of such sites.
• Zeneca Corp. U.S. Patents Nos. 5,660,612 and 5,660,613 disclose the remediation of soil contaminated with DDT by anaerobic composting followed by aerobic composting under specific conditions of water content, temperature, redox potential and the presence of unacclimated soil-indigenous anaerobic and aerobic microbes capable of transforming DDT into harmless materials.
• the present invention provides a process of decontaminating soil and/or sediments containing HCB contaminant by converting the contaminant into harmless materials. This process is capable of decontaminating the soil to whatever extent desired, either partial decontamination or complete remediation. More particularly, the invention provides a process of decontaminating soil contaminated with HCB comprising:
• the process comprises preparing an inoculant that contains populations of viable anaerobic and aerobic microbes that are capable of degrading the contaminants into harmless materials, and are viable under both the anaerobic and aerobic conditions throughout the process.
• the soil is then treated with these microbes by solid state composting as hereinafter described.
• the inoculant is prepared by mixing a small amount of contaminant, preferably from 10 to 100 by wt. of the final inoculant mixture, with a conventional highly nutritious aqueous formulation normally referred to as a heterotrophic medium. Desirably, some soil is added to this mixture, which remains liquid. The liquid mixture is then stored at a temperature from about 20°C. to 30°C. for about 6 to 10 weeks. During this time, acclimated indigenous anaerobic and aerobic microbes form that are capable of transforming the specific contaminants into harmless materials and are viable under both the anaerobic and aerobic conditions throughout the process.
• the decontamination treatment comprises mixing the HCB contaminated soil with amendment material to form a solid compost mixture containing organic nutrient materials; adding to the compost mixture 1 to 20% v/v of the inoculant to the compost mixture in the range of about 20 to 65 °C. and the water holding content of the compost mixture in the range of about 40% to 100% water holding capacity (WHC); during this composting maintaining the redox potential of the compost mixture below about negative 200 mV until a significant amount of contaminant is degraded, mostly to toxic HCB metabolites; thereafter raising the redox potential of the compost mixture above about positive 100 mV until a further significant amount of contaminant is degraded.
• WHC water holding capacity
• composting means transforming by degradation contaminants in the soil and/or sediment to harmless materials utilizing biological activity, the process being carried out in the solid state in the presence of organic nutrient material.
• Amendment means material comprising organic nutrient for the microbes that is added to the soil to be decontaminated.
• Constants means HCB and toxic metabolites from the degradation of HBC.
• Degrade when used with HCB and its metabolites, means the removal of one or more chlorine atoms and/or cleaving the benzene ring.
• Hard materials means materials that are unobjectionable in the concentrations present in soil or sediment for its intended use.
• Toxic metabolites of HCB means degradation products of HCB from which one or more chlorine atoms have been removed from the HCB molecule, but the HCB has not been completely degraded to harmless materials.
• Decontamination means ttansforming the contaminants to harmless materials, including biodegrading the contaminants and/or binding the contaminants to soil or other material.
• Remediation means decontamination to an unobjectionable level of the contaminants in the soil/sediment for the intended use of the soil/sediment.
• Solid means earth, i.e. humus, sand and paniculate rock, and includes sediment from above and beneath the surface of water.
• the soil to be decontaminated must contain throughout the process appropriate types of viable microbes capable of degrading the contaminants.
• Both anaerobic and aerobic microbes must be present. These microbes must be viable under both the anaerobic and aerobic conditions to which they will be subjected during the present process.
• the microbial populations must not be killed or adversely affected so that they will not adequately degrade HCB.
• the microbes normally are bacteria, fungi, actinomycetes, and to a lesser extent protozoa.
• the first step in the present invention is to prepare an appropriate inoculant from microbes, normally microbes initially present in the soil to be treated, that have been adequately acclimated to the contaminant that is to be subject to decontamination. Additional active microbes can be recycled from decontaminated soil.
• a solid compost mixture is prepared by mixing appropriate soil amendment into the soil to be decontaminated to give a total amount of amendment material of at least 10%, and up to about 95% by weight of the mixture, and preferably from about 30% to 70% by weight of amendment material, some of which may already be present in the soil.
• the soil amendment material comprises a conventional source of organic nutrients for the microbes during composting.
• the preferred amendment nutrient materials are agricultural waste and municipal waste sludge, preferably a manure such as horse, cow, sheep, turkey, chicken or fish manure, or activated sludge. Alfalfa, hay, sawdust, peat, grass and other bulking materials may be present in the compost mixture, originate in manure or be specifically added.
• the inoculant is added to the compost mixture in sufficient quantities to significantly influence the microbial population. At least 1 % is needed, and 1 to 20% inoculant v/v added to the amendment is preferred. In some cases it may be desirable to include in the soil amendment a surfactant to render the contaminants more available to biological degradation. Suitable surfactants include polysorbates, octoxynols, anionic alkyl sulfates, anionic alkyl aryl sulfates and ethoxylates.
• surfactants examples include TWEEN ® nonionic surfactants commercially available from ICI Americas, Inc., TRITON ® nonionic surfactants commercially available from Union Carbide and DAWNTM detergent commercially available from Procter & Gamble.
• a suitable mixture of surfactants is TRITON ® and DAWNTM.
• the amendment material may also contain, or be supplemented with, liquid or solid organic or inorganic nutrients. Organic materials high in nitrates and phosphates are normally used.
