Changes Of Heavy Metals During Life Cycle Of Municipal Sludge And Effects On Phytoremediation Of Contaminated Soil

Nowadays,the sludge production in our country is increasing day by day,and the treatment of sludge needs to be solved urgently.The land use of sludge is the most promising development direction,but the distribution of heavy metals in sludge is unstable,the application of farmland will bring heavy metal pollution to farmland,and once farmland is polluted,it will be difficult to recover.In view of the widespread distribution of heavy metal contaminated soils in our country,the sludge was treated and applied to contaminated soil,it can not only improve the nutrient matrix in polluted soil,but also can reduce the soil pollution by adding sludge.Plants with strong resistance to heavy metal pollution can be selected,and even plants with certain biomass and heavy metal extraction can be selected,which can play a certain role in the remediation of contaminated soil at the same time of sludge treatment.Alfalfa(Medicago sativa L.)grows widely in loess area in the west of China,and and its growth cycle is short.It can harvest multiple crops in a year,which indirectly leads to its increased biomass,and is beneficial to extract heavy metals.Finally,the sludge compost was applied to contaminated soil planted with alfalfa.The main results are as follows:1.When sewage sludge pH ranges from 7 to 9,the concentration of Cd,Pb,Ni,Cu and Zn solutions of heavy metals is 0.01 to 1,0.1 to 10,0.05 to 10,0.5 to 10 and 1 to 20mg/L.the activated sludge can stabilize all of Pb > 96% of Zn and Cd,> 94% of Cu,and > 61% of Ni from wastewater.2.In the process of sludge composting,the total amount of the heavy metals accumulated in the sludge did not change significantly.By the end of the sludge composting,a proportion of active fractions of heavy metals were transformed to stable forms.3.In light polluted soil,when Cd concentration of sludge was less than or equal to soil pollution level,soil Cd bioactivity was promoted.Sludge with Cd concentration larger than soil pollution level inhibited Cd activity in soil.When applied to moderately polluted soil,sludge played a certain inhibitory role on Cd bioactivity in soil.In severe polluted soil,sludge application had a slight promoting effect on soil Cd bioactivity.4.Through the analysis of contaminated soil Cd concentration contrast before and after the restoration,such as to add sludge promoting plant remediation of soil contaminated by heavymetals,for mild and moderate polluted soil,need to control the sludge concentration of Cd is less than or equal to the corresponding soil pollution level and the proportion is more than or equal to10% sludge.For heavy metal heavily polluted soil,it is necessary to control the Cd concentration of sludge less than or equal to the level of soil pollution,or when the Cd concentration of sludge is greater than the level of soil pollution is less than 40mg/kg.The proportion of sludge added to control sludge should be less than 10%.5.Through the analysis of contaminated soil Cd concentration contrast before and after the restoration,such as to add sludge promoting plant remediation of soil contaminated by heavy metals,for mild and moderate polluted soil,need to control the sludge concentration of Cd is less than or equal to the corresponding soil pollution level and the proportion is more than or equal to10% sludge.For heavy metal heavily polluted soil,it is necessary to control the Cd concentration of sludge less than or equal to the level of soil pollution,or the Cd concentration of sludge is greater than the level of soil pollution.The proportion of sludge added to control sludge should be less than 10%.Study on the whole life cycle of the change of heavy metal sludge used in the repair of soil pollution of plants,the sludge added to the contaminated soil and planting alfalfa can better repair effect on Cd soil pollution,get a new technology for remediation of soil pollution of plant.The concentrations of Cd,2.3,4.5 and 9.9mg/kg were mild,moderate and severe in soil.To promote the repair of the plant sludge added in soil,mild and moderate pollution to control sludge added ratio greater than or equal to 10% and the concentration of sludge Cd were less than or equal to2.3,4.5mg/kg,severe pollution to control soil sludge Cd concentration is less than or equal to9.9mg/ kg.Therefore,the sewage treatment plant activated sludge concentration is 2500 ~4000mg/L,with the Cd adsorption efficiency was 75%,corresponding to the phytoremediation of soil heavy metals in mild,moderate and severe Cd pollution,produce city sludge from sewage Cd accumulation concentration limit value corresponding to 7.6 ~ 12.12mg/L,15.1 ~ 24.16mg/L theory and 33.13~53.01mg/L.