Before you write off a plant as a weed, read this

Encouraged by the Celebrating Singapore's BioDiversity blog set up by Ria and gang in tandem with the International Year of Biodiversity 2010, I decided to write about an important but not so cuddly class of living things - plants AND their effects on environmental quality. (They are important not just for their production of oxygen or role as primary producers.)

“What is a weed? A plant whose virtues have not yet been discovered” Ralph Waldo Emerson

“Natural wetland systems have often been described as the ‘earth’s kidneys’ because they filter pollutants from water that flows through on its way to receiving lakes, streams and oceans.” EPA on constructed wetlands

South East Asia is blessed with lush greenery consisting of a vast diversity of plants. Remarkably, this greenery can be utilised in the fight against environmental pollution. Imagine your dish water being piped into a garden of bougainvilleas for treatment before returning to your water closet to flush your waste. Or imagine a plot of industrial land contaminated by a former plating plant being sown with Indian Mustard (Brassica juncea) to suck out the copper from the soil.

The two scenarios above are examples of phytoremediation, a method of cleaning up soil or water using plants. It is an emerging technology attracting significant interest in U.S.A., Canada, Europe and the former U.S.S.R. Though much of phytoremediation is still under research and experiment, many cases of successful application have been documented.

Actually, phytoremediation has been practised by Mother Nature to clean up human and animal waste in wetlands and other water bodies since time immemorial. Phytoremediation as a technology is simply enhancing Nature’s ability to treat soil and water. This element of “naturalness” enables it to be more readily accepted by the public as an environmentally friendly and safe way to clean up pollution.

Not only is phytoremediation environmentally benign, its capital and operating costs are typically low. This makes the technology particularly appealing to developing countries having large polluted areas but limited budgets for cleaning up.

For land contamination, phytoremediation is capable of in situ operation without a need to excavate the entire contaminated site. Unlike traditional pump-and-treat methods, phytoremediation is aesthetically pleasant, having rows and rows of greenery instead of masses of mechanical equipment, hence appearing more like a plantation rather than a wasteland.


The primary disadvantage of phytoremediation is the long time required to effectively clean up pollutants. Operational duration is generally in terms of years. In addition, the depth of contamination should be within reach of the plant roots (choice of plants becomes important).
 
Phytoremediation may be broadly classified into four different mechanisms - accumulation, degradation, volatilisation and stabilisation.


Accumulation
Most of the work being done on phytoremediation focuses on accumulation, especially in the treatment of heavy metals e.g. lead, chromium, zinc, copper, nickel, cadmium. Certain plants can extract heavy metals from soil or water partly because these heavy metals are chemically similar to the elements required for plant growth and nutrition. The heavy metals are subsequently stored within the plant tissues or on the roots.

Hydroponics systems utilising Indian Mustard were proven effective in removing the above heavy metals. Even radioisotopes such as Caesium-137 and Strontium-90 were successfully removed by Sunflowers (Helianthus annuus). When the plants have matured and reached their accumulation capacity, they are harvested and sent for drying, composting or incineration. The resulting residue still contains the heavy metals hence it has to be appropriately disposed. Many cycles of growth and harvest are necessary before a site is sufficiently cleaned.

Figure: Continuous hydroponics system cultivating Brassica oleracea. Instead of growing vegetables for consumption, can such systems be used to treat wastewater using the same vegetables?

Figure: Phytoremediation experiment using Indian Mustard grown by batch hydroponics in week 4. This was the control which has no heavy metal in the nutrient solution.

Figure: Phytoremediation experiment using Indian Mustard grown by batch hydroponics in week 4. Nickel was present in the nutrient solution. Notice the gradual deterioration of the plant due to the toxic effects of nickel.

Degradation
Degradation is usually applicable to organic contaminants, including domestic wastewater, agricultural wastewater, oil, explosives and solvents. Certain plants are capable of absorbing these organic compounds through their roots and metabolising them into harmless substances.

Usually, these plants receive help from the friendly microorganisms thriving in the rhizosphere (root system zone). The rhizosphere tends to support a large population of bacteria and fungi attracted by the rich environment there. This environment is influenced by root exudates containing proteins, organic acids, alcohols, carbohydrates etc. most of which are beneficial to the microorganisms. Besides exudates, the root system also gives out an important product of photosynthesis – oxygen. Oxygen is particularly useful in promoting aerobic decomposition of the organic contaminants by the surrounding bacteria and fungi into carbon dioxide, water and other harmless chemicals.

Floating plants such as water hyacinth (Eichhornia crassipes), water ferns (Azolia spp.) and duckweeds (Lemna spp., Wolfia spp.) have treated wastewater which is allowed to pass through basins holding these floating plants. More elaborate systems exist in the form of constructed wetlands involving cattails (Typha angustifolia), bulrush (Scirpus spp.), reeds (Phragmites australis), rushes (Juncus spp.) and sedges (Carex spp.). These systems mimic the cleansing ability of natural wetlands and can effectively treat wastewaters of high organic loading e.g. human and agricultural wastewater.

