DuckweedsDuckweeds -A promising method for waste water Treatment systems..

A.Adaikala Arul Rajanathan

IInd M.Sc. Eco-Biotechnology

What is Duckweed?

Duckweeds or Lemnaceae, are small floating aquatic plants, which can be found worldwide under varying environmental conditions.

The plant morphology is simple and consists of short root and a frond. The plant measures only a few millimeters. Common Duckweed has 1 to 3 fronds .

Growth rate:

Since stems and leaves are absent the tissues are actively involved in photosynthesis hence these duckweeds are considered as the most vigorously growing plant species on earth.

The average yield of biomass will be 15-30 tn Dm/ha/yr.

Duckweeds grow 20 times faster than a corn and is able to cover 1 acre in just 45 days .

Composition:

Protein content of 14-26% was observed in natural water bodies, whereas 29-41% of protein was observed in waste stabilization ponds.

As it consists of high amount of protein it is used as an animal feed in some countries and some species of duckweeds are used as feed for humans in Burma, Laos and thailand.

BOD Removal

Alaerts et al. (1996) reported BOD removal efficiencies of 95-99% in a 0.7 ha sewage lagoon in a time span of 20 days.

DeBusk and Reddy (1987) observed BOD removal rates as high as 300-400 kg/ha/day.

As oxygen in prevented from entering the

water, aerobic pathogens are reduced

which ultimately leads to BOD

removal/reduction.

TSS Removal

Algae results in an increase of Total Suspended Solids (TSS) to over 200 mg/l, Since algal growth is greatly suppressed in Duckweed ponds the amount of TSS would also be greatly reduced – Taylor et al.,1995

Removal of Heavy metals

Heavy metal removal by duckweed is by plant uptake, chemical precipitation and adsorption.

Landholt and Kandeler(1987) suggested that duckweed could effectively be used to remove metals from waste water.

Gellini and Piccardi(1981) reported that Lemna causes a 75% reduction in Copper content of water containing 5 mg/l within 48 hrs.

Xenobiotics removal

The removal of various xenobiotics compounds such as phenol, naphthalene and chlorodibromomethane in waste water along with microbial biota results in removal of xenobiotics by accelerating efficient biodegradation.

-Federle and Schwab(1989)

Pathogen Removal

Islam et al.(1996)observed that Faecal coliform level was reduced from 4.5X10 per 100ml in the influent to 100/100ml in the effluent.

The reason for the reduction of the fecal coliform was unknown but the relation of duckweed in pathogen removal is confirmed and extensive studies is underway to find out the connection of duckweeds.

(+)ve Impacts:

They grow rapidly and shows high nutrient uptake rate when exposed to sewage.

They can tolerate high concentrations of detergents present in domestic sewage.

They are capable of absorbing and accumulating a wide variety of substances including heavy metals.

Because of high protein content and low fibre it is highly nutritious hence given to animals as feed

Cont…A complete duckweed cover may effectively

prevent the development of algal bloom in water.

Presence of duckweed cover has decreased the development of mosquitoes.

In semi arid regions it is believed that there is less evaporation of water when compared to ponds without duckweed cover.

Duckweeds does not require any fertilizers for their growth and hence they can be grown in water bodies without fertilizers.

(-)ve Impacts:Even though there are many (+)ve impacts about duckweeds It also have some (-)ve impacts like, As these duckweeds cover the entire water

surface there are possibilities of unavailability of oxygen and sunlight for fishes, phytoplankton's, etc.

If these duckweeds are allowed to absorb heavy metals then they cannot be used as animal feed.

It is a time consuming process, hence cannot be incorporated in urban areas.

As it prevents the entry of oxygen into water there is possibilities of suppression of the growth of aerobic bacteria.

Harvesting of Duckweeds:

Harvesting of duckweeds is much simpler when compared to other macrophytes like water Hyacinth (Tamil: Agaya Thamarai), etc., as there is no extensive rooting system below the water.

If duckweeds are cleaned completely it may result in algal bloom

These plants produce biomass faster than any other flowering plant, serve as high-protein feed for domestic animals and show clear potential as an alternative for biofuel production

"We can kill two birds -- with one stone, (biofuel production and wastewater treatment) (i.e.) using duckweed," -- Dr. Cheng, (North Carolina State University)

Setup in Rural Areas:

Rural Community

Biogas Plant

Duckweed pond

Fish pond

Irrigation

Crops

Fish

Animal Husbandry

Future prospects:

Due to the high starch content of duckweeds they can be used to produce ethanol which can be used as biofuel, thus paving way to reduce global warming.

Proteins can be extracted and supplied for protein deficient people.

Duckweeds can be genetically modified so that heavy metals and pesticides are absorbed and degraded within the system.

Should be checked for any medicinal values.

References:

Online:

http://www.clean-flo.com/lakeweeds/duckweed.html http://www.thetechherald.com/article.php/200915/3414/ http://en.wikipedia.org/wiki/Lemna

Book:

Environmental Biotechnology and Cleaner Bioprocesses - E.J.Olguin

Gloria Sanchez

Elizabeth Hernandez

Thank you