Advantages of the Bioremediation Technique

Start with a clear definition

• Biodegradation is the breakdown of a substance by living things. It occurs naturally when suitable microbes meet suitable chemicals.
• Bioremediation puts that process to work at a polluted site.
• Workers may improve conditions, add microbes or use plants.
• Some soil microbes use parts of petroleum as food. They can turn those chemicals into simpler products.
• Success depends on which compounds are present. It also depends on oxygen, nutrients, moisture and time.
• Source: US EPA bioremediation guide.

Follow the pollutant, not just the organism

• Lead and cadmium are elements. Microbes cannot destroy an element by breaking its chemical bonds.
• A plant may take a metal into its shoots. Workers must then harvest and manage that plant material.
• Other methods hold a metal in place. It can become mobile again if site conditions change.
• One lab study reported about 91% cadmium removal by a bacterial strain. The result depended on test conditions. It is not a promise for a polluted field.
• The study measured cadmium leaving a test solution. It did not show the element was destroyed.
• Sources: US EPA phytoremediation review and biosorption study.

Three useful verbs

• Transform: change a pollutant’s chemical form.
• Remove: take it out of soil or water.
• Immobilize: stop it moving so easily.
• A safer form is still present. A removed metal still needs safe handling.
• Green plants alone do not prove a site is safe. Workers must measure the result.

In place or after removal?

• In-situ treatment happens where the pollution lies. Wells may bring air or nutrients into polluted groundwater.
• Ex-situ treatment moves polluted material first. Workers may place excavated soil in a pile or tank.
• A biopile is an engineered soil pile. A bioreactor is a vessel for water or sludge.
• Landfarming spreads contaminated soil in thin surface layers. Workers turn and monitor it.
• In-situ work can disturb a site less. Yet dense soil may stop air and nutrients reaching the pollution.
• Ex-situ work offers more control. Digging and transport add cost, dust and exposure risk.
• Source: EPA site guide.

Feed local microbes or add new ones?

• Biostimulation helps microbes already at the site. Workers add oxygen, nutrients or food.
• Bioaugmentation adds selected microbes. It may help when the right ones are absent or too few.
• Adding microbes is not a shortcut. They still need suitable conditions.
• Aerobic microbes work with oxygen. Anaerobic microbes work without it. Some chemicals need both stages.
• Tests must also look for harmful products between stages.
• A fall in smell or colour is not enough. Workers should measure the chemical itself.
• Source: US Geological Survey.

How can plants help?

• Plants need space, seasons and time to grow. They may fail where pollution kills them.
• Source: EPA introduction to phytoremediation.

A cleanup follows a chain of decisions

1. Map. Find the source and spread.
2. Test. Check chemicals and site conditions.
3. Pilot. Try a small treatment.
4. Treat. Control the chosen method.
5. Verify. Compare samples with cleanup goals.

• A falling concentration can have several causes. A chemical might evaporate, wash away or move deeper.
• So monitoring needs more than one sample point. Workers may compare treated ground with an untreated area.
• Some sites need further checks after treatment stops.
• Source: EPA technical guidance.

Three grounded examples

• Oily soil: workers mix it with air and nutrients in a managed pile. Tests must check the remaining oil and vapours.
• Drain water: selected microbes may be added where sewage runs through an open drain. A pilot does not replace a working sewage treatment system.
• In its 2013–14 annual report, India’s environment ministry described pilots in Bharatpur and Indore. It reported lower pollution loads in the treated stretches.
• Those dated pilot results do not prove every drain will improve.
• Metal-rich soil: plants may take up some metals or help hold them in place. Stabilized soil still needs care because the metal remains.

When is this a poor choice?

• Bioremediation can take months or years.
• Cold, dry, very toxic or tightly packed sites can slow living organisms.
• Some chemicals break into harmful middle products.
• Too much oil can block oxygen. Deep pollution may lie beyond plant roots.
• Floods can carry pollution away.
• A fast physical removal may be needed when exposure is urgent.
• Biological treatment can cost less at a suitable site. It is not always cheaper. Excavation, monitoring and long treatment times all add cost.
• Landfarming needs enough land and runoff control. Turning soil can release vapours.
• Source: EPA landfarming guide.

Exam traps to avoid

• Natural biodegradation can occur without a planned cleanup. Bioremediation uses biology for a cleanup goal.
• In-situ means in place. Ex-situ means the material moves first.
• Biostimulation feeds local microbes. Bioaugmentation adds microbes.
• A metal can be removed or held in place. It cannot be biodegraded like oil.
• Landfarming uses managed surface soil. It is not deep burial.