2.4 Wastewater treatment using duckweed

Technical description

This is a relatively new technology in which small-scale wastewater treatment can be achieved using duckweed (Lemna spp. or Spirodela sp.). Duckweed is a self growing plant abundant in the tropical countries. It is commonly used as a fertilizer in paddy fields, but has recently been used in the treatment of wastewater in Bangladesh. In Mirzapur, Bangladesh, this technology has been implemented at the village level as part of a UNDP project examining the potential of duckweed-based wastewater treatment and fish production.

Operation and Maintenance

Use of this technology is simple, being based upon a modification of conventional maturation lagoon technology (see Wastewater Treatment Using Lagoons below). Maintenance consists of removal of excess biomass to encourage continued growth of the duckweed community, and thereby removal of nutrients from the wastewater, and maintenance of the containment structure of the pond.

Level of Involvement

This technology can be implemented at either the individual farm or community levels.

Costs

No data are available, but costs are estimated to be low.

Effectiveness of the Technology

Since 1989, PRISM, Bangladesh, has developed farming systems using duckweed-based technology and tested their potential for wastewater treatment and fish food. The results have been promising and, together with similar activities in Lima, Peru, have succeeded in generating interest among multilateral as well as bilateral donors in further examining the potential of this technology.

Suitability

This technology is suitable in tropical climates.

Advantages

This technology is inexpensive to construct and operate, and easy to implement. Duckweed is a prolific plant, especially in nitrogen-rich environments, and can be easily used as mulch or a natural soil organic enrichment.

Disadvantages

If the flows through the oxidation pond are not properly controlled, there is a possibility that the duckweed will flow out with the effluent. Treatment capacity may also be lost during high floods, if the area is not protected.

Cultural Aspects

No problems relating to the use of this technology are known to occur.

Further Development of the Technology

More research through pilot projects is needed in order to refine the sizing of the ponds used and to determine the correct innocculum of plant material to achieve a predetermined effluent quality.

Information Sources

Gert van Sanden, EMTAG, INUWS. The World Bank, 1818 H Street NW, Washington DC.

Jan van der Laan, DGIS, Royal Netherlands Embassy, New Delhi, India.

Erik S. Jensen, Danida, Royal Danish Embassy, Road 51, Gulshan, Dhaka, Bangladesh, Tel. 880 2 881799, Fax 880 2 883638.

Mohammed Ikramullah, PRISM, Bangladesh, House 67, Road 5A, Dhanmondi, Dhaka, Bangladesh, Tel./Fax 880 2 861-170.

Paneer Sehvam and Arun Mudgal, UNDP-World Bank Regional Water and Sanitation Group, 53, Lodi Estate, Post Office Box 416, New Delhi 110 003. India, Tel. 91-11 469 0488/9, Fax 91-11 462 8250.