Biotechnology against pollution: an introduction to bioremediation
Every living thing breaks down some of what it meets. Bioremediation is the attempt to put that ordinary talent to deliberate use on polluted water, soil and waste.
The idea in one sentence
Wikipedia's article on bioremediation defines it broadly as any process in which a biological system, typically bacteria, microalgae, fungi or plants, living or dead, is used to remove environmental pollutants from air, water, soil, fuel gases, industrial effluents and similar settings. The article explains the attraction plainly: organisms can adsorb, accumulate and degrade many pollutants, and compared with conventional physical and chemical treatment the approach aims to be sustainable, cheap and scalable.
The same article makes a point that surprises many readers: most bioremediation is inadvertent. Native organisms in soil and water break down a great deal of pollution without anyone asking them to. Research in the field therefore concentrates on helping that process along, for example by adding organisms to a polluted site or by supplying the nutrients that let resident microbes grow faster.
Who does the work
Different organisms bring different abilities. Bacteria are the chemists of the group, able to use an enormous range of substances as food or as a source of energy, and many of the classic examples of pollutant breakdown involve bacteria that can consume hydrocarbons or other organic compounds. Fungi, with their spreading hyphae and powerful secreted enzymes, are good at attacking tough plant-derived material and have their own name in the field, mycoremediation. Microalgae take up nutrients such as nitrogen and phosphorus from water as they grow, which is why they appear in discussions of effluent treatment. Plants, finally, can draw substances up through their roots and hold them in their tissues, the basis of what Wikipedia's article calls phytoremediation.
Breaking down versus locking up
An important distinction runs through the whole subject. Organic pollutants, those built on carbon skeletons, can in principle be taken apart, step by step, until little of the original molecule is left. Metals are different. As the Wikipedia article puts it, heavy metals cannot be degraded, only oxidised or reduced; in some cases they can be immobilised, but they do not disappear. A biological treatment of metal-contaminated soil is therefore usually about changing the metal's chemical form, trapping it, or concentrating it in plant tissue that can then be harvested and handled, rather than making it vanish.
On site or off site
The article divides bioremediation techniques into two broad groups. In situ techniques treat polluted material where it lies, in the ground or in the water. Ex situ techniques are applied to material that has been dug up or pumped out and moved somewhere it can be managed. In both, extra nutrients, minerals or pH buffers may be added to support the organisms, and sometimes particular microbial cultures are introduced, an approach the article names biostimulation. Its list of related technologies includes phytoremediation, bioventing, biosparging and composting in piles, which gives a sense of how varied the practical forms are.
Effluents from industry
Wastewater from factories is a common target for biological treatment because it is concentrated, continuous and often rich in organic matter. Textile dyes, food-processing residues, tannery waste and many other effluents have been studied as candidates. The appeal is that a treatment step using organisms can run at ordinary temperatures and pressures, and can sometimes be combined with producing something useful, such as algal biomass or electricity, a theme taken up on the pages about microalgae and microbial fuel cells.
The limits
Bioremediation is not a universal answer, and its own advocates are careful about this. Wikipedia's article states that one drawback is rate: the processes are slow. It also records that bioremediation is rarely employed, that heavy metals and radionuclides simply do not biodegrade, and that in some cases microbes do not break a pollutant down completely and can leave behind a compound that is more harmful than the one they started with. Conditions at a real site are also far less tidy than in a laboratory flask: the soil may be too dry, too cold, too poor in nutrients or too patchy for introduced organisms to thrive.
This is why published work in the area tends to be careful about scale. A laboratory result showing that a microbe can degrade a substance in culture is the start of a long road, not the end of one. Field conditions, other organisms, the mixture of pollutants actually present and the time available all decide whether a treatment is practical. A reader should look for which of those questions a study actually addressed.