The present invention relates to a method and means for

remediation of contaminated soil by biodecontamination

techniques.

W095/22374, W095/22375 and W095/22418 of common ownership

herewith describe processes for the decontamination of

material and land in-situ. Whilst these processes operate

satisfactorily, they require considerable capital

investment in plant and manpower and are unsuitable for

larger sites needing decontamination.

It is an object of the present invention to provide an

efficient and economic method and means for the

biodecontamination of soil having one or more of: toxic

heavy metals; metals; and, organic contaminants, without the necessity for heavy capital plant expenditure.

According to a first aspect of the present invention,

there is provided a method for the remediation of soil

polluted by one or more organic species and/or one or

more metal species by biodecontamination techniques, the

method comprising the steps of providing trough means,

said trough means being impervious to acidic and alkaline

liquors; providing support means at an opening of said

trough means, said support means having a permeable

membrane associated therewith; heaping soil to be

decontaminated on top of said permeable membrane; said

soil having therein sulphur oxidising bacteria micro¬ organisms; optionally providing aerobic conditions and

conditions to promote growth of micro-organisms to

initially degrade organic contaminants within said soil

heap where one or more organic contaminant species are

present; maintaining said optional conditions until said

one or more organic contaminants have been degraded;

then, providing a supply of nutrient and liquid to said

soil under conditions to promote growth of said sulphur

oxidising bacteria micro-organisms and generation of

sulphuric acid to promote conversion of said one or more

metal species to a sulphate; collecting a sulphate

leachate in said trough means which has been provided

with sulphate reducing bacteria and nutrient under

conditions to promote growth of said sulphate reducing

bacteria; and converting said sulphate leachate to a

metal sulphide.

According to a second aspect of the present invention

there is provided a composite bioreactor for the

remediation of soil containing one or more organic

species and/or one or more metal species, the bioreactor

including a combination of a sulphur oxidising bacteria

(SOB) bioreactor directly on top of a sulphate reducing

bacteria (SRB) bioreactor; the SOB bioreactor comprising

a heap of said soil to be remediated and having suitable micro-organisms therein to react with and degrade

contained contaminants; said SRB bioreactor comprising

trough means having support means to support said soil

heap associated with an opening thereof, said support means having a permeable membrane associated therewith to

allow passage of liquid but substantially no soil and,

sulphate reducing bacteria within said trough means; and,

liquid and nutrient supply means to promote growth of

said sulphur oxidising bacteria and said sulphate

reducing bacteria within said composite bioreactor.

It will be understood that the sulphur oxidising bacteria need a source of oxidisable sulphur to provide an energy

source. Where this is not already present in the soil to

be decontaminated, such a source will need to be added to

the soil, e.g. elemental sulphur or other sulphur

compounds. Similarly, it will be understood that sulphur

reducing bacteria normally require a carbon source which

could be, for example, ethanol, lactate, certain organic

pollutants (including, for example, volatile organic compounds - VOCs, phenol (s) , chlorinated aromatic

compounds) or combinations thereof; where these are not

present, for example due to destruction by the optional

organic degradation step, they will need to be added to

the leachate in the trough. The SRB reactor can thus be

arranged to act to degrade polluting organic materials

which have survived the SOB and any preliminary

degradation within the soil heap.

In addition to the production of sulphides by the SRB, it

is possible that soluble sulphur compounds will be

produced, typically as sulphites. These may be returned

to the SOB to provide sulphur, either directly or after a

preliminary oxidation step, as appropriate for optimum

operation of the bioreactor.

Preferably, the trough means may comprise a pit excavated

into the ground surface and, for example, lined with a

suitable impermeable plastics sheet material membrane

resistant to the effects of, for example, sulphuric acid,

ethanol, alkaline sulphide and hydrogen sulphide. The

dimensions of the trough are dependent upon the amount of

soil to be remediated and the size of the site on which the remediation is to be carried out.

Alternatively, the trough means may be constructed of

concrete slabs or some other suitable cheap material on

the ground surface.

The trough support means may be any suitable packing

material such as rubble, pebbles, broken masonry, stones or gravel or a mixture of any or all of them for example.

The support packing material may be approximately level

with the ground and be of sufficient porosity to allow movement or circulation of liquor therein at least over

restricted areas of the trough means.

Alternatively, the support means may comprise a metal

grid spanning the opening of the trough and having a

permeable membrane to substantially prevent soil from

entering the trough.

Preferably, the support means also comprises or is

associated with an impermable membrane to maintain the

integrity of the two reactors. When this is absent, it is possible to allow leachate to pass through the

permeable membrane directly to the trough, but when it is

present, it is necessary to make provision for passage of

the leachate from the soil heap to the SRB, e.g. by

passages through, or bypassing, the impermable membrane,

or by conduits and pumps.

