Study On Municipal Sewage Sludge As Fertilizer For Fruit Trees

With the improve of municipal sewage treatment rate, the output of sewage sludge increased rapidly. The outlet of sewage sludge faces a severe problem in big cities increasingly. How to dispose so many biosolids safely is the special pressing task currently. Land utilization is increasingly considered as a positive and effective method for sludge disposal. This method take use of a large number of rich nutrients and organic matter from sewage sludge to improve soil structure and supply crop nutrient, which can not only take the appropriate use of the sludge resources and solve the problem of sludge, but also will not endanger human's health because toxic and harmful substances such as heavy metals and organic pollutants can not enter the food chain.As an example for Xiamen City, study has been done on municipal sewage sludge as fertilizer for fruit trees. In Xiamen City approximately 260m~3 dewatered sewage sludge (Contained 76 percent of water) was produced evry day, which was transported to refuse dump of Dongfu for landfill. But it was rejected by refuse dump now because it couldn't reach the demand for landfill. New outlet for municipal sewage sludge must be cried for. In this paper, lots of experiments were carried out on the main constituent of sludge from municipal sewage treatment plant of Xiamen City. To offer guidance of theory and technology for land utilization and insure the safe reuse of sludge, experiment was done on grapefruit trees fertilized by arerobic compost of sludge from Shiweitou wastewater treatment plant in Xiamen Island. The study focused on the impact of heavy metals transfer from soil to grapefruit leaves and pulp. The paper includes analysis on the content of heavy metals in sludge of municipal sewage treatment plants in Xiamen City, analysis on nutriments and value on recycling of sludg, experiment on grapefruit trees fertilized by compost of sludge from Shiweitou wastewater treatment plant, safety evaluation of soil on heavy metals after sludge application, and study on transfer characteristic and model of heavy metals from soil to plant system.Results showed that it was feasible for municipal sewage sludge from Xiamen City as fertilizer for fruit trees. Sludge composition of Shiweitou wastewater treatment plant was simpler than that of the outside of the Island. Content of heavy metals was low and nutrient of TN and TP was high in sludge of Xiamen Island. Further more there was an increase of TN and TP. The foreground of land application of sludge of Xiamen Island was very considerable. The nutrient content in sludge of Shiweitou treatment plant were slightly higher than the nutrient content of farmyard manure, fertilizer efficiency of which was better. After the grapefruit trees were fertilized sludge compost for a year, the efficiency of TP was increaced in surface soil. Sludge application could promote the growth of grapefruit tree. The dimension of fruits increased and the period of fruits prolonged. Heavy metals mainly transfered and concentrated in surface soil (0～15 cm), also in leaves and fruits but with little affect. Leaves of grapefruit trees could concentrate more heavy metals than pulp, and therefore leaves contained much more heavy metals than pulp, with the content of Cr, Cd, Pb, Cu, and Ni beyond National Food Sanitation Standard. The concentration of heavy metals in pulp is quite low, with only the content of Ni beyond National Food Sanitation Standard. Sludge compost couldn't be used for rootstalk and leave edible plants. The content of Ni must be reduced and the nutriment of TK must be supplemented before land utilization for sludge.The study of content and transfer characteristic of heavy metals in soil-plant system showed that Cd had high enrichment in soil; the downward transfer ability in soil was Pb>Zn>Hg>Cr>As~Cd>Ni; Pb and Zn had greater ability and Ni relatively weak; Cu largely concentrateds in the surface soil layer of 0～15 cm. Heavy metals tended to transfer and concentrate much more to leaves than to pulp. Their transfer and concentration ability from soil to pulp is Ni>Cu>Zn>Cr>Pb>As. Cd was not teste in pulp which indicated that basically Cd didn't transfer to pulp and the heavy pollution of Cd in soil would not harm the pulp. The utilization of sewage sludge to fruit trees showed that among the five elements that were mostly highly contained in sludge, Zn and Cu were the required nutriments for plants growth and would exert no harm in short period, but monitor was a must if they would be used for long term. Sludge fertilization increased the pollution of Cd in the soil, and the transfer ability of Cr from soil to leaves, and Ni to pulp. The three elements all had large potential harm in sludge utilization in soil and therefore long term tests should be followed.In short period that sludge was fertilized to grapefruit trees, heavy metals mainly transfered and concentrated in the surface soil (0～15 cm). The content of heavy metals didn't show significant changes and had no pollution to soil except that the content of Cd in depth was significantly increased and burden the pollution. By adopting set pair analysis model, this paper evaluated the quality of soil environment after sludge application and the results showed that after one year of sludge application, 0～15cm layer of soil and 15～30cm layer of soil were both relatively clean and thus it proved the safety of sludge application. Cd in the soil was considered as the limiting factor whose largest concentration decided the age of urban sludge land application, if the largest continuous application was 30 t / (hm~2.a) to the farmland, the largest application period would be 20 years.By adopting regressive analysis and the software of Eview 5.1 for Windows, the paper further established the correlation model of heavy metals like As, Hg, Cu, Pb, Cr and Ni between grapefruit trees comparable soil sample and experimental soil, as well as the transfering model of heavy metals from soil to leaves and pulp. It also predicted the concentration of heavy metals in soil-plants system after sludge application. This paper combined the analysis of transfer mechanism and statistics and provided a very practical research method for sludge utilization in soil.Finally, some suggestions about the shortages of the research and its possible expansion were proposed for future study.