Silent genius - the world beneath our feet
An exploration of in situ remediation, by Dr. Jeremy Birnstingl, of Regenesis.
Change and demand breed opportunity. In our brave new world post 16-7, three strategies appear set to grow. These are: (a) off-site treatment facilities or 'soil hospitals', especially for soil that must be removed for development purposes; (b) on-site treatment approaches that enable soil to be cleaned for on-site re-use or pre-treated to enable its disposal to landfill; and, (c) in situ treatment, where the contamination is addressed in place, without excavation. The in situ approach potentially carries the distinct advantages of:
Clean up of residual plumes following in situ remedial approaches such as pump-and-treat, air sparging or even in situ chemical oxidation tend to rely on one thing: natural attenuation - the natural disappearance of the contamination over time. And among the variety of processes that through their common influence are collectively termed natural attenuation, there is one overriding process that ultimately achieves the destruction of the contaminants and is frequently responsible the lion's share of the attenuation. This process is biodegradation, the microbial transformation of contaminants into harmless products.
To make the most of in situ remediation, and more importantly to complete the remedial process to target levels with the minimum time and cost, its clear we need to understand this vital link. The time is ripe to take a look at it in more detail.
First, let's get this into perspective. Soils and rocks have been there a long time - any geologist will tell us that. But to get a mental grasp of the true meaning of geological time in relation to our human experience is no easy feat, and is the subject of a host of colourful analogies. But if you think rocks are old, bacteria as a group have been around longer than most of the rocks we see - and many times longer than the oldest fossils. In fact they may even have generated some of the original materials that now form familiar rocks. One theory attributes the original generation of carbonates to ancient microbial transformation of methane. Now that's a long time ago!
With this much time to practice, the bacteria that have evolved to be with us now have become pretty adept at what they do, and this includes the destruction of contaminants in soils. This has been appreciated for many years. Ronna in 1878 hailed the total ability of soil as a transformer of any type of sewage waste, irrespective of content . But the omnipotence of bacteria has perhaps been best formulated by Gale in 1952 in a phrase that has since become known as the Principle of Microbial Infallibility .
"It is probably not unscientific to suggest that somewhere or other some organism exists which can, under suitable conditions, oxidise and substance which is capable or being oxidised."
So following from Gale, the task of the bioremediation engineer can be summarised as ensuring the "suitable conditions" and the appropriate "organism" are available with the contaminant in the same place at the same time. To do it effectively, you must first know something about your work force, and how they behave where they live, not just in a test tube. You need to ask a microbial ecologist.
And when asked, a microbial ecologist will generally tell you that for the complete destruction of a given contaminant, "it takes a village". One bacterium will deal with this part of the problem, another with that part. The first makes its living dismantling this, letting its neighbour make its living in turn from the pieces left over, and so forth down the street - a microbial consortium.
So what makes for an effective village, a healthy microbial economy?
At the end of the day, it comes down to one thing: available energy - microbial cash. For hydrocarbon plumes, the energy comes from the contaminant itself, and is therefore plentiful in the plume. But there were two words: "available" and "energy". So the key factor to look at must be the availability of this energy. And for the bacteria, the principal ingredient they need to gain access to the energy locked up in a hydrocarbon plume is oxygen.
. It's the supply of oxygen that determines whether the microbial village surrounded by the contaminant plume is a thriving, diverse city centre, or a run-down set of boarded-up shops - dormant, and waiting for the good times to return. It is oxygen that determines whether the bacteria will voraciously devour the plume or whether they will simply take an occasional bite out of the tastiest contaminants that float by, leaving tougher nuts like benzene and MtBE to one side, or leaving a host of partial degradation products to accumulate and fester, not having the energy to get up and use them.
Oxygen is the key to energy availability, just like in the engines of our cars, just like in our boilers at home. Without oxygen, the boiler goes out; the car stalls; the rubbish in the furnace remains un-burnt. Bacteria also use oxygen as their first choice for oxidising fuel, their food, but microbial oxidation is metabolic - they derive the same energy but within themselves, without burning.
