| Wastewater treatment plants (WWTPs) are increasing rapidly in China. By 2011, the number of WWTPs has reached about 3038 with the amount of treated wastewater being 1.34×108 M3/d. Sewage sludge production is estimated to close to about 6,000,000 dry tonne per year. Sewage sludge reduction, harmless and utilization has become a considerable problem in a sludge treatment and disposal field. Sludge high-efficiently dewatering to dewatered sludge moisture centