Effect And Mechanism Of Bioleaching On Enhancing Dewaterability And Heavy Metals Removal Of Sludge

With economic development,population growth,and increasing demand for high quality of environment,the number and scale of the wastewater treatment plant is increasing.Hereby,a huge amount of excess sludge has been produced.The excess sludge has the characteristics of high water content,heavy metals,and serous pathogens pollution,which make the proper disposal of sewage sludge become more and more urgent.The most commonly used method of sludge treatment is pretreating the sludge by adding flocculating agents(organic,inorganic or their mixture),followed by mechanical dewatering.However,the water content of the treated sludge just can be reduced to around 80%,while heavy metals,pathogens,and odor of the sludge can not be resolved by these methods.Recently,bioleaching method has been developed as an attractive method to remove heavy metals from sludge.It was also reported that bioleaching not only has an important role in removing heavy metals but also in improving sludge dewatering by Zhou et al,and they also had successfully achieve a continuous sludge reactor operation,and the moisture content of the bioleached sludge could reduce to less than 60%after dewatered.However,the dissolved organic matter(DOM)(such as organic acids)in sludge strongly inhibited the growth of Acidithiobacillus species and thus delayed bioleaching processes.Meanwhile,the mechanism of the sludge dewatering was still not clear.The heavy metals removal and dewaterability of sludge facilitated by bioleaching was investigated through batch experiments with co-inoculation of different acidophilic thiobacilli(Acidithiobacillus thiooxidans TS6 or Acidithiobacillus ferrooxidans LX5),and results showed that co-inoculation of two acidophilic thiobacilli was the best choice.In order to investgate the mechanism of sludge dewatering,chemical acid pretreatment and added Fe3+ conditioning were used to research the dewaterability of sludge.Then,different methods were utilized to extracted extracellular polymeric substances(EPS)from sludge,and the mechanism of EPS on the sludge dewatering by bioleaching was also studied extensively.Finally,an acid-tolerant heterotrophic microorganism Galactomyces sp.Z3 was isolated from the sludge in order to shorten the bioleaching time,and then the effect and mechanism of the heterotrophic microorganism Galactomyces sp.Z3 on enhancing the bioleaching efficiency were also investigated.The main results were presented as follows:The removal efficiency of Cu and Zn exceeded 85%and 90%after bioleaching involved inoculation of A.t,A.f and A.t+A.f,respectively,and A.t+A.f was better than other treatments.The dewaterability of sludge after bioleaching was also improved.Specific resistance to filtration(SRF)of sludge with inoculation of A.f and A.t+A.f was reduced from 81.20×1013m/kg to 5.56×1013m/kg and 5.40×1013 m/kg,separately.The solid content rates of CK,sludge with inoculation of A.t,A.f and A.t+A.f after centrifuged at 4000 r/min for 3 min were 4.30%,5.57%,9.89%,and 10.06%,respectively.Meanwhile,the settleability of sludge was also improved after bioleaching,and the supernatant proportion of CK,A.t,A.f,and A.t+A.f treatments were 2%,2.1%,26%,and 25%,respectively.Taking dewaterability,settleability,and heavy metals removal efficiency into account,the treatment co-inoculation of A.thiooxidans and A.ferrooxidans was better than other treatments.CST values of original sludge,chemical reagent acid-sludge(pH 2.67),2 g/L Fe3+ added sludge and bioleached sludge were 356 s,68.9 s,25.5s and 19.9 s,respectively.In addition,the solid content of dewatered sludge after centrifuged at 40000 g for 30 min of the above sludge were 12.1%,15.2%,16.2%,and 21.3%,and the solid content of dewatered sludge could increased to 12%if the above sludge were centrifuged at 40000 g,17000 g,11000 g and 5000 g,respectively.Obviously,the bioleaching method was better than other treatments.It was found that the structure of sludge floc was also broken down,and bioleaching maybe make the sludge bound water released thoroughly compared to other methods,because of the bioleached sludge particles were minimum among these treatments.Formaldehyde+NaOH and 2%EDTA methods were proved to be effective in extracting EPS from sludge,and the total EPS production was 128.86 mg/gSS and 42.38 mg/gSS,respectively.However,the cell destructed seriously after extracted by Formaldehyde+NaOH and 2%EDTA methods.Heating method was a gentle and efficient method,and the total EPS production was 21.97 mg/gSS.Sludge dewaterability was largely improved after extracted slime,loosely bound EPS and tightly bound EPS from the sludge floc.It was also found that the higher content of EPS in slime but the worse dewaterability of sludge,and sludge dewaterability would be deteriorated when EPS was released from other layers to slime.Capillary suction time(CST)of A.t+A.f was reduced from 255.9 s to 25.45 s within 48 h,and the solid content rate of sludge after centrifuged at 4000 r/min for 3 min was 11.6%at 48 h.CST had significant positive correlation with the content of protein and polysaccharide in the slime,loosely bound EPS and tightly bound EPS of sludge.Therefore,the main reason of the improvement of sludge dewaterability was due to the decrease of protein and polysaccharide in the slime,tightly bound EPS and total EPS.It was also found that pH had a positive correlation with CST,while Fe in the sludge solid phase distribution rate had a negative correlation with CST.In brief,variations of EPS,pH,and Fe in the sludge solid phase distribution rate were the main mechanism of sludge dewaterability improved during bioleaching.In order to shorten bioleaching time,an acid-tolerant heterotrophic microorganism was isolated from pig slurry,and it was identified as Galactomyces sp.Z3,which could grow well at pH 2.5 to 7 and degrade pig slurry DOM from 1973 to 942 mg/L within 48 h.During the successive multi-batch bioleaching systems,the co-inoculation of pig slurry degrader Galactomyces sp.Z3 and the two A cidithiobacillus species could improve pig slurry bioleaching efficiency compared to the control system without Galactomyces sp.Z3.Not only the bioleaching time was shortened but also the heavy metal removal efficiency of the pig slurry was increased in the cooperative system.It was also found that co-inoculation of organic acids degrader Galactomyces sp.Z3 and two acidophilic thiobacillus(A.ferrooxidans LX5 and A.thiooxidans TS6)in the bioleaching system could improve sewage sludge bioleaching efficiency,4.5 days were shortened compare to the single system,and as exhibiting that the removal efficiency of Cu and Zn and oxidation rate of Fe2+ and S0 were higher than those in the single system with the inoculation of only acidophilic thiobacillus.The Galactomyces sp.Z3 could use the DOM(organic acids)and reduce its toxic to Acidithiobacillus species.The initial concentration of acetate,propionate,iso-butyrate,butyrate,and iso-valerate in sludge were 109.5,28.8,7.7,34.3,and 18.4 mg/L initially,and reduced to 10.20、0.61、0.63、19.40 and 1.30 mg/L in 12 h,which were much lower than CK.Meanwhile,it was found that EPS of the acid-tolerant heterotrophic microorganism Galactomyces sp.Z3 had certain surfactant ability and could promote sulfur oxidization rate by improving the hydrophility of sulfur.It was concluded that bioleaching involving two acidophilic thiobacillus(A.ferrooxidans LX5 and A.thiooxidans TS6)was an effective technique in improving sludge dewaterability and removing heavy metals from sludge.The dewaterability of sludge was greatly improved during bioleaching by changing sludge floc structure,particle size,impacting the content and distribution of EPS,reducing pH and increasing Fe in the solid phase.Galactomyces sp.Z3 could use DOM(organic acids)and reduce its toxic to Acidithiobacillus species.Meanwhile,the EPS of Galactomyces sp.Z3 had certain surfactant ability and could accelerate sulfur oxidization rate by A.thiooxidans.