After many experiments, Hebei Academy of Forestry has screened seven kinds of heavy metal-enriched plants from 85 plants, which laid the foundation for the use of biological engineering to treat sludge from sewage treatment plants, especially Shijiazhuang City, where the cadmium content exceeded the standard sludge.
At present, the three major sewage treatment plants in Shijiazhuang City produce an annual output of 82,000 tons of dewatered sludge. The tests showed that the total cadmium content of sludge reached 64.253 milligrams per kilogram, which is more than three times the cadmium content of agricultural fertilizers. Shijiazhuang currently uses landfill methods for sludge, and the investment and operation costs are huge. The cost per ton of landfill is up to 500 yuan.
According to reports, from the 1980s onwards, developed countries began to study the use of phytoremediation technology to treat heavy metals polluted soil and sludge. Phytoremediation technology has the advantages of low project investment, no damage to the ecological environment of the soil, and reduction of secondary pollution. Screening plants with enrichment capability is a prerequisite for phytoremediation.
In 2005, Hebei Academy of Forestry Science and Technology undertook the bidding project of Shijiazhuang Municipal Science and Technology Bureau, “Sewage Treatment and Application Technology for Municipal Sewage Sludgeâ€, and adopted bioengineering to remove or reduce the content of heavy metals in sludge. After more than three years of screening and a large number of experiments, Hebei Academy of Forestry obtained more than 10,000 data. Seven types of heavy metal-enriched plants were screened from 85 plants. These plants can not only grow normally in soils with excessive heavy metals. , And can absorb heavy metal components in the formation of agglomeration of the branches and leaves, the heavy metals transferred from the sludge and soil.
Among the seven heavy metal-enriched plants screened this time, there is only one type of cadmium-rich plant, namely the garden plant Safflower, which is almost unaffected by growth in soils rich in cadmium, and the amount of cadmium per kilogram of above-ground parts of the plants. It can be as high as 23 mg and the enrichment factor reaches 0.36, while other plants have a maximum of 0.05.
According to Guo Weizhen, a senior engineer at the Hebei Academy of Forestry, the discovery of cadmium-rich plants provided the basis for the further transformation of sludge into valuable materials through biotechnology. "In addition, Hebei Academy of Forestry Sciences has systematically conducted studies on sludge composting techniques, bioleaching tests, and the application of sludge forest land. This will provide a variety of harmless use pathways for excess heavy metal sludge."
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