Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1999-06-21
2001-03-20
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S170050, C435S176000, C435S180000, C405S128350, C405S263000
Reexamination Certificate
active
06203703
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems for biological remediation of water contaminated with hydrocarbons, and more specifically, to such systems operating underground, typically under oxygen-limited conditions.
BACKGROUND OF THE INVENTION
Gasoline and some other petroleum-derived hydrocarbons are among the most common contaminants of soil and groundwater. These compounds often mix with drinking water causing serious health concerns. Development of effective and economical remediation techniques is essential since groundwater is a source of drinking water in many areas of the world.
Known treatment techniques involve pump-and-treat methods, air stripping and steam flushing combined with vacuum extraction. Natural attenuation processes have been shown to reduce the concentration of contaminants in soil and groundwater, but these processes are usually slow and may take many years to decontaminate an affected area.
Recently, permeable under-ground walls and barriers have gained recognition as an alternative for the treatment of groundwater. Such techniques are more economical when implemented in-situ rather than ex-situ. The former also removes the need to transfer the affected material from the site thus eliminating the risk of phase transformation of volatile compounds. The underground biobarriers are passive reactors as they operate by natural hydraulic gradients of the underground stream, and they do not need pumping equipment to supply the contaminated stream to the reactor nor to pass the treated stream back underground. There is therefore a substantial difference between the in-situ passive underground biobarriers and above-ground ex-situ biological reactors for similar purposes (bioremediation of underground contaminated streams), the latter exemplified by U.S. Pat. No. 5,080,782 to Caplan.
Biological in-situ techniques and systems are particularly attractive compared with chemical ones since they have the potential to completely destroy the target contaminants leaving non-toxic chemicals as the products of biodegradation. The so-called in-situ bioremediation techniques have been successfully applied in the remediation of groundwater contaminated with petroleum hydrocarbons and chlorinated compounds.
Starr and Cherry (Ground Water, Vol. 32, NO. 3, May-June 1994) discuss various types of in-situ reactors and the packings used. Some reactors use a packing that modifies pH or Eh conditions in the subsurface. Others use a material (e.g. hydroxyapatite) that dissolves or causes precipitation of a mineral phase that immobilizes the contaminant. Another type of reactor removes contaminants mostly by sorption. Activated carbon and peat moss are the most commonly used materials in this category. Zeolites or synthetic ion exchange resins can also be used.
A number of patents pertain to various structures and features of the in-situ biological barriers for removal of organic contaminants. Exemplary in this regard are U.S. Pat. No. 5,057,221 to Bryant et al.; U.S. Pat. No. 5,384,048 to Hazen et al.; U.S. Pat. No. 5,624,552 to Vales et al.; U.S. Pat. No. 5,628,364 to Trenz; U.S. Pat. No. 5,518,620 to Eguchi et al.; and U.S. Pat. No. 5,389,248 to Pare et al. Bryant et al. deal with halogenated hydrocarbons and use activated carbon bed to support methylotrophic and heterotrophic microorganisms thereon. The specification mentions, without elaboration, that other substrates can also be utilized.
SUMMARY OF THE INVENTION
While the prior art in-situ biobarriers are useful to remove organic contaminants to a certain degree from underground plumes, they use mostly organic, particulate, compressible, highly porous packing such as peat moss, zeolite or activated carbon. The removal of contaminants is due in part to microbial transformation due to the presence of microorganisms that develop on the packing, and in part to physical-chemical sorption. As is commonly known, sorption may reach saturation necessitating a periodic removal of the packing for regeneration.
It has now been found that unexpectedly good performance of an underground-disposed, gravity in-situ biological reactor, a so-called biobarrier, may be assured by using, as a packing of the reactor, a material having the following characteristics:
solid, rigid (i.e. practically non-compressible as opposed to peat moss, activated carbon or zeolite) and non-sorptive, preferably consisting of discrete separable pieces or elements;
preferably non-corrosive in the prevailing conditions of an underground bioremediation process, e.g. made of stainless steel; and
when deposited in the reactor, having a free space of more than about 75%.
The biological reactor, or biobarrier, has the following characteristics:
The reactor has an underground-disposed chamber having an inlet adapted so as to allow a flow by gravity, or hydraulic forces naturally-occurring underground, of an underground mixture or stream of contaminants to be treated, through the chamber. The reactor can therefore be devoid of any pumping equipment which would be necessary if the reactor was disposed above-ground and the underground stream of contaminants was to be lifted to the above-ground reactor level.
Further, the reactor has an outlet for the stream after treatment. The reactor contains a volume of a solid, substantially rigid, non-sorptive packing disposed in said chamber, said packing disposed such as to leave a free space of at least 75%, and an amount of microorganisms disposed on and within the packing, said microorganisms effective to at least partially biodegrade said hydrocarbons from said mixture or stream.
Since the flow of the contaminants through the chamber and the packing with the microorganisms is by gravity, and considering that the pressure gradient in the underground contaminant stream (plume) is typically relatively small, the packing is stationary during operation, even if it consists of discrete pieces or elements.
Exemplary packing materials meeting the above conditions are rigid, non-sorptive plastic shapes (as opposed to flexible films and sheets), non-corrosive metals and ceramic shapes. A number of packings suitable for the above purpose is already known on the market and used in chemical industry. It is hereby proposed to use those in the underground bioremediation process of the invention.
In accordance with another aspect of the invention, there is provided a method of at least partially biodegrading petroleum derived hydrocarbons present in an underground aqueous mixture or stream, the method comprising
supporting a population of microorganisms specific to petroleum-derived hydrocarbons on a substrate packing confined in a chamber disposed underground in the path of an underground stream to be treated, the chamber configured to allow said mixture or stream to flow therethrough by gravity or naturally occurring hydraulic forces, such that said stream or mixture contacts said microorganisms, and
passing said aqueous stream or mixture through said packing such that said microorganisms decompose at least partly said hydrocarbons in said mixture or stream as it flows through said packing,
wherein said packing comprises solid, substantially rigid non-sorptive elements which leave a free space of at least 75%.
The process is carried out in predominantly microaerophilic conditions in said mixture or stream during its passing through said packing. “Microaerophilic” denotes a low dissolved oxygen concentration in the stream, less than about 2 mg/liter.
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Guiot Serge R.
Yerushalmi Laleh
Barry Chester T.
National Research Council of Canada
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