Research On The Stability Of Heavy Metals(Cu、Zn) In Sludge With The Pretreatment Conditions Of Thermal Hydrolysis

With the further popularization of water treatment facilities, the sludge production is increasing progressively. In sludge the heavy metals are considered as the main contaminants, so the disposal of heavy metals has been paid more and more attention. In order to explore the effects of thermal hydrolysis conditions on the species distribution of heavy metals in sludge and ultimately determine the best conditions, the temperature and time were chosen in this test to study the changes they caused on the concentration and stability of heavy metals(Cu、Zn) in sludge by using the modified BCR sequential extraction process. Then, the detection results of floc and cell structure, organic functional groups, mineral crystallinity and leaching rate before and after thermal hydrolysis were compared. The main conclusions could be drawn as follows:(1) The concentration of Cu increased from 162.4mg/kg to 193.3mg/kg by heating the sludge up to 210℃, while the Zn concentration accumulated from 779.7 mg/kg to 975.7mg/kg, and both of them got a 20% ascent. With time elapsing, the concentration of Cu rose slightly and the greatest amount was 218.4 mg/kg at 90 min, but Zn achieved the peak of 975.7 mg/kg at 30 min.(2)The best thermal hydrolysis conditions were 210 ℃, 30 min, since the stability of Cu and Zn reached 83.0% and 47.4%, having increased by the maximal 11.9% and 7.3% when compared with raw sludge. Moreover, the results determined by means of SEM, FTIR and XRD showed that the thermal hydrolysis process at 210℃, 30 min was beneficial to the stability of Cu and Zn in sludge.(3)The leaching rate of Cu in the raw sludge and the sludge heated at 210℃ for 30 min decreased with the rise of the extractant pH, and the lowest leaching rate was 0.304% at pH=12. Zn had a similar law with Cu, but its lowest leaching rate appeared at pH=10, the rate was 0.412%-0.753%. After the leaching test was conducted in the acid extractant for the same period of time, the leaching rate of both Cu and Zn in sludge decreased after thermal hydrolysis, and the leaching rate of Zn had an apparent decrease of 5.847%-6.591% when the sludge was submerged in the extractant at pH=2.(4) No matter the contaminant in sludge was Cu or Zn, it had an ascent of leaching rate in acid extractant with the leaching time increased before and after sludge thermal hydrolysis. After the 15-day-leaching test in the acid extractant, the leaching rate of Cu in sludge after thermal hydrolysis kept 1.752%-1.978%, reduced about 2.250% when compared with the raw sludge; while the leaching rate of Zn reduced from 11.914%-14.307% to 6.706%-7.716% after thermal hydrolysis under the same leaching condition. However, the law became less apparent when the extractant was alkaline.(5) A linear relation between the proportion of acid soluble state(P) and the leaching rate(L) of Cu and Zn after the sludge was submerged in different extractants for 15 days was proposed as follows: L=aP+b.