Distribution Characteristics And Status Of Heavy Metals In City Wastewater Treatment Process

As the amount of city sewage disposal increases,the quantity of sludge,a side product from the process,is on a rapid rise.Sludge not only contains abundant nutritive elements,but also heavy metals,which are non-degradable toxic substances.If not handled properly,further pollution may occur.The presence of heavy metals prevents sludge from being used for agricultural purpose or brick manufacturing.Therefore it is quite necessary to monitor sludge produced from urban sewage treatment and the transfer and distribution of metals in the treatment process.By means of microwave digestion sample pre-treatment technology and inductively coupled plasma atomic emission spectrometry(ICP-AES),we studied the water sample and sludge from the treatment unit of Jinan City Water Purification Plant.ICP-AES was applied in determining the content of eight kinds of heavy metals:Cd,Cr,Cu,Mn,Mo, Ni,Pb,and Zn.Thereby we are able to gather information on aqueous phase and distribution of metals in each treatment unit,as well as their intake and transformation in the sewage plant.The application of BCR method facilitated the research on the states (acid-soluble state,reductive state,and oxidizable state)of heavy metals in primary sludge,aeration pool sludge,double precipitation sludge,dewatering room.With data processing based on mathematical methods,we discover the characteristics of the heavy metals in various states.Those data can be used for the purpose of heavy metal control, urban sludge disposal and utilization.The research suggested:(1)the content of Cd,Cr,Cu,Pb,Mo,Zn,Ni,and Mn from the discharge of the plant complied with GB18918-2002 pollutant discharge requirements on sewage treatment plant,and Cd,Cr,Cu,the Zn content meets GB5084-92 standards on agricultural irrigation water.(2)At the sewage treatment plant,most metallic elements (Cd,Cu,Mn,Mo and Pb)precipitated in the primary pool,while a portion was absorbed by activated sludge.The content of those elements in the primary sludge is higher than that in the excess sludge.In contrast,the content of Cr,Ni and Zn in the excess sludge is higher than that in the primary sludge.(3)In the sewage treatment process the metallic element that deposits to the sludge does not change much.The morphological advantage of the same metallic element varies with different sludge samples,which can be modified in the treatment process.(4)With BCR method,we gained insight into the distribution pattern of the metallic elements:Cd,Zn,Pb mainly exist in deductive state,while Pb doesn't have acid-soluble state.Mn mainly exists in acid soluble state,whose content in oxidization state is rather low(9.3—11.8%).Cr,Cu and Mo are mainly in the oxidable state,whose content in acid soluble state is relatively low(＜1.2%).The content of Ni varies slightly with each state.