So with respect to degradation and attenuation of a contaminant plume, a plentiful supply of oxygen is the determining factor between a microbial furnace and slow smoulder. For example, the degradation of benzene in groundwater under aerobic conditions (i.e. in the presence of oxygen) has been estimated to be some 73 times faster than under anaerobic conditions (no available oxygen) - a half life of ten days as opposed to two years!
For bioremediation purposes, a variety of means of supplying oxygen have been attempted in the field. Early attempts included direct injection of compressed air or oxygen-saturated water, but the principal limitation of this was found to be the low solubility of oxygen in water and the high oxygen demand of the hydrocarbon degrading organisms. As early as the 1980's this led other workers to explore more bold approaches such as direct injection of molecular oxygen, ozone or hydrogen peroxide, but these were subject to limitations of expense, adverse soil reactions, inhibitory effects on microbial activity (excess oxygen can sterilise), and handling difficulties.
The key for effective microbial stimulation and therefore an effective bioremediation tool was found to be the constant, steady supply of oxygen - not too much or the bacteria are stressed or killed; not too little or no purpose is served. Focusing on this, and aware of the practicalities required by the remediation industry, Regenesis developed Oxygen Release Compound (ORC®) back in the early 1990's; a simple, slow release source of dissolved oxygen. Most commonly supplied in powder form, ORC can be mixed with water to form a slurry and injected into hydrocarbon plumes with direct-push drilling equipment and a simple grout pump, added directly into open excavations or suspended in wells in specially designed 'filter socks'. Now ten years old, ORC has been used on close to 10,000 sites in twenty countries around the world including dozens of sites in the UK - for the simple reason that it works.
The chemistry of ORC is relatively simple; it is a purpose-built, patented formulation of phosphate-intercalated magnesium peroxide. When wet, it releases oxygen at the steady rate required by the soil micro-organisms, and continues to do so for up to a year. In most contaminated aquifers, this results in an increase in microbial degradation rate of one to two orders of magnitude - in other words, a 90% to 99% reduction in plume degradation time.
What does this translate to for a remediation engineer? It means a plume that would take 30 years to disappear will be gone in about eighteen months. It means that more concentrated plumes can be addressed by natural attenuation, or more specifically, by accelerated natural attenuation. It means a cut-off barrier can easily be created by a few slurry injections to contain a hydrocarbon plume without disturbing groundwater flow. It means that extractive remediation systems can be used at their greatest efficiency for quick contaminant mass removal, and then decommissioned and replaced by a lower cost approach entirely able to completing the task - no more failed targets, extended treatment durations and pump-for-ever scenarios.
The ease of use of ORC and the low cost and minimal site disturbance that follow from its intrinsic controlled release feature, have made it the remedial tool of choice for a wide variety of sites including filling stations, development sites, industrial facilities and domestic residences . These and other case studies are available on the Regenesis website (http://www.regenesis.com).
And its not just hydrocarbon plumes that can be addressed in this way. Chlorinated solvents, pesticides and other oxidised species that typically degrade very slowly under normal groundwater conditions can also be addressed. But these species are poorly amenable to aerobic degradation at best (i.e. with oxygen), therefore another approach is needed if their bioremediation is to be successful. Once more the key is available energy. But this time there is little energy in the contaminants themselves, as by virtue of their composition, they are already oxidised. This is clearly evidenced in their pure form - chlorinated solvents such as PCE and TCE don't burn.
Nevertheless, the omnipotent soil microflora can still consume them, given the right conditions, but in this case the energy must be supplied from elsewhere. If an oxidisable food source (electron donor) is in plentiful supply and oxygen, this time, is absent, specialist groups of soil bacteria can destroy the contaminants by using them to 'breathe' in place of the oxygen (electron acceptor) while they consume the energy source. This is a similar process to hydrocarbon degradation but turned on its head, with the contaminant taking the place of the oxygen whilst something else provides the 'food'.
However, in the energy stakes, this metabolic approach is pretty impoverished. Where it happens at all it is often slow. Therefore to sustain it at a rate that becomes practical for remediation purposes the available energy must be increased. How to do this? The contaminant side of the equation, the electron acceptor, is fixed - we can't change that; it's the plume we are trying to deal with and its there whether we like it or not (not, probably). So we have to work on the other side of the equation, the food source, the electron donor - we have to find a high energy food source, a high energy electron donor to turn up the metabolic 'voltage' (Gibbs' free energy to the biochemist) and get the microbial metabolic wheels turning fast, fast enough to destroy the plume in an acceptable time frame. And there's no higher energy electron donor than hydrogen - it's the cream of the cream, microbial ambrosia. With hydrogen as a food a talented microbial 'industrial park' can rapidly consume a chlorinated solvent plume, and keep going through the often more stubborn break-down products also.
