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Abstract:Based on the hypothesis that reclaimed water for irrigating is the cardinal source of heavy metals pollution in soil, and the heavy metals leave from soil mainly by plant harvest, the article constructs a differential equation and simulates the cumulation of Cd in wheat soil of southeast suburb in Beijing based on simulink of matlab. With the data relative Cd mass proportion in soil as predictor parameter in stems and leaves as response ones, the fitting curves are produced respectively. The curves show that the amount of Cd leave from soil is very small compared with the amount entering into soil via irrigation water. But in the condition of lower concentration in soil, the proportion of Cd left from soil is bigger relatively. The prediction shows that the content of Cd will exceed the national standard of China after 40 years in the condition of the content of Cd in reclaimed water with 10mg·m -3 and concentration in soil with 0.1333mg·kg -1 , but it should be 100 years if the concentration of Cd in reclaimed water for irrigation reduced to 5mg·m -3 .

The difference of reclaimed water qualities between in summer and in winter were proved considerable. Reclaimed water derived in spring or summer could be applied irrigating in some arid region, which have been considered a rational renewable resource of water. Yet in winter, reclaimed water would run off into river or stream in vain. The idea of storing it in winter and being ready for the next year's irrigation is proposed, however, the effects on crops are maintained sealed. Experiments processed using raw sewage, primary treated water, and secondary treated water in Gaobeidian Sewage Plant of Beijing to irrigate maize and soybean in spring and summer from 2006 to 2007, based on which the irrigation effect derived from reclaimed water of winter was predicted by ANN. The results validated the strong relations between irrigation effect and qualities of water, which indicated the reclaimed water stored in winter was grossly similar to secondary treated water in summer for irrigation.