Experimental Study On Emergency Treatment Of Source Water Accidentally Polluted By Heavy Metals

Heavy metal is a common type of pollutant during accidental pollutions. In recent years, the frequency and intensity of accidental heavy metal pollution events tends to increase in China. Due to high damage of heavy metal on human health and poor removal of soluble heavy metal by the traditional process of water plant, the drinking water safety of residents will be in great danger if heavy metal pollution happens in drinking water sources.The methods for treating source water accidentally polluted by Cr(VI), Cu(II) and Cd(II) are investigated in this paper, including: (l)Evaluation of their removal effects by current water treatment process; (2)Powdered activated carbon adsorption for Cr(VI) removal; (3)Ferrous sulfate reduction for Cr(VI) removal; (4)Enhanced coagulation-sedimentation for Cu(II) and Cd(II) removal. These proper methods for heavy metals' removal are fixed through experiments. And according to the experiment results the technical schemes for dealing with these three heavy metals are initially proposed to improve the emergency management of accidental pollutions in drinking water sources.The main conclusions were listed as follows:(1)The result of the experiment which simulates the removal of Cr(VI) ,Cu(II) and Cd(II) by traditional process of water plant shows that: there are some limits in heavy metals' removal by current water treatment process, where PFAC and sodium silicate are used as coagulant and its aid. And the maximum pollution levels of Cr(VI), Cu(II) and Cd(II) are 0.100mg/L, 3.0mg/L and 0.020mg/L, respectively.(2)The result of the experiment in which Cr(VI) was removed by powder activated carbon adsorption shows that: when the concentration of Cr(VI) is lower than 0.5mg/L, powdered activated carbon adsorption can be adopted to produce up-to-standard water and the maximum removal rate is 90%.(3)Compared to powdered activated carbon adsorption, ferrous sulfate can greatly increase the removal rate of Cr(VI) due to its reduction of Cr(VI) to Cr(III). Additionally, Fe²⁺ can be oxidized into Fe³⁺, which enhances Cr(VI) removal effect. When Cr(VI) in source water is 0.10mg/L～2.00mg/L, the dosage of ferrous sulfate is 3.0mg/L～16.0mg/L. After enhanced coagulation-sedimentation the total amount of Cr(VI) in treated water can be decreased below 0.02mg/L and the removal rate will get to 99.1% or more.(4)When using enhanced coagulation-sedimentation to deal with Cu(II) and Cd(II) pollutions, pH value of source water is an important factor. The proper pH values for Cu(II) and Cd(II) removal are 7.5～8.0 and 8.0～9.0, respectively. When pH is 9.0, source water with Cd(II) of 0.050mg/L can be treated up to the standard by enhanced coagulation-sedimentation and the removal rate is 90%. Adding sodium carbonate can not only change pH value of water, but also promote the production of cadmium carbonate and cadmium hydroxide, which can increase Cd(II) removal rate. When the dosage is 20mg/L source water with Cd(II) of 0.080mg/L can become up-to-standard after enhanced coagulation-sedimentation and the removal rate can exceed 94%.