Feasibility And Risk Assessment Study Of Biosolids Agricultural Application

Biosolids, as the sediment of the sewage treatment plant wastewater treatment process, which is complex non-homogeneous body composed of organic debris, bacteria, inorganic particles and colloidal. Along with the acceleration of urbanization and improvement of the sewage treatment rate in China, biosolids production is increasing dramatically and in urgent need for harmless treatment and resource disposal. Comprehensive comparison of various kinds of biosolids disposal, biosolids agricultural utilization is a kind of economic and effective method. In our country, toxicology database based on the pollutants of biosolids agricultural utilization is absent. The rsearch on risk assessment of environmental effects caused biosolids agricultural application is also relatively weak. The laboratory experimental protocols are known to lead to several artefacts, so it is important that research on the control standards of bioslids agricultural application under field conditions should be undertaken. Morever, the soil pollution risk caused by nitrogen, phosphorus accumulation in bioioslids agricultural application be evaluated for lack of a long-term field experiment research. Therefore, it is an important and urgent task to research the above issues under field conditions.A field experiment was designed for the rational application biosolids and control standards of bioslids agricultural application. The biosolids added to a calcareous field soil was studied in order to assess the effects of biosolids agricultural application on soil quality and crop growth, and to determine the safety amount of the applied biosolids. The field experimental results were analyzed for assessing the environmental and agricultural risk of biosolids application in order to supply scientific basis for the feasibility of biosolids agricultural application. Bioaccomulation factors of trace elements for bioslids agricultural application were collected to extablish database and conbined with species sensitivity distribution to develope the cadmium criteria in soils and biosolids in order to provide a scientific basis for the rational application biosolids and control standards of bioslids agricultural application. The main results and conclusions are as follows:First, the dynamic changes of nutrients and trace elements in bioslids from Beijing were studied, and the results showed that the biosolids presents the characteristics of high nitrogen, phosphorus contents, and low potassium contents. Also the contents of nutrients and organic matter, pH in biosolids can meet the organic fertilizer standards of agricultural industry (NY525-2002). The mean concentrations of Zn, Cu, Cr, Pb, Cd, Ni, As, Hg and Cd in biosolids from Beijing sludge disposal plant are 1075,96,78,39,33,13.6,10 and 1.6 mg g-1, respectively. The contents of trace elements such as Cr, Cu, Cd, Ni, Pb, and As are much lower than the control standards for biosolids agricultural use (CJ/T309-2009), however, the contents of Zn were close to limits for the class A biosolids (1500 mg kg-1), Hg contents were over the limits for class A biosolids (3 mg kg-1). Zn and Hg in biosolids in Beijing are the elements of high risk for soil environmental quality, especially for Hg, which is 3.6 times of the average content in biosolids Hg of Chinese urban, so that Beijing biosolids are applied to agricultural field should pay attention to biosolids Hg pollution.The field experiment was carried out for five years in Dezhou, Shandong province. The results showed that the contents of TN and TP for wheat and maize grain were significantly increased by biosolids treatment. Crop yields by biosolids treatment maintain at a high level. biosolids can be a replacement for a part of nitrogen fertilizer or chicken manure. The optimum biosolids application rates was 18 t ha-1 yr-1, which derived from the wheat and maize grain yield effect functions by biosolids application.In wheat-maize rotation system, application of biosolids to agricultural soils based on the N requirement of crops will oversupply P. A similar trend of Olsen-P accumulation was found in soils treated with soluble P fertilizers only or with biosolids P as well, which provides evidence that the behaviour of biosolids P in field soils is similar to that of inorganic P fertilizers. In soils with the range of 78.6-2097 kg P ha-1 added by biosolids, the concentrations of Olsen-P in soils with wheat-maize rotation systems and the P accumulation rates could be predicted by a model that depended upon the initial concentration of Olsen-P in the soil, P input rate, crop yield, soil pH, and cultivation time.All of the eight kinds of trace elements were significantly increased in the plough layer (0-20cm) of Dezhou calcareous soil after biosolids application for five years, especially of Zn, Cu, Cd and Hg, and there were no significant changes in trace elements of soils applied with chicken manure. There was a significant linear relationship between the concentrations (mg kg-1) of Zn, Cu, Cd and Hg in plough layer soils and the biosolids application rates (kg ha-1); the increased concentration of Zn, Cu, Cd and Hg in plough layer soil were 1.10,0.128,0.00099, 0.0062 mg kg-1, respectively, when biosolids of 1 t ha-1 per year was added to the soil. According to the accumulation rates of different trace elements and the control standards for biosolids agricultural use (CJ/T309-2009), soil Hg content was up to the soil environmental quality standards (GB15618-1995) required the shortest time (18 years), followed by Zn (30 years), and other trace elements required more time (＞30 years).The contents of Zn in wheat and corn grains were significantly increased by biosolids applied. The contents of all trace elements except Cr and Pb in wheat and corn grains were under the national food hygiene standards (GB 2762-2005). The bioaccumulation factor (BCF) of all trace elements for wheat and maize grains were less than 0.5, the BCF of trace elements for wheat grains were in the order:Zn＞Cd＞Cu＞Cr, Hg＞Pb＞Ni＞As; the BCF of trace elements for wheat grains were in the order:Zn＞Cu＞Cd＞Cr＞Pb＞Hg＞Ni＞As.The cadmium of concentration in soils which prevents 95% plant edibile parts in safety (HC5) was mainly affected by soil pH and OC. Meanwhile, soil pH is the most important predictor for HC5, which explains 62.7% of variances while soil OC explains 25.2% of variances.The prediction model was set up by the relationship between soil pH, OC value and cadmium HC5 can well predict the cadmium threshold values in different soil types, which supplies a scientific basis for formulation and implementation the environmental quality standard of soil cadmium. The model for biosolids agricultural application based on the limits of soil total cadmium content can well control the amount of biosolids applied and the cadmium content of biosolids in different types of soil, which provide the theory basis for the safety and rational of biosolids agricultural application.Main innovative points:using a five-year continuous field experiment to investigate the feasibility of biosolids application to agricultural soils in a wheat-maize rotation system; the accumulation of the nutrient elements and trace elements in soils after biosolids application was quantified; the BCF values of trace elements in soil-plant systems integrated with species sensitivity distribution (SSD) were used for derivation of cadmium thresholds in soils and biosolids when biosolids were applied to soils; and finally the feasibility with cautions for biosolids application in agriculture was presented.