Potential risk assessment of heavy metal in biogas slurry irrigation

【Objectives】The biogas slurry contains abundant nutrients and physiological activators. Irrigation with biogas slurry is a cost-effective and environmental friendly way for biogas slurry disposing. However, there is always concern about the residual heavy metals brought through forage or animal feces, the contents of which sometimes exceed the up limit of standards. The risk of biogas slurry for irrigation is assessed to provide the basis for the selection of proper treatment methods. 【Methods】 One hundred samples of biogas slurry, from household marsh gas tank and biogas engineering in northern and southern parts of China were selected, the contents of total Cd, Cr and Pb are determined by atomic absorption spectrometry, Hg by cold atomic absorption spectrometry. The risk degree of heavy metal from different source of biogas slurry is evaluated using Geometric mean index, Nemerrow index and the unit vector index. 【Results】 1)The heavy metal contents from both of household marsh gas tanks and biogas engineerings are not conforming the normal distribution. The mid-value contents of As, Cd, Cr, Cu, Hg, Pb and Zn from household biogas slurry are 0.312, 0.0195, 0.183, 0.75, 0.025, 0.33 and 1.15 mg/L, respectively; from engineering biogas slurry are 0.110, 0.009, 0.218, 0.985, 0.004, 0.131 and 0.857 mg/L, respectively The average contents of the latter are higher than the former, except for As. In household biogas slurry, the Cd and Hg contents are varied strongly, As, Cr and Pb contents are less strong. All of heavy metal contents in the engineering biogas slurry are varied strongly. 2)The contents of heavy metals in the biogas slurry from different sources are different. The heavy metal contents from piggery are generally higher than from cattle farms, showing higher risk. 3)Evaluated by comprehensive risk index, the unit vector index of Pb and As in household biogas slurry is low in risk potential, and other ones are less than 0.2, indicating no risk. The Arithmetic Average Index, the Geometric Mean Index, Nemerrow Index and the Unit Vector Index of Hg in engineering biogas slurry are the highest. The content of Hg in engineering biogas slurry assessed by the Nemerrow Index is high in risk potential, however, it is low while assessed with the Arithmetic Average Index and the Unit Vector Index. The content of Cd in engineering biogas slurry assessed by the Nemerrow Index is low in risk. All the risk index values of other heavy metals are different when assessed with different methods, they are all in low degree of risk. 4)The risk of the biogas slurry among the investigated provinces is different for irrigation. The Nemerrow Index from Henan and Anhui Provinces is the highest (high risk), suggesting that physical and chemistry treatments are needed before the biogas slurry is used for irrigation. Those from Zhejiang and Hunan Provinces are low in risk, and from other provinces have no risk. In addition, the Nemerrow Indexes of the household biogas slurry form Inner Mongolia, Liaoning, Fujian, Sichuan Provinces are lower than 0.2, means only simple physical methods are needed before the biogas slurry irrigation. 【Conclusions】The heavy metal contents in the biogas slurry are mainly influenced by the sources of the raw materials. Although there are some differences in the risks of heavy metal among different areas, the difference between the northern and the southern China is not existed in the engineering biogas slurry. The Hg and Cd contents in the biogas slurry are low in concentration but still higher in risk, suggesting that they should be removed before application.