• the compost mixture is maintained in a moist but solid state. Throughout the process the moisture level is maintained at less than 100% of the mixture water holding capacity (WHC), preferably in the range of about 40 to 70% WHC.
• WHC mixture water holding capacity
• the temperature of the mixture is thereafter maintained within the range of about 20-65 °C. This is easily done by controlled air movement through the compost mixture (e.g. through pipes) and/or by the addition of nutrient material. Below about 20 °C, the biodegradation proceeds uneconomically slowly; above about 65 °C excessive microbe kill may take place. The preferred temperature range is within about 20-50 °C.
• the anaerobic microbes in the compost mixture remain viable for the subsequent aerobic degradation steps and the anaerobic microbes remain viable for any needed subsequent anaerobic degradation steps. Thus, it is essential that viable aerobic and anaerobic degradation microbes be maintained during the process of the present invention.
• a low redox potential level is maintained in the compost mixture, at least about negative 200 mV, and preferably in the range of negative 300 mV to 500 mV. This level has been found to be optimum for the anaerobic process.
• the redox potential level can be maintained within this range by moist air movement through the compost and/or by the addition of conventional reducing agents such as sulfite and acetate reducing compounds.
• the first anaerobic step and any subsequent anaerobic steps are continued until a significant amount of the contaminant is degraded in the step. This can be determined by analysis. Typically, in the first anaerobic step degradation of about 30% to 70% of tile initial content of the HCB contaminant is desirable.
• the mixture is oxygenated by any suitable means, preferably by air fed through and/or mixed with the mixture to achieve aerobic conditions. Sufficient oxygenation is applied for the redox potential level during the aerobic treatment to be maintained above about positive 100 mV.
• the aerobic conditions activate further degradation, yielding harmless materials.
• the aerobic degradation step is continued until a significant amount of the contaminants is degraded by the aerobic treatment.
• the desired degree of biodegradation of the contaminants for acceptable remediation will not be achieved in the first anaerobic/aerobic treatment sequence.
• the sequence is repeated one or more times as needed for acceptable soil decontamination. Substantially complete remediation is achievable by this multi-sequence preferred process.
• a desirable feature of this process is that the degrading microbes are maintained viable throughout the anaerobic/aerobic treatment cycles, so that it is not essential that microbes be supplemented before repeating the treatment cycle.
• redox potential levels of the compost mixture in the anaerobic and aerobic steps is necessary for effective practice of the present process.
• the appropriate redox potential levels can be maintained by the addition of conventional nutrient materials and/or reducing agents such as sulfite and/or acetate compounds. Absolute anaerobic and aerobic conditions are needed (although short localized excursions can be expected).
• a redox potential level of less than about negative 200 mV is considered anaerobic, and is required for the anaerobic steps; and a redox potential level greater than about positive 100 mV is considered aerobic and is required for the aerobic steps.
• the preferred redox potential level is in the range of about negative 300 to 500 mV; and during the aerobic steps it is in the range of about positive 200 to 300 mV.
• the redox potential level from about negative 200 mV to about positive 100 mV is considered anoxic.
• anoxic conditions are present in the compost. During this period, degradation of some amounts of the contaminants appears to take place, but at low rates. Thus, speedy transition from one state to the other expedites overall degradation.
• microbe counts are present, preferably up to 10 8 aerobic colony forming units (cfu) per gram, as measured by standard plate count techniques, and up to 10 6 anaerobic cfu/g. These microbe counts of course include microbes other than those that degrade the contaminants.
• the present process is conducted in a compost container, normally in a container cell or windrow.
• the soil to be treated can be analyzed and composted in the laboratory to determine optimum conditions of amendment composition, and anaerobic/aerobic treatment times and number of sequences.
• the present process involves an anaerobic composting stage followed by an anaerobic stage.
• Example I An inoculant is prepared for use in decontamination of soil containing HCB by mixing together 25 mL of dilute heterotrophic* medium; 625 micro liter of a 2 mg/mL methanol solution of HCB and 50% of an uncontaminated soil. The inoculant is then stored under anaerobic conditions for 6 weeks at 22 degrees C inside an anaerobic chamber. 1 kg (60ml) of soil containing HCB at approximately 25 ppm is then mixed with a compost mixture containing 4000 ml of cattle manure and 50 ml straw and the above described 50 ml of inoculant.
• Duplicate samples of the mixture are composted anaerobically in a chamber for 6 weeks.
• the water holding capacity, redox potential and temperature of the anaerobic compost mixture during this time averages about 70% , below 250 mV, and 22 degrees C, respectively.
• Significant degradation of the HCB takes place.
• the compost mixture redox potential is raised by oxygenation to aerobic conditions of the compost mixture.
• This aerobic phase is continued for 2 weeks.
• the WHC, redox potential, and temperature averages respectively about 70%, above 200 mV and 34 degrees C.
• Significant further degradation of the contaminants occurs during this aerobic phase.
• Example 1 The procedure of Example 1 is followed except that the amendment (compost mixture) ingredients, the composting times and the compost conditions are as indicated in the following Table.
• the inoculant is prepared by the following procedure of Example 1.

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• 1999
  • 1999-05-10 TW TW88107546A patent/TW446585B/zh active
  • 1999-05-14 WO PCT/CA1999/000440 patent/WO1999059743A1/en not_active Application Discontinuation
  • 1999-05-14 AU AU38068/99A patent/AU3806899A/en not_active Abandoned
  • 1999-05-14 EP EP99920501A patent/EP1079944A1/de not_active Withdrawn

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