Figure: Basins of water hyacinths in Johor. These can be converted to treat wastewater from domestic or agricultural sources.

Figure: Cattails are common emergent aquatic plants in wetlands (Singapore). They have been incorporated into wetlands to treat organics in wastewater.

Terrestrial plants offer another option as they tend to have more extensive root systems and can potentially grow to larger sizes. Therefore they are able to treat wastewater more efficiently. One example is the Earthship concept by Earthship Biotecture (which designs and builds self-contained houses (Earthships) in the U.S.A. and other countries.

Figure: Outdoors botanical cell next to an Earthship. This cell is used to treat black water.

Their treatment units are designated “botanical cells” occupied by bananas, bougainvilleas, grapes, lemons etc. These botanical cells are either enclosed in greenhouses (integrated with the living quarters) or sited outside next to the house. Grey water from the sink and the shower and black water from the water closet go into separate botanical cells. As black water has a higher organic loading and is a potential biohazard, it detours into a septic tank before heading for the botanical cells.

Figure: Indoors botanical cell. The greenhouse is incorporated into the living quarters. This cell is used to treat grey water.

Volatilisation
Certain toxic contaminants such as mercury and selenium can exist as gaseous compounds. In volatilisation, plants take in mercury and selenium and release them as gaseous compounds into the atmosphere via the leaves. Members of Brassicaceae and cattails (Typha latifolia) have been proven to remove selenium from soil in this manner. Thale cress (Arabidopsis thaliana, Brassicaceae) and Tobacco (Nicotiana tabacum) were genetically modified to incorporate bacterial genes so that they can absorb mercury compounds from the soil and release mercury vapour.

Obviously, the main concern is the fate of the gaseous products – where will these gases ultimately go? After all, volatilisation simply transfers mercury and selenium from soil and water into air. Most experts agree that volatilisation should not be carried out near human populations or under meteorological conditions that do not favour the dispersion of gaseous pollutants.

Stabilisation
Stabilisation is only appropriate for heavy metals in soil. Unlike other mechanisms, it does not remove the contaminants from the environment. Instead, it immobilises the contaminants by root sorption. The plants also provide soil cover to prevent the movement of contaminants via water and wind erosion. Vertical transport of contaminants into groundwater is minimised by controlling the downward movement of water. Ideally, these plants should not transfer heavy metals from their roots to their aerial parts in order to reduce the potential of exposure to humans and animals.

Two cultivars of Colonial Bentgrass (Agrostis tenuis) and one of Red Fescue (Festuca rubra) are commercially available to stabilise lead, copper and zinc in soil.

Stabilisation is usually employed if there is no urgency to clean up a site or the contaminated area is simply too large. Alternatively, it may act as an interim measure before a decision is made on the final method of contaminant removal.


Is this technology for us?

Absolutely.

Even though South East Asia (SEA) lags behind many countries in research and application of phytoremediation, it should not discount phytoremediation as an option to clean up pollution. An inspiration for phytoremediation in SEA is the huge commercial potential as large parcels of land and water are polluted in the wake of SEA countries’ efforts to modernise and develop their economies.

Plants ARE the system in phytoremediation. Before even contemplating whether they can tolerate and clean up contaminated environments, they must be able to grow in that environment. Climate and soil (or water) conditions must favour the plants. All these imply that most studies done in the U.S.A. and other temperate countries cannot apply to the environment in SEA. SEA will have to come up with its own unique phytoremediation solution suitable for its unique conditions.

Wetlands are well known as repositories of biodiversity and pit stops for migratory birds. Constructed wetlands can also perform these functions as documented in many of such systems in U.S.A. Can we incorporate mangrove genera such as Avicennia, Sonneratia or Rhizophora into our very own constructed wetlands to perform wastewater treatment and at the same time provide sanctuary to plants and animals? It certainly warrants research to answer such intriguing questions.

Figure: Abandoned prawn farms in Singapore. These can be adapted as constructed wetlands for phytoremediation.

Figure: Typical mangrove wetlands habitat with Rhizophora sp., Singapore. The potential of Rhizophora for phytoremediation is as yet unclear.

On a more personal level, low impact living is becoming more of a necessity than a lifestyle option. Rural households may be able to implement some form of constructed wetland (albeit closer to the size of a swimming pool), perhaps working in tandem with a septic tank. Water is thus conserved by recycling wastewater back into the house for non-potable uses, effectively closing the loop on water usage.

In closure, phytoremediation is undoubtedly a natural and viable method to treat waste but it is NOT the solution to all environmental problems. It is simply a possible tool in a repertoire of environmental remediation tools.