Fractional Transformation Of Heavy Metals In Sewage Sludge During Hydrothermal Treatment

Sewage sludge is a kind of solid waste and at the same time a kind of resource. Generally, for resource recovery, land application is regarded as the best alternative for sewage sludge disposal. However, pretreatment must be carried out to stabilize the sludge and control the risk of the contained heavy metals(HMs). Hydrothermal treatment(HTT), especially at temperatures lower than 300°C is widely recognized as a promising method for the stabilization of sewage sludge because of its simplicity and cost effectiveness. However, the behavior of HMs during HTT process should be investigated, and their environmental risk after HTT should be evaluated.In this study, the behavior of HMs during HTT process was investigated based on the change in concentration, fractionation and leachability of the HMs. The effect of temperature, the content of lignocellulosic materials and phosphorus on the behavior of HMs was also investigated. The results showed that:The dewaterability of the sewage sludge was greatly improved. After vacuum filtration, the moisture content of sludge was sharply decreased from 85% to 75%, 62% and 45% when the sludge was treated at 170°C, 200°C and 280°C, respectively.HMs was mainly accumulated in the solid residues after HTT, though partly dissolved into the process water. HTT resulted in the fractional transformation of HMs in sludge from weakly bound fractions to more stable state, and therefore the bioavailiability and leaching toxicity of HMs was greatly decreased. 280°C was preferable for risk reduction of HMs, and under this condition the content of metals in the weakly bound fractions decreased by 68.48%, 78.78%, 91.10%, 75.62%, 98.93% and 74.29% for Cr, Ni, Cu, Zn, Cd and Pb, respectively. Furthermore, the leachable content was decreased by 73.33%, 71.93%, 97.46%, 86.14% and 93.91% for Cr, Ni, Cu, Zn and Cd, respectively.The behavior of HMs in sludge was affected by rice husk(RH) addition. When the sludge was treated at 200°C, RH addition enhanced the transformation of Cr, Cu and Cd from unstable fractions(F1 to F4) to stable state(F5) while increased the direct toxicity of Zn and Pb. When the sludge was treated at 280°C, RH addition can help transform all the metals except for Pb from relatively stable fractions(F3 and F4) to both weakly bound fractions(F1 and F2) and stable state(F5). The leachable content of all the metals except for Pb was further decreased after RH addition. These implied that the behavior of HMs during HTT process was affected by the contained lignocellusic materials in sludge, and this kind of effect changed with metals species, HTT temperature and the content of lignocellulosic materials.About 89% of phosphorus in the sludge was found to be apatite phosphate(AP), and all the other forms of phosphorus were almost completely transfromed into AP after HTT. During HTT, the behavior of HMs was changed after hydroxyapatite(HAP) addition. Some metals(Cr, Ni and Cd) transformed into more stable state. However, the direct toxicity was increased for Cu, Zn and Pb. This result indicated that the behavior of HMs during HTT process was affected by phosphorus in sludge, and the effect was related to phosphorus content, treatment temperature and metal species.Totally, HMs in sludge can be stabilized after HTT process, and this kind of stabilization was affected by treatment temperature, the content of lignocellulosic materials and phosporus in sludge.