Study on the form of Heavy Metals in the Sludge Co-compost with Steel Slag
|Keywords||sewage sludge heavy metal chemical forms aerobic composting steelslag organic matters|
In recent years, with the accelerated process of urbanization and the increasing population, the scale of sewage treatment speeds up and yields huge sludge year by year, while its resources utilization rate is low. As effective biological resources, sludge is very suitable for using as a soil conditioner, especially for poor soil region loess. Organic matters in the sludge can be returned to the field, which not only solves the disposal of sludge and pollution problems, but also starts a kind of organic source of manure and improves its fertilizer effect, but the heavy metals have become the key factors to limit its land using. The mobility, the bioavailability and the toxicity of heavy metals in the soil depend on their specific chemical forms as well as their binding state. As a solid waste, steel slag is produced in the making process and it owns characters of high basicity, large surface areas, and adsorption capacity and curing heavy metals, then the steel slag has gradually become an important material for heavy metals passivation. On the other hand, steel slag itself contains elements which can change the soil by increasing the physical and chemical properties, what makes the application to the land come true.·In order to make the sludge be safely applied to soil, this experiment is aimed at changing the form of the5heavy metals of Cd、Pb、Cu、Zn and Ni and reaching the effect of its passivation in the sludge through the way co-compost with steel slag. According to the different amount of steel slag, the experiment designed six groups of composting and lasted54days. It sampled regularly throughout the whole composting process, and analyzed the changes the form of the5heavy metals in sewage sludge and organic matters to determine the best passivation time and the required composting time.The experimental results showed that the steel slag had a different influence on the5heavy metals’form. Generally it indicated a passivized effect on the easy bound of Cd and Pb, and to the metals of Cu, Zn and Ni, the easy bounds were activated, what’s more, with the unequal of increased amount of steel slag, neither the effects to the5heavy metals.Through the comparison of the heavy metals’ forms change after composting sludge, co-compost with steel slag have passivation effect of Cd, Cu and Ni in the exchangeable and carbonate binding state was obvious, and the proportion of the easy bound reduced. To Zn the exchangeable state lost mainly converted into Fe-Mn oxidation state and organic-bound matters, however, carbonate-bound state change a little before and after composting. While it didn’t work well on Pb, but Pb’s activation effect had been suppressed under the condition of the co-composting, and therefore reduced the risk of sludge land utilization.By comparison of the heavy metals’forms proportion in the composting process, Cu、Ni、Zn’s exchangeable reduction was mainly converted into the organic bound、 the Fe-Mn oxidation state and organic bound、carbonate-bound state and Fe-Mn oxidation state. To Pb the decrement in exchangeable, carbonate and organic combination state transformed the Fe-Mn oxidation state and residual state; To Cd, the exchangeable and carbonate bound states mainly converted to Fe-Mn oxidation state and organic bound, and residual increased a bit;Form the analysis adding steel slag composting, we obtained the steel slag can shorten the passivation time, compared with the compost without steel slag the consuming time18days, Cu、Zn and Ni achieve the best passivation effect on the10th day, and that of Cd was on the18th day in the comparison of26th day. And the passivation effect was much better than the compared experiment GZO.