Study On Bioleaching Combined With Fenton-Like Reaction To Remove Heavy Metals From Sewage Sludge

With popularization of sewage treatment, the production of excess sewage sludge greatly increases, which makes the proper sludge treatment and disposal technologies become more and more important. Land application is considered as one of the most economical and effective ways for sludge final disposal. Unfortunately, the sludge containing heavy metals shows serious potential risk to human health, and the heavy metals become the main limiting factor for sludge land application. Bioleaching is one of the microbial technologies for sludge treatment, which has gained widespread attention since it is environmentally friendly and economical. However, the bioleaching time is much longer than that of chemical method, and the long reaction time has become the main barrier for its application. To overcome this problem, bioleaching combined with Fenton-like reaction was investigated in this paper to remove the heavy metals from sewage sludge.Indigenous mixed iron-oxidizing bacteria, which mainly contain Thiobacillus ferrooxidans, were selected as experimental strains. They were enriched and cultured by three time continuous cultivation, and FeSO₄ was used as substrate. The combination of bioleaching and Fenton-like reaction, leaching with inorganic acid and single bioleaching of the sludge were compared to evaluate the feasibility and heavy metal removal efficiency of the combined sludge treatment. The bioleaching combined with Fenton-like reaction was the most effective method for heavy metal removal from sewage sludge. The combination method is significantly more effective than inorganic acid leaching, and is slight more effective than single 15-day bioleaching; on the other hand, the whole reaction period was shortened from 15d to 5d. After the combination treatment of bioleaching and Fenton-like reaction, the residual heavy metals in treated sludge meets the requirement of Control Standards for Pollutants in Sludges from Agricultural Use（GB4284-84） of China for acidic soil.The mechanisms of bioleaching combined with Fenton-like reaction were investigated from two aspects: the basis of combination processes, and change in iron ion species. In the system of bioleaching combined with Fenton-like reaction, the bioleaching not only provided the acidic condition and catalyst for Fenton-like reaction, but also reduced the heavy metal load for Fenton-like reaction. Meanwhile, the Fenton-like reaction shortened the bioleaching period and increased the heavy metal removal efficiency. Iron ion played an important role as a link between bioleaching and Fenton-like reaction. Fe²⁺ served as energy substrates during the bioleaching stage, which provided energy for the growth of indigenous mixed iron-oxidizing bacteria, and Fe³⁺ participated in heavy metal dissolution by indirect leaching. During the Fenton-like reaction stage, Fe³⁺ was firstly reduced to Fe²⁺, which then functioned as catalyst to decompose H₂O₂ and generate ?OH. In addition, in the system of bioleaching combined with Fenton-like reaction, the iron ion mainly existed as the form of Fe³⁺ after 5d bioleaching, which reduced the probability of rapidly consuming ?OH and increased the oxidation efficiency of H₂O₂.Reaction conditions for bioleaching combined with Fenton-like reaction was optimized from several aspects separately, including reaction temperature, sludge solid content, FeSO₄ dosage, H₂O₂ dosage, initial p H and reaction time of Fenton-like reaction. The optimal conditions for bioleaching combined with Fenton-like reaction were as follows: reaction temperature of 30℃, sludge solid content of 3%, FeSO₄ dosage of 10.0 g/L, H₂O₂ dosage of 7.5 g/L, initial p H of 2.5 for Fenton-like reaction, reaction time of 90 min for Fenton-like reaction. Under the optimal conditions, solubilization of Cu, Zn and Cd were 77.5%, 75.8% and 69.7% respectively.After bioleaching combined with Fenton-like reaction, the sludge properties changed. The content of protein, polysaccharide, EPS and SCOD obviously increased, which indicated that bioleaching combined with Fenton-like reaction obviously disintegrated the EPS of sludge. The content of Cu、Zn、Cd、Pb in the treated sludge met the requirement of the Control Standards for Pollutants in Sludges from Agricultural Use（GB4284-84） of China and the Disposal of Sludge from Municipal Wastewater Treatment Plant-Control Standard for Agricultural Use（CJ/T 309-2009） of China, moreover, the stability of heavy metal in the treated sludge increased. The loss of TN, TP and organic matters were high, but the content of TN, TP in the treated sludge were still higher than the average value in pig manure. The dewaterability of treated sludge obviously enhanced, and the SRF reduced from 1.85×1013 to 1.56×1012 m/kg, the moisture of sludge cake reduced from 89.7% to 80.1%, and the efficiency of centrifugal dewatering increased from 74.5% to 84.7%. The number of total coliforms and fecal coliforms drastically reduced, and their reduction were respectively 99.98% and 99.92%, leading to an improved security of treated sludge.