Effect Of Vermicomposting With Rice Straw On Fertility And Heavy Metal Bioavailability Of Sewage Sludge

With the increase in urban population and urban sewage treatment rate, the amount of municipal sewage sludge has been gradually increased. The sewage sludge utilization has a widespread concern. The use of earthworm in sludge treatment often refers to as vermicomposting. In this paper, rice straw as a conditioner in the sewage sludge, investigations were made to explore the effect on sludge nutrient content, heavy metals and its forms after treatment by earthworms (ie, earthworm Eisenuia faetida). Pot experiments were studied the different proportions after applying sewage sludge vermicompost on crops (lettuce) yield, quality, heavy metals contents and the physicochemical properties of the soil after harvest crops. The main results are as follows:1. The parameters such as pH, organic matter, the contents of NH4+-N and Kjeldahl nitrogen(TN) decreased after vermicomposting, while the conductivity, the contents of NO3--N, available P, TP and available K increased. It indicates that earthworms could enhance the fertility of the sludge.2. The final result indicated that the total concentration of heavy metals and its bio-concentration decreased after vermicomposting, and with the added amount of rice straw increase gradually decreased. The content of heavy metals and its bio-concentration is the biggest decline, when rice straw in the20%of the total weight (dry weight). The present study showed earthworms can change the bioavailability of heavy metals in sewage sludge, and heavy metals from the sludge after treatment has a stabilizing effect, and is not conducive to crop uptake.3. Sewage sludge or vermicompost amended soil, can make the lettuce production increases. Under the same proportion, the application of vermicompost increased the lettuce yield more significant. Compared with control, application of sewage sludge or vermicompost caused the nitrate content of lettuce increased significantly. The lettuce nitrate content in vermicompost treatment is lower than in sludge treatment. When the application of vermicompost rate was20g·kg-1(soil), the nitrate content of lettuce was lower than other treatments.4. After the application of sewage sludge or vermicompost, the heavy metals content of lettuce leaves and roots was higher than the control. And the heavy metals content in lettuce roots was higher than in lettuce leaves. Heavy metals content first increased and then decreased with the proportion of sludge or vermicompost. When the amount of vermicompost applied to20g·kg-1(soil), heavy metals content of the lettuce leaves and roots was lowest.5. The result showed that the application of sludge or vermicompost can reduce the pH of the soil after harvest lettuce, and the pH value decreased with the proportion of sludge or vermicompost. While the parameters such as the conductivity, organic matter, TN, TP and TK content increased.6. It was found that when the application of vermicompost rate was20g·kg-1(soil), the heavy metals contents (included Zn, Cu, Pb, Ni, Cd and Cr) in soil after lettuce harvest were lower than other treatments, and were less impact on later crops.7. The final result indicated the application of sludge or vermicompost amended soil make the phosphatase, urease and catalase activity increased. With the proportion of sludge or vermicompost, the phosphatase, urease and catalase activity gradually.Under the same proportion, the application of vermicompost increased these three enzymes activity more significant. The increase in the phosphatase activity was highest.