World Environment Day : How trees can purify wastewater and provide economic benefits

Plants and trees have been traditionally valued for their role in cleaning the air, generating wood and generating biomass. In recent years, their property of purifying water has emerged to be useful in bringing down pollution.

Tree such as eucalyptus, poplar and salix are used in short rotation for biomass energy. These species and agricultural crops are usually irrigated with fresh water. But what happens if they are irrigated with grey water (domestic wastewater from laundry, bathing and dishwashing)?

Domestic wastewater is enriched with nitrates, phosphates, sodium and potassium. When this water is used for irrigation, the roots of plants and trees suck up the pollutants and leave behind cleaner water.

The pollutants in wastewater act as nutrients for the trees and plants and also help yield higher fuel wood biomass. The process of uptake of pollutants from wastewater through tree roots and convertion into less toxic form is called dendroremediation. When plants perform the same thing, it is known as Phytoremediation.

These techniques are also more cost-effective than conventional wastewater treatment. Casurina and dendrocalamus can reduce nitrogen by 61-76 per cent, phosphorus by 18-70 per cent and biological oxygen demand by 80-94 per cent from wastewater.

Dendro- or phytoremediation techniques involve extraction, degradation, rhizofiltration, dendrostabilisation, dendrovolatilisation and rhizodegradation:

• Dendroextraction / Phytoextraction: Organic and inorganic contaminants are absorbed by the root hairs in this technique. This system can be used for recovery of sites and water contaminated with heavy metals as well in a method called phytomining and can be used for mineral extraction from their ores. Gold and nickel is regained through the process in the United States though some studies suggest it is not cost-friendly. More than 400 species of Asteraceae, Laminaceae, Euphorbiacea and Brassicacea families are identified, which have high potential in accumulation of heavy metals.

• Dendrodegradation / Phytodegradation: Through this system organic pollutants are degraded by the compounds that are released by the trees during metabolic processes. The trees enzymes are able to degrade hazardous substances like chlorinated solvents, herbicides. The beneficial point of this method is these all process of reduction and degradation occur inside the trees and do not depend on microorganisms.

• Dendrostabilisation / Phytostabilisation: Roots of trees have the capacity to tolerate the heavy metal concentration in polluted water. These metals are immobilized through sorption, sedimentation and reduction of metal valences.

• Dendrovolatilisation / Phytovolatilisation: Roots absorb toxic metals through their root hairs and transform them into a less toxic form of substance. The absorbed form of metals moves through the vascular system and are removed by transpiration or evaporation. This mechanism of removing heavy metal pollutants is well-established in poplar species. This mechanism, however, has the disadvantages of releasing heavy metals into the atmosphere.

• Rhizodegradation: The organic pollutants present around the rhizosphere in soil or polluted water is degraded by microorganism activities. There are several compounds like sterol, fatty acids, nucleotides amino acids, flavanone and other enzymes are released by roots of trees and degrade the contaminants around the rhizosphere.

• Rhizofiltration: Aquatic and terrestrial plants with strong root system are used in this method. The species are planted directly on the contaminated site or for the treatment of polluted water using raft system. It is effective in removal of radioactive substances from polluted water.

This natural technology performs two functions simultaneously: Purifying wastewater while also getting higher biomass from trees, leading to greater economic benefits.

It has been reported that eucalyptus hybrid k-143 has shown 25 per cent higher biomass growth when irrigated with polluted water. The biomass of poplar and salix trees have also doubled and quadrupled when fertigated with domestic wastewater.

Views expressed by the author do not necessarily reflect that of Down To Earth.

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