Study On Risk Assessment Of Sludge And Heavy Metal Stablization In Weihui Water

With the improvement of urbanization, people's daily water consumption continually increase, and the urban domestic sewage and industrial waste water discharges are also showed exponential growth year by year. As by-product, sludge also increased rapidly. To properly handle the issue of the sludge, it was first necessary to analysis its physical and chemical indexes, while the treatment and disposal of heavy metals in sludge were important direction in the field of environmental engineering.In this thesis, the physical and chemical indexes of the sludge from Wei Hui Water Affairs were analyzed It was found that the water content of the sludge was 83.2%, pH was 7.8, and the organic matter was 467g/kg. Moreover, the content of N, P and K were 9.2g/kg, 4.13g/kg and 10.6g/kg, respectively. Water content and pH value were in the statistical range of the country. Organic matter, N and P contents were higher than that of farm manure, while K content was close to the farm manure, which suggested the sludge had a higher agricultural value.The total contents of heavy metals in the sludge were determined by adopting H₂O₂^-HCl- HON₃^-HF microwave digestion system, and 11 kinds of heavy metals including Ni, Hg, Cd, Pb, As, Zn, Cu, Fe, Ba, Mn, and Cr were extracted. The method could realize the full digestion of heavy metals under high temperature and high pressure. The total amount of heavy metals and the form content of all kinds of heavy metals were determined by using ICP-AES. Fe was found as 15955mg/kg, which was the highest content of the measured elements. Zn, Mn, and Ba were also high, which were above 1000mg/kg. Pb, As, Cu, Ni, and Cr were relatively low and at about 100mg/kg or below. And Cd content was below the detection range. All the contents of Pb, Zn, As, Cd, Cr, Cu, and Ni, were lower than the standard limit of the agricultural shale?CJ/T309-2009?.The modified BCR state extraction method was used to study the morphology of heavy metals. Zn and Mn in the sludge showed very strong mobilities, and more than 90% were in the presence of water soluble state, acid soluble state, reduced state and oxidation state, which would be moved with the rain and surface runoff. The potential mobilities of Pb, Cu and Ni were relatively strong. When the natural environment changed, it would also change, such as the change of pH. Residue contents of Fe, As, Ba and Cr were more than 70%, and they were mostly in the stable forms and had low migration abilities. The biological effective state of Mn was determined as 85.25%, suggesting that Mn could be easily used by plants and microorganisms efficiently. Both the biological effective state and the potential effective state of Zn were high, indicating Zn had strong biological effectiveness with the changed external conditions. The unavailable state of Fe, Pb, As, Ba, and Cr reached more than 50% and generally would not be used by biology.The ecological risk of heavy metals in sewage sludge was evaluated. The assessments for Pb, As, Cr, and Ni using single pollution index method were found at the safety level, and no harm to the environment. The pollution levels of Cu, Mn, and Zn were assessed as mild, moderate and serious harmness, respectively. The sludge was assessed as heavy pollution based on the Nemero pollution index?P? and the cumulative index method?Igeo?. However, the three methods only took heavy metal contents as main indexes and could not fully reflect the actual situation. Potential ecological risk index method was introduced to calculate the toxicity coefficient, which was more scientific and reasonable, and the assessment result was found at low hazardous level.The effects of heavy metals on the health of adults and children were evaluated in hand-mouth feeding and skin exposure ways. The main exposure methods for children and adults were hand-mouth feeding. Noncarcinogenic risk for adults and children were calculated to be 3.43E-02 and 9.36E-02, which were far less than the warning value of 1. The cancer risk were 5.29E-05, 2.48E-04, which were in children's cancer risk range of 1E-04 to 1E-06 and would have potential carcinogenic hazard to children.The stabilization of heavy metals were carried out by fly ash, diatomite, Na₃PO₄ and Na₂S, respectively, using leaching toxicity method. The diatomite showed little stabilization ability for Zn, Cu, Ni, Cr, and Pb. The fly ash had a certain effect on the removal of 5 kinds of heavy metal ions. The removal rates of Zn, Cu and Ni were 96.6%, 89.7% and 82.3% by 3g Na₂S, however its effects on Pb and Cr were relatively weak. The removal rates of Pb, Zn and Cr reached 94.6%, 83.9% and 78.3% by 3gNa₃PO₄, while its effects on Ni and Cu were poor. When the dosage mass ratio of fly ash to sodium phosphate was 1:10, all the removal efficiency of 5heavy metal ions would reach more than 65%.The stabilizer also had an effect on the morphologies of heavy metals. The morpholog percentage of the 5 heavy metal ions had different changes. Ni, Pb, and Cr transformed into stable morphology, while Zn and Cu have little changes.