3.7 Lemna-based wastewater treatment system

Technical Description

Lemna, or duckweed, is a small, green plant that grows on the water surface, and is especially prevalent in conditions that are rich in nitrogen. When Lemna is used for wastewater treatment, the plant, which floats on the top of the wastewater, uses the nutrients and other organic substances contained in it. It grows very fast, is not very sensitive to changes in climate or wastewater quality and quantity, and can be found all over the world. A Lemna-based treatment system can be created in new artificial ponds or can be introduced into existing lakes within a couple of weeks. The ponds can be planned as individual treatment areas or can be joined together to reach the necessary treatment efficiency. The depths of the ponds should be between 1.2 m and 3.6 m.

Before discharge into the duckweed pond, bulk materials, sand, and grit must be removed by screening and preliminary settling in a grit chamber, and the clarified effluent may received further treatment in anaerobic, facultative, or aerobic ponds. In some cases, to ensure a final dissolved oxygen concentration of over 5 mg/l, cascade aeration could be considered. After discharging the treated effluent into the Lemna ponds, the resultant plant biomass must be collected with special harvesting machines. This biomass has high protein and mineral contents, and, after composting, is usable in agriculture without restrictions.

Figure 3. Biology of a Lemna pond.

Extent of Use

This technology has limited application in Poland and Hungary.

Operation and Maintenance

Maintenance of the system is simple; namely, maintenance of facilities, biomass harvesting, and composting. It does not require highly trained personnel, and uses predominantly natural processes. [Caution: Non-native species should not be introduced into waterways.]

Figure 4. Construction of a Lemma treatment plant.

Level of Involvement

This technology is implemented at the local community level.

Costs

Costs are generally low, but include the cost of land, construction materials, and labour.

Effectiveness of the Technology

This technology naturally removes not only BOD₅ but also phosphorus and nitrogen at an affordable price. Retention time, depending on the removal efficiency desired, is between 15 and 30 days. Average effectiveness is 90% to 95% removal of BOD₅ and suspended matter. Reductions in nitrogen average 25% and 80% for phosphorus.

Suitability

This technology is suitable for the treatment of household wastewaters in medium sized municipalities, pert-urban districts, and rural areas.

Advantages

Treatment can be carried out in natural or artificial existing ponds, and the treated water can be reused for irrigation. The biomass produced makes a good fooder for animals or compost. The energy demand is minimal, as the technology uses natural energy sources.

Disadvantages

The disadvantage of this system is its dependence on favourable climatic conditions. During winter or cold weather, the treatment process is slowed down. In this situation, it is necessary to create additional storage to handle the same volume of wastewater as during warm weather.

Cultural Acceptability

This is an acceptable wastewater treatment technology.

Further Development of the Technology

The technology is well tested and fully developed.

Information Sources

PetKovac and Dr Korna H. Kocsis, FelsTisza - Vid Kezetvlmi Fels 4400 Nyiregyh, Szenyi u.19, Hungary, Tel. (36-42) 310 155, fax: (36-42) 310 713.