| In recent years, with the rapid expansion of urban capacity in China, The capacity of urban sewage treatment increased significantly. Domestic sewage has been purifieec in municipal sewage treatment plant, and most of the respective pollutants left in sludge, therefore sludge has become the "shelter" of various contaminants in sewage.Sludge,as a product of sewage treatment, is becoming a new environmental concerns. Treating scientifically and utilizing rationally is the last part of the treatment of urban sewage,as well as the key issues to be solved urgently. This paper, relying on the project《Sludge disposal characteristics and practical disposal technology research in urban sewage treatment plant of Sichuan 》 born by Sichuan Academy of Environmental Sciences, explores and researches the rules of heavy metal migration and conversion in the process of breeding earthworms by using sludge when dealing with municipal sludge.Raising earthworms by using sludge, which has an ability of breeding earthworm products with economic value. What’s more, the earthworms have strong digestive ability and through the grinding of their gizzards and digestive tracts, organic matter in sludge is resolved and converted quickly under the combined metabolizing effect of enzymes and aerobic microorganisms.Earthworms excrete granular and aggregate compost with pH neutral, water-gas reconciling and high porosity, making the sludge dewatering, stabilizing, deodorizing, harmless, recycling. In this transformation process, the changing rules of heavy metal elements in the sludge before treatment and after treatment have important comment effect to the the practical application of this technology. This paper, on the basis of previous research, field sampling and laboratory analysis, identify the heavy metal characteristics of the sludge samples in eight sewage treatment plant surrounding Chengdu, studies the changing rules of heavy metal elements before and after treatment comparing sludge samples in two experimental zone, explores the heavy metals content in earthworm body and the rules of absorbing and converting in different physiological stages and evaluate the quality. Through research, achievement and understanding achieved as following:1.PH value of the sludge samples ranges from 5.62 to 6.71, the average content of organic matter is 46%. The coefficient of variation of heavy metals in sludge samples has a relationship: Zn>Cu>Cr>Pb>Ni>Cd> As>Hg.The big variation of heavy metal content of sludge in kinds of sewage treatment plants is related to differences of urban planning, environmental background, electroplating enterprises in longquanyi district and leather-dyes industry in shuangliu district, which may be the cause of leading Zn, Cr and other heavy metals content of sludge in sewage treatment plant in these areas to a higher level. The organic matter content and pH value of sludge in two experimental points are fit to the standards of breeding earthworm. Heavy metals content in sludge samples is close to the content of national average, which shows that studying the heavy metal content in sludge is universal and representative.2. Referring to the standard of 《Agricultural sludge in urban sewage treatment plant sludge disposal》(CJ/T309-2009), samples W01(shuangliu 1), W02(shuangliu 2), W04(Chengdu),W05(Pi county), W07(Deyang)all meet B standard for agricultural sludge, heavy metal content of sludge samples W03(Wenjiang), W06(Jintang) and Ws(Shuangliu experimental Zone) meet the A standard for agricultural sludge, heavy metal content in Wj(Wenjiang experimental area) does not meet any standards for agricultural use only for landscaping; In W08(Longquan), the content of Zn is too high to use for agriculture. Vermicompost Fj and Fs after treated, pH value increases and the organic matter content decreases, meanwhile heavy metals content reduces significantly. Heavy metals content of two samples meet A standard of agricultural sludge, and they are substantially lower than the limits of standard.3.The chemical forms of heavy metals in sludge samples Hg, Pb, As, Cr, Cu exist as residual and organically bonded state with low mobility and bioavailability. And the chemical forms of Zn, Ni, Cd exist in water soluble, carbonates bound, humic acid with strong mobility and bioavailability. After breeding earthworms, the bound humic acid state of Cu, Zn in manure increases; the state of ion exchange, carbonate bound and bound humic acid of Cd increase; the state of humic acid bound, iron and manganese oxides bound of Pb increase. These elements are activated, mobility and biological activity have been enhanced. Changes of the chemical forms of As, Hg, Cr Ni before and after treatment are not obvious.4. The potential ecological risk of heavy metals in vermicompost and sludge form large to small exist in orders: Cd>Cu> Hg> As> Zn> Pb> Ni> Cr. Potential ecological risk of sludge before treatment is highly hazardous or moderate.After disposed,the potential ecological risks of earthworm manure reduces to slight. Cd and Hg make a bigger comtribution to the potential ecological risk index(RI) value. Combining with Heavy Metal characteristics, Cd in sludge and vermicompost is the largest element causing danger.5.In the process of using earthworms to dispose sludge, comparing to the content of heavy metals in vermicompost, earthworms showed an ability of bioconcentration for Cd, Pb, and the ability for concentrating heavy metal from strong to weak is: Cd> Pb>As>Hg>Cu>Zn>Ni>Cr. In the process of growing up, earthworms(the young earthworm-middle earthworm-adult earthworm) heavy metals content was mostly have trend of increasing, in which earthworms have a stronger absorbing ability towards Ni,Cd comparing to other elements. In the period of middle earthworm-adult earthworm the rate of absorbing Zn is higher than the period of young earthworm, while the rate of absorbing Pb and Cr decreases slightly. |
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