| With the rapid increase of sewage treatment rate in China,there are a large amount of municipal sludge needed to be disposed urgently every year.At present,the frequently-used disposal methods of sewage sludge include compost,landfill and incineration.From the perspective of sustainable development,resource utilization of sludge is the best way of disposal,and land application is one of the most promising methods.Up to now,the research on land use of sludge has been mainly focused on application of sludge after composting or other pretreatments.However,after high temperature degradation of compost,the organic matter and other nutrients will be largely lost,and the pretreatment such as composting will take a long time,which increases the difficulty of sludge treatment.At the same time,due to the limitation of relevant standards of sludge land use,the sludge application rate has been maintained in a relatively small range,and it is difficult to explore the upper limit of sludge application rate.Therefore,fresh dewatered sludge was directly applied to farmland without pretreatment and with a large amount of application in this experiment.They were studied in relation to effects of different amounts of fresh sludge on soil physical properties,nutrients and heavy metals contents,feasibility of direct agricultural use of sludge and greater sludge farming amount,in order to reduce the cost of sludge disposal time and increase the capacity of sludge disposal.Fresh dewatered sludge of Tongchuan Xinyao Wastewater Treatment Plant were directly applied to farmland soil with 6 dosages included 30t/hm~2,60t/hm~2,90t/hm~2,140t/hm~2,200t/hm~2and 260t/hm~2.Then the effects of sludge application on soil physical and chemical properties were analyzed and discussed.The main conclusions were as follows:(1)With the increase of sludge application rate,soil bulk density decreased gradually,and soil saturated hydraulic conductivity increased significantly.Soil water characteristic curve was measured by centrifuge method and was fitted under different sludge application rates.Then the main soil water constants of different treatments were compared,and the specific water capacity of soil under different suction was analyzed.It was concluded that the water holding capacity and water supply capacity of soil were both enhanced after fresh sludge application.(2)With the increase of sludge application rate,the contents of organic matter,ammonium nitrogen,nitrate nitrogen,total phosphorus,available phosphorus and available potassium increased significantly,and the soil pH did not decrease significantly.But the content of soil total potassium did not change significantly.(3)After sludge application,the content of heavy metals in soil treated with different sludge dosages increased to different extents,but the contents of Cu,Zn,Cr and Pb did not exceed the Risk screening value of Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land(GB 15618-2018).The Cd content of the soil exceeded the risk screening value but was lower than the risk control value,and did not cause heavy metal pollution to the soil.The safe continuous application period of sludge direct agricultural use was estimated.It was concluded that the heavy metal element was Zn limiting the direct agricultural use of sludge,and sludge could be applied for 1 year with the dosage of 140t/(hm~2·a),therefore it could reduce the sludge dosage appropriately to prolong the continuous application time of the sludge. |
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