The main function of the support means or packing in the

SRB bioreactor is to support the soil heap in the SOB

bioreactor above. It also facilitates the development and growth of an SRB biofilm.

The soil heap may be up to 2m in height or any suitable height consistent with the width of the trough and

stability of the soil heap.

The trough means and the soil heap may be provided with

extraction and/or supply conduits therein to extract or

supply liquids, nutrients, acids, alkalis and gases (such

as air for example) as appropriate. The conduits may have

suitable pump or suction means associated therewith to allow supply or extraction of liquids and/or gases. For

example, the trough means may have conduits arranged to

extract and pump back liquors to promote circulation and mixing thereof in the SRB bioreactor.

The SRB bioreactor may have supply/extraction conduits

arranged in two or more different levels of the trough means. For example, conduits may be provided to encourage circulation/distribution of ethanol, for example, as a

source of carbon for the SRB in the trough; and, the

distribution of an alkali such as sodium hydroxide

throughout the trough.

The SRB bioreactor will also be provided with liquid

extraction conduits near to its base and near to its top

for recirculation and so as to be able to take-off liquors for introduction (after treatment if necessary)

into the soil heap. With respect to the lower liquid off¬

takes, it is advantageous if the base of the trough

slopes slightly towards such off-takes.

Similarly, the soil heap of the SOB bioreactor may have

supply conduits arranged therein by burying, at several different levels (optional) so as to control, for example, addition of reduced sulphur form, i.e. recycle

liquor or gases from the SRB reactor. Such supplies may

be provided in a continuous or discontinuous form.

A particular advantage of the bioreactor of the present

invention is that the soil heap around the edges of the

trough means may be used to effect a gas seal to

substantially prevent hydrogen sulphide resulting from

the conversion of sulphate to sulphide in the SRB

bioreactor from escaping into the atmosphere. This is effected by extending the impermeable membrane inside the

trough means to the outside thereof to lie flat on the

ground and to have the outer periphery of the soil heap

overlying the flat area. Hydrogen sulphide may be

directed back into the soil heap where it reacts with

other chemical constituents to provide a sulphur source

to promote growth of the SOB. Thus, contamination of the

atmosphere surrounding the composite bioreactor is

substantially negated.

With the passage of time the organic contaminants in the

soil heap are first degraded into harmless or less

harmful species and the metal contaminants are eventually

leached out of the soil and fed into the SRB bioreactor from where they may be retrieved in the form of a

sulphide sludge and further processed for recovery or

disposed of in a known manner. Once the contaminants have

been removed from the soil heap it may be removed and

reutilised by spreading on the land and a further batch

of contaminated soil heaped onto the SRB bioreactor and

processed as above. This process may be repeated until

all the soil on the site for remediation has been

decontaminated. If necessary, further treatment is

carried out on the decontaminated soil before it is

reutilised, e.g. pH adjustment using lime, and/or

addition of composting materials.

It will be understood that although mention has been made

of a soil heap over the SRB, the composition and/or

distribution of the soil may differ across the heap in a

continuous or discontinuous manner, or there may be one

or more further soil heaps arranged to feed the same SRB.

In order that the present invention may be more fully

understood, examples will now be given by way of

illustration only with reference to Figure 1 of the

accompanying drawing, which shows a schematic transverse

cross section through a composite bioreactor according to

the present invention.

Referring now to the drawing, the composite bioreactor 10 comprises an upper SOB bioreactor 12 and a lower SRB