As with oxygen in hydrocarbon plumes, for this approach to be an effective remedial tool the availability of dissolved hydrogen must sustained and constant (by the way, dissolved hydrogen is H2(aq) not H+ 'acidity'). With such a supply, the microbial economy is able to thrive - precisely what the remediation engineer needs, as a thriving microbial population soon consumes a contaminant plume. Boom and bust cycles of hydrogen supply are not efficient and generate a range of problems of their own.
How to do this? Direct injection and handling of hydrogen is challenging and presents a number of safety concerns owing to the explosive flammability of hydrogen and ease of leakage of this gas. In situ hydrogen generation is expensive and requires power supplies and maintenance in addition to the above.
From a sympathy with field requirements and learning once more by asking the microbial workforce themselves (with the help of microbial ecologists) Regenesis developed Hydrogen Release Compound (HRC®) in the late 1990's. HRC contains no dissolved hydrogen itself. It's much smarter than that. It is made from and slowly releases an ideal substrate from which other families of soil bacteria can generate dissolved hydrogen which is then used by their dechlorinating colleagues - it takes a village, remember? Modelling its field application on the highly successful ORC, HRC is once more a purpose built, non-toxic, environmentally benign remediation product that can be injected into contaminant plumes using standard industry equipment (e.g. direct-push rig + grout pump). Like ORC, it increases the degradation rate of chlorinated solvent or similar plumes by one to two orders of magnitude. Like ORC, it has a sustained, controlled release of the critical factor, dissolved hydrogen, on which the specialist microbial dechlorinator populations are dependent and eagerly await. And the controlled release of dissolved hydrogen by HRC really is sustained - standard HRC releases hydrogen steadily for between one and two years whilst its bigger brother, the extended release HRC-X designed to tackle residual DNAPL, lasts for an impressive three to five years from a single application.
Let's look at this once more from a remediation perspective; this means degradation rates are increased by one to two orders of magnitude (e.g. half lives reduced 10x to 100x). And then these rates are sustained. For years. From one application. This gets through a lot of plume, for very little disturbance!
And indeed it does. HRC and HRC-X have been used to tackle a broad range of contaminants from solvents and dry-cleaning fluids to explosives and timber preservatives. Since they are not reliant on permeability for the effectiveness (the distribution of hydrogen is diffusive) they have been used successfully not only in sands and gravels but also in porous and fractured rock systems and dense clays. This has made it the remedial tool of choice on over 500 sites around the world including the UK since 1997.
The key to all of this is an understanding of the genius in the world beneath our feet and the natural attenuation mechanisms that take place there. These processes are already heading in the same direction the remediation engineer is trying to go, and are therefore sustainable, almost by definition. To really get them to work for us we need to help them move faster by taking the brakes off. ORC and HRC do just this. It's a matter of harnessing a natural process, and using it to work for us - as simple as a horse and cart. But like a horse and cart, the really smart bit is the design of the horse&
Dr. JEREMY BIRNSTINGL is European Technical Manager for Regenesis, and provides technical support and design assistance for ORC®, HRC® and MRCTM projects. He has 15 years experience in research, development, and commercial application of bioremediation and other in situ and ex situ remediation technologies from within the academic, industrial and consulting business sectors. He is based in London. firstname.lastname@example.org
For more information on Regenesis products including ORC®, HRC® and the award winning new Metals Remediation Compound (MRCTM): http://www.regenesis.com
1. Ronna, A. (1878). Irrigation ou Épuration Chimique. Imprimerie A. Lagarde. Reims.
2. Gale, E.F. (1952). The Chemical Activities of Bacteria. Academic Press. New York.
3. Howard, P.H.; Boethling, R.S. et al. (1991). Handbook of Environmental Degradation Rates. Lewis Publishers. Boca Raton.