Not all pollutants can be physically removed from the environment by humans. In the case of some pollutants like some heavy metals and petroleum products, bioremediation may be the only way.

Bioremediation refers to the use of microorganisms which remove pollutants by metabolising them. Last week I looked at several methods of bioremediation, namely phytoremediation bioventing, land-farming and composting. This week I will go into some detail about some other bioremediation techniques; rhizofiltration, bioleaching, biostimulation, bioaugmentation, and the use of bioreactors, as well as the advantages and disadvantages of their use.

Rhizofiltration is an in-situ bioremediation technique where plant roots are used to filter water and remove excess nutrients or toxins. It is similar to phytoextraction or phytoremediation, mentioned in part one of this article; however, the former removes toxins from the soil, while rhizofiltration is used in aquatic ecosystems. It is used to treat run off from urban and cultivated areas.

Also, like other methods of bioremediation, rhizofiltration has its limits, one of which is that it cannot grow in highly contaminated soil. This method does not work overnight, so that long term maintenance and monitoring of the site is required, especially to prevent animals from feeding on the plants and becoming contaminated. Therefore, plants that are consumed by animals as fodder should not be used in this remediation process, so as to prevent contamination from spreading to animals, some of which are a food source for humans. Another disadvantage is that the contaminants found in the roots cannot be extracted.

Another method of bioremediation is bioleaching which is used to remove metals from their ores, such as gold and copper. The ore is directly exposed to the bacteria or kept in an external pond and the leaching chemicals that are produced, at a distance. Bacterial microorganisms act as a catalyst speeding up the natural processes inside the metal ore, whereby oxidation turns it into pure metals. Over time enough material builds up in the waste solution produced which is concentrated into the pure metal. It is a water based process so it generates less dust. The bioleaching process also takes place at atmospheric pressure (that is normal pressure found on the planet's surface) and at low temperatures, so that it does not need extra energy to produce specialised conditions, which mean that the combustion of fossil fuels is not necessary. This is also advantageous over the other extraction processes which use cyanide. There are also no emissions of sulphur dioxide which cause acid rain.

But rather than introduce microorganisms, one can modify the environment to stimulate the bacteria that naturally occur in the area to carry out bioremediation. This process is called biostimulation and nutrients such as; oxygen, phosphorous, nitrogen and carbon are injected into the subsurface of the soil.

The advantage of this method is that no exotic or non-native species are introduced in to the area which could potentially outcompete the native species thus decreasing the biodiversity of the area. However the delivery of the stimulant substances and therefore use of this method is dependent on the geology of the area, that is, the permeability of the soil.

Biostimulation can be enhanced using bioaugmentation, where more of the bacteria native to the area can be introduced. Another method is to use genetically engineered microbial strains. This is usually used to treat municipal wastewater. This method is advantageous as the microorganisms are able to attack both solid and soluble pollutants. Another advantage is that the waste or pollutants act as food for the microorganisms while the waste that is produced is used for food by fish and other invertebrates, therefore supplying food to allow our food chain to continue.

Using a bioreactor is another form of bioremediation, which is usually used in the treatment of sewage and wastewater. This uses a container or vessel usually constructed of stainless steel. There are two main types of reactors; aerators which supply oxygen while submersible mixers agitate the mixture thereby keeping the solids in suspension so that the bacteria can interact with the pollutant and break it down.

Therefore it can be seen that bioremediation is not a "quick fix" solution to environmental pollution, with some techniques yielding results after years of use and monitoring. Also some pollutants can only be removed from the environment through specific bioremediation techniques.

But this article should be taken as a beam of hope in that pollutants can be removed from our environment. However, prevention is better than cure, so not allowing pollutants to enter our environment will always remain the best method of protecting our planet.