bioreactor 14 separated by a permeable membrane 16 which

allows the passage of liquid but substantially no

significant amounts of soil and an impermeable membrane

17 to maintain the integrity of the two reactors. The

lower SRB bioreactor 14 comprises a trough 18 excavated

from the surface 11 of the ground. The trough 18 is lined

with a plastics material sheet 20 which is impervious to

the liquors and chemicals employed. The trough is filled

with a bed of packing material 22 of rubble and pebbles

on which the impermeable membrane 17 rests. The bed

supports the weight of soil in the SOB bioreactor and

provides a substratum for the organisms to grow on. The

SRB bioreactor trough 18 is provided with conduits 24, 26

at different levels, the conduits being connected to

various pumps so as to be able to withdraw or inject

liquids as appropriate into the SRB bioreactor. The

conduits 24, 26 also serve to promote circulation of

liquors in the trough, to supply and distribute a carbon

source such as ethanol, and to supply and distribute an

alkali such as sodium hydroxide in the trough so as to

control pH in the SRB bioreactor. Pump 50 is coupled to

the conduit 24 and is able to pump the extracted

liquor/sludge either to a storage tank (not shown) for

further processing or recirculate liquor to a conduit 28

where it may be supplied to the SOB bioreactor 12 at a

desired level depending upon the stage which the

remediation process has reached. Air pump 30 is used to

supply air to aerate the SOB bioreactor 12 via conduit 32

to promote growth of bacteria and bioleaching of

contaminants in the soil heap. Nutrients may be supplied

to SOB reactor 12 from vessel 34 via pump 52 and conduit

28. The SOB bioreactor essentially consists of

contaminated soil which is to be remediated. The soil may

have mixed therewith micro-organisms, nutrients and sulphur to initiate the bioremediation process .

Although only two sets of supply/distribution conduits

are shown, there may of course be more than this number depending upon the size of the composite bioreactor and

upon the degree of control it is desired to exercise over

the process. Where buried in the soil heap the conduits

themselves may be of perforated form so as to distribute liquid or air as appropriate over as wide an area of the

soil heap as possible.

Operation of the composite bioreactor will now be

described assuming that the soil heap is contaminated

with both organic and metal species pollutants.

The initial stage of remediation will be to degrade the

organic molecules present and will be controlled to

provide aerobic conditions by supplying air and nutrients

mainly through the conduits 28, 32 to the soil heap such

that they slowly drain down through the SOB. Take-off

conduit 38 may be used to recirculate liquor to the soil

heap, e.g. as shown via pump 52, vessel 34 and conduit

28. The chemistry of degradation of organic molecules by

biochemical processes is known and reference is made to

our copending patent application W095/22375, the content

of which is included herein by reference.

While the organic contaminants are being degraded, the

SRB bioreactor 14 is filled with a dilute sulphate liquor of pH 6 and the trough contents inoculated with a consortia of SRB. Ethanol or other suitable carbon source

is fed to the SRB bioreactor and conditions produced to

promote healthy growth of SRB. This growth period will

require sulphate ions, nutrients and carbon source, any

or all of which may be provided from one or more vessels

such as 60 with an associated pump 58 coupled to the

conduit 26. Mixing may be achieved by withdrawing liquor

by the conduit 24 and pump 50, and re-injecting into the

trough 18, e.g. via pump 50 and the conduit 26 to promote

mixing and fluid movement. In order to sustain growth of

the SRB, it may be necessary to remove some liquor to

prevent conditions toxic to the SRB from arising.

Once the organic contaminants have been degraded, the

process conditions may be adjusted to commence leaching

out of the metal contaminant species in the soil .

Appropriate nutrients are introduced into the soil heap

from vessel 34 via pump 52 and conduit 28 to stimulate

growth of SOB. As sulphuric acid is formed from the SOB

growth, an acid front passes down the soil heap, thus liberating the metal species by conversion into the metal

sulphate. Sulphide containing liquor from the base of the

SRB bioreactor is recycled ahead of this acid front into

a more alkaline region of the soil. Metal sulphides will be produced in the soil from the recycled liquor and the

heavy metal pollutants in the soil without substantial

evolution of hydrogen sulphide. The effect of this is to

minimise evolution of hydrogen sulphide, thus allowing

the SOB to oxidise the sulphide more efficiently and

preventing significant quantities of hydrogen sulphide

from leaking into the atmosphere.

The acid leachate which is percolating down through the

soil heap may be collected in vessel 40 before being

introduced into trough 18 at a controlled rate by pump

56. Effluent from the SRB bioreactor 14 may be fed into a

storage vessel (not shown) and may be treated (depending

on the reactor parameters) before being recycled back

into the soil heap via conduit 28. The recycled SRB liquor contains soluble sulphur containing compounds

which provide nutrient for SOB growth and improve the

economics of the process since the separate addition of

sulphur is comparatively expensive. As the acid front eventually passes all the way through the depth of the

soil heap, the metals will be removed as sulphates and

converted into sulphides in the SRB bioreactor and

precipitate out to form a sludge in the base of the trough from where it may be removed.

Also provided is a source 42 of an inert gas such as

nitrogen, which is passed to the SRB via a conduit 36.

This can be used to keep the SRB free of oxygen, e.g.

when setting the SRB up, and also to remove any hydrogen

sulphide gas formed in the SRB. Conduits (not shown) may

also be provided between the SRB and the SOB, to conduct

gases from the SRB for distribution (and recycling of

sulphur when the gas is hydrogen sulphide) within the

soil heap.

Once the soil heap has been decontaminated it may be

removed for disposal as desired and replaced with a new

contaminated soil heap and the process repeated.