WERNER, David (29 Felton Avenue, Newcastle-upon-tyne, Tyne And Wear NE3 3NX, GB)
SINGLETON, Ian (54 Hartley Avenue, Whitley Bay, North Tyneside NE26 3NT, GB)
JONES, Martin (8 Edlingham Close, South GosforthNewcastle-upon-tyne, Tyne And Wear NE3 1RH, GB)
WERNER, David (29 Felton Avenue, Newcastle-upon-tyne, Tyne And Wear NE3 3NX, GB)
SINGLETON, Ian (54 Hartley Avenue, Whitley Bay, North Tyneside NE26 3NT, GB)
Claims
1. A method for treating soil contaminated with at least one pollutant comprising the steps of
(a) mixing the soil with an amount of biodiesel in the range 1-20% by weight;
(b) adding nutrients in an amount in the range 0.1-10% by weight of biodiesel added;
(c) forming the resultant mixture into a heap; and
(d) leaving the heap for a period of up to 12 months.
2. A method as claimed in Claim 1, further comprising the step of mixing the soil with activated carbon after step (d).
3. A method as claimed in Claim 1 or 2, wherein the at least one pollutant is a polycyclic aromatic hydrocarbon (PAH).
4. A method as claimed in any of Claims 1 to 3, wherein the biodiesel is selected from the group comprising vegetable oil esters and fat esters.
5. A method as claimed in any preceding claim, wherein the nutrients are selected from the group comprising phosphorous and nitrogen containing fertilisers.
6. A method as claimed in Claim 5, wherein the nutrients are in an oleophilic form.
7. A method as claimed in Claim 5 or 6, wherein the carbon - nitrogen - phosphorous ratio is substantially 100:10:1, wherein the carbon is provided by the biodiesel.
8. A method as claimed in any preceding claim, wherein the mixture is formed using a mixer.
9. A method as claimed in any preceding claim, wherein the heap is in the form of a windrow.
10. A method as claimed in Claim 9, wherein a windrow turning machine is used to form the mixture.
11. A method according to any preceding claim, wherein the mixture is inoculated with a microbial innoculant.
12. A method according to any preceding claim, wherein the windrows are formed upon a base, the base being substantially impermeable and including leachate collection means.
13. A method according to Claim 12, wherein the leachate is removed from the collection means and applied to the contaminated soil in the windrow.
14. A method according to Claim 12, wherein the base comprises a substantially impervious sheet and leaching is also controlled by covering the soil with a sheet material.
15. A method according to any preceding claim, comprising the further step of mixing a bulking agent with the soil.
16. A method according to Claim 15, wherein the bulking agent is selected from the group comprising: straw, wood chip and compost.
17. A method for treating soil contaminated with pollutants substantially as shown in, and as described with reference to, the drawings. |
Bioremediation Method
Field of the Invention
The invention relates to a method of bioremediation of contaminated soil, particularly of soil contaminated by polycyclic aromatic hydrocarbons (PAHs).
Background of the Invention
Land sites contaminated with pollutants such as hydrocarbon by-products of the coal, oil or automotive industries are unable to be used for other purposes such as residential housing development unless the contaminants are removed or reduced to acceptable levels. Methods of treating such contaminated sites include removing the contaminated soil to landfill sites, incineration of contaminated soil, or, more recently, through bioremediation of the soil to remove the pollutants.
Removal of contaminated soil to landfill is very costly and only shifts the problem to another location. Incineration of contaminated soil is also very costly and itself an environmental problem.
Bioremediation processes use micro-organisms to break down pollutants in contaminated soil. Typically a bioremediation process involves enhancing the growth of organisms already present in the soil through the addition of fertilizer and oxygen. Additionally, specialised microbes, for example white rot fungi, may be added to the soil to increase rates of biodegradation.
From an economic point of view, incineration of contaminated soil costs around £300- 900 per ton, whereas costs for bioremediation of the same soil are likely to be in the region of £15-150 per ton. In one example in the UK 34,000m of soil contaminated
with PAH tar and diesel was successfully bioremediated at a cost saving of /1.26 million pounds over landfill.
Coal tars are by-products of the coal carbonization process and are often present as contaminants at the sites of coke production and manufactured gas plants. Coal tar includes abundant PAHs with low aqueous solubility. Traditional bioremediation methods for removing this type of pollution are often unsuccessful. This is because a pollutant having a low aqueous solubility has low bioavailability, and therefore the rate of biodegradation by micro-organisms is low.
The inventors have discovered that the addition of biodiesel together with nitrate and phosphate to soil containing coal tar, in laboratory and field experiments, resulted in degradation of coat tar PAHs that was not apparent when nutrients alone were added (L.T. Taylor, D.M. Jones, Chemosphere 44 (2001) 1131-1136). The experiments were only conducted on a small scale and were not suitable for large scale land regeneration projects.
It would be desirable to provide an improved method for the large-scale bioremediation of contaminated soil.
Summary of the Invention
One aspect of the invention provides a method for treating contaminated soil as specified in Claim 1.
Preferred aspects of the invention are specified in the claims dependent on Claim 1.
The present invention offers an improved method that mitigates the above-identified problems with low aqueous solubility pollutants, and provides more effective bioremediation of contaminated soil.
Addition of biodiesel to soil contaminated with PAHs solubilises the PAHs and makes them more bioavailable for bioremediation by microbes, which are present in soil,
although additional microbes may be added to the soil if desired. Biodiesel is a nontoxic, diesel equivalent fuel made from vegetable oils or animal fats by transesterification. Rapeseed oil methyl ester (RME) and pure vegetable oil methyl ester (PME) are examples of biodiesels. It is envisaged that biodiesel will be used in its pure form (known commercially as the BlOO form) because in this form biodiesel has very low toxicity and is easily biodegradable.
The method of the invention involves mixing contaminated soil with biodiesel, preferably in an amount in the range 1 to 20% biodiesel to soil by weight. More preferably, the contaminated soil is mixed with biodiesel in the range 8% to 16% biodiesel to soil by weight. Nitrogen and phosphorous nutrients such as nitrates and phosphates can also be added to enhance the bioremediation of pollutants by naturally occurring microbes. Preferably nitrogen is added in an amount in the range 1 to 10% by weight of biodiesel added and phosphorous in an amount in the range 0.1 to 1.0% by weight of biodiesel added. An acceptable C:N:P ratio is 100:10:1, where the carbon is supplied by the biodiesel, which is approximately 77% carbon by weight.
Nutrients in an oleophilic form may be used to enable the nutrients to mix well with the biodiesel in the mixture. An example of a commercially available product is Inipol EAP22. In this product nitrogen and phosphorous nutrients are contained as urea and trilaurethphosphate together with oleic acid as an organic hydrophobic phase and 2- butoxyethanol as a surfactant and emulsion stabiliser.
The soil mixture is then formed into heaps, which advantageously are long heaps or piles known as windrows. Typically a windrow is 3m wide, 2m deep and several metres long.
Alternatively, contaminated soil may be piled into windrows, and the mixture formed by turning the soil with a windrow turner and adding biodiesel and nutrients. The biodiesel and nutrients may be added prior to, or contemporaneously with mixing.
Microbes or fungi, for example white rot fungi, may also be added to the soil mixture to enhance the rate of bioremediation.
Bulking materials, for instance straw or compost, may also be added to the soil to improve soil texture.
The heaps may be formed on top of an impermeable membrane or other impermeable base to prevent leaching of any pollutants back into underlying soil. A suitable impermeable base would include a concrete base with a drainage system to collect any leachate from the heap. Recovered leachate could then be sprayed onto the heap to allow for further bioremediation.
The heap may also be covered with sheet material, for example a compost cover or fleece. Such a cover allows moisture to escape from the windrow, but reduces water passing into the heap. This minimises leaching of pollutants from the heap into the surrounding environment.
The heaps are then left to allow the bioremediation reactions to occur. The heaps may be turned over at intervals using a standard windrow turning machine in order to promote an aerobic environment within the heap.
Typically the bioremediation reactions will reduce pollutant concentrations to within acceptable limits within a period of between two and twelve months. In fact, in most cases the bioremediation is complete within two to three months.
Bioremediation reactions are unlikely to go fully to completion and after the bioremediation step there may still be undigested pollutants or metabolites in the soil, albeit at low levels. However, despite the low concentrations, the presence of residual biodiesel in the soil means that these remaining pollutants may be more bioavailable than prior to remediation, albeit at a reduced overall concentration in the soil. In such instance activated carbon or another suitable sorbent may be added to absorb and stabilize remaining pollutants or metabolites. Preferably activated carbon is added in an amount in the range 1 to 2% by weight. This step lowers the bioavailability of pollutants remaining in the soil. The step of addition of activated carbon may be complete by spreading the material onto the surface of the heap and then mixing the heap.
Brief Description of the Drawings
In the drawings, which illustrate preferred embodiments of the invention and are by way of example:
Figure 1 illustrates a standard windrow turner;
Figure 2 illustrates the windrow turner of Figure 1 straddling a windrow;
Figure 3 illustrates a site for performing bioremediation of soil according to the invention; and
Figure 4 illustrates a windrow covered to prevent ingress of rainwater.
Detailed Description of the Pr efer r ed Embodiments
Referring now to Figures 1 and 2 a windrow turner is shown generally at 1. The turner 1 includes several spray outlets 2 for spraying matter onto a windrow. As the windrow turner is driven through a windrow a rotatably mounted rotor 3 is rotated in order to mix the soil. Figure 2 shows the windrow turner 1 straddling a windrow 4. A biodiesel/nutrient mixture 5 is sprayed onto the windrow 4 through spray outlets 2. The windrow turner 1 is driven along the windrow 4 and the rotating drum 3 mixes the biodiesel/nutrient mixture 5 with the soil in the windrow 4. Mounted on top of the turner 1 is a tank 6 containing the biodiesel/nutrient mix 7. A pump 8 draws the biodiesel/nutrient mix from the tank 6, pressurises the fluid and forces the pressurised fluid through the spray nozzles 2.
Referring now to Figure 3, there is shown a site 10 upon which bioremediation of soil is performed, the site 10 including a windrow 4 located on an impermeable base 11 formed of concrete. The base 11 is constructed so as to have a fall in both the "X" and "Y" directions. Rain falling on the base 11 collects in a drain 12 as does leachate emanating from windrow. The fluid collected in drain 12 is transferred via a pipe 13 to a collection
and sedimentation chamber 14, which is itself provided with an outlet pipe 16. A pump 15 is located adjacent to the chamber 14 and is connected to one end of the pipe 16 so as to draw fluid out of the chamber 14. The pump 15 delivers the fluid via pipe 17 in a manner desired by the operator of the site 10. For example, the tank 6 of a windrow turner 1 could be filled for application of the leachate to the material in the windrow 4 during turning, or the leachate could be applied directly to the surface of the windrow 4. Any PAH's in the leachate are then subjected to further bioremediation.
Referring now to Figure 4, the windrow 4 is covered with a sheet 20 which reduces water ingress to the windrow from rainfall but does permit moisture transfer. In the example of Figure 4 the sheet 20 is in the form of a fleece.
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