Cleaning up the Soil and Water
Treatment of Wastewater by MSL
Wastewater is a major cause of soil contamination, especially in irrigated rice areas. Asian countries badly need a low-cost method of cleaning wastewater.
One promising new technology is the treatment of wastewater by a combination of physical filtering and biological decomposition. This is called the MSL (multi-soil-layering) method. Pilot projects have been operating in Japan for some years, and have now been established in Thailand.
Blocks of soil mixture (soil mixed with organic matter and iron particles) are alternated with zeolite in a brick-wall pattern. Aerobic conditions occur in the zeolite, and anaerobic conditions in the saturated soil blocks. This mixture of aerobic and anaerobic conditions allows for the efficient purification of wastewater. Nitrogen is removed by both (aerobic) nitrification, and reduction under anaerobic conditions. Phosphate is removed by forming insoluble compounds.
The cost depends partly on the materials used. These can be adapted to suit local resources. For example, zeolite is expensive in Thailand, but can be replaced by charcoal, which is cheaper. The cost of the pilot plant in Thailand is US$0.08 per cubic meter of treated water.
As to replacement costs, there is no need to replace the components, which can be used for at least ten years. (This is the length of time the MSL plant has been operating in Japan). Two tanks are needed, which are used alternately. If one tank clogs, the second tank is then used, while the organic matter in the first tank decomposes. The system does not produce any unpleasant odor.
Remediation of Polluted Soils
Levels of heavy metals in some soils are so high that food crops cannot safely be grown in them. The problem then is how to use the land economically, and how to clean the soil. The easiest form of remediation is to convert the contaminated land to non-food crops such as flowers and ornamentals. However, this may not be acceptable to farmers. Lime applications may help, by increasing the soil pH and thus reducing the uptake of heavy metals by the crop.
Phytoremediation
Another approach is phytoremediation, planting selected species to take up the heavy metals. Suitable plant species not only tolerate the presence of heavy metals, they have a strong appetite for them, and take them up so they accumulate in the plant.
More than 400 different kinds of plants are known to accumulate toxins. Research is now being carried out in many countries in Asia to identify the most promising species.
Plants Used for Phytoremediation
The best plants should have a high uptake, and the ability to grow in contaminated soils. Ideally, they should also translocate the contaminants from the root zone to the leaf, so that the leaves can be cut, burned and treated as solid phases. In this way, the contaminants are removed in the cuttings.
Vetiver grass combined with applications of zeolite is being used to remove boron, cadmium and lead from Indonesian soils contaminated by sludge from the textile industry. In Thailand, the silverback fern Pityrogramma calomelanos is being used to remove arsenic from the soil.
Another promising plant species is Thlaspi caerulescens, one of the many species of stone cress. This plant is a super accumulator of copper, lead, cadmium and zinc. Another member of the Crucifer family, Brassica juncea, has been tested in Taiwan and shown to be a super accumulator of lead and selenium.
Index of Images
Figure 1 Diagram of the MSL Treatment Plant, Thailand
Figure 2 Phytoremediation: Testing the Uptake of Heavy Metals by Different Plant Species
Figure 3 Stone Cress (Thlaspi Caerulescens)
Figure 4 Vetiver Grass (Vetiveria Zizanioides)
Figure 5 Leaf Mustard (Brassica Juncea)
