Research On Utilization Of Dewatered Municipal Sludge To Soil And Its Environmental Effects

The dewatered municipal sludge in Guangzhou municipal wastewater treatment plant was taken as experimental material in the present study. On the basis of analyzing the sludge characteristics, the growth response of different plants, the basic properties changes of compounded soil and transport and transformation of nitrogen, phosphorus and heavy metals in compounded soil were investigated by pot experiment method. And then the possibility of using the dewatered municipal sludge to soil was evaluated. The phytoremediation efficiency of heavy metals by plants during the utilization of sludge for non-agricultural purposes and effects of agricultural use of sludge after phytoremediation on biomass and quality of crops were also studied. The results showed:The contents of organic matter, nitrogen and phosphorus in dewatered sludge were relatively high, which showed that the sludge had the relatively high use potential to soil. However, the concentrations of Zn were higher than the criterion, meanwhile, Cd and Pb existed in active fraction were 20% and 30% respectively, so the sludge must be treated by appropriate method before agricultural utilization. Taking account of the high contents of nutrients, the dewatered sludge could be used for non-agricultural purposes first and during the utilization, phytoremediation could be used to reduce the contents of heavy metals in sludge.The compounded soil mixed by Ferralsols and sludge under different proportion was used to cultivate Canna indica, Turf Grass, Dockleaf Knotweed and Goosegrass Herb in pot experiments. The results showed that the Canna growth pattern exhibited a pronounced positive growth response under the certain application rate and the higher loading rate, the more number of plants, number of leaves and stem length. The application of sludge could not only increase the overground biomass of Turf Grass, but also promote the development of roots. Goosegrass Herb and Dockleaf Knotweed, which could adapt quickly to the environment and belonged to the important weeds in Guangdong, could survive in the sludge, and the sludge application could increase the biomass of Goosegrass Herb and promote its growth.During the experiments, chlorophyll fluorescence parameters were analyzed to test the impacts of sludge on plants. The results showed that sludge did no harm to plant although the heavy metal contents in plant were increased, while under the conditions of barren soil alone, the plants were put into nutrients stress conditions.The analysis of heavy metal contents in compounded soil showed that Canna indica, Turf Grass and Dockleaf Knotweed were effective in removing Cd, Ni and Zn; and Goosegrass Herb was effective in removing Cd, Ni, Zn and Cu, it also had certain removal efficiency with Pb and Cr. Based on biomass, the contents of heavy metals in plants and the removal efficiency of heavy metals, the application rate of sludge should controlled beyond 50% (330 t·hm-2).The compounded soil after phytoremediation by Canna indica, Turf grass and Goosegrass herb were used to plant Brassica Chinensis, Romaine Lettuce and Asparagus Lettuce respectively. Seed germination and root elongation experiments indicated that the biological toxicity of sludge had decreased after phytoremediation and it could be used for agriculture. The appropriate sludge application rate could increase the biomass but couldn't decrease the quality of these crops, which indicated that healthy risk could be greatly reduced when the sludge was used for agriculture after phytoremediation by plants. The continuous utilization of sludge (non-agricultural use—agricultural use) could be used as an effective method for treatment and application of sludge.The treatment ability of soil was calculated with mathematic model for soil environmental capacity and the results showed that the contents of heavy metals in soil couldn't exceed the critical value when the application rate of sludge was 322.2 t·(hm2·a)-1. The market investigation of sludge utilization to soil showed that sludge has wide application prospects in respects of using it in forestry and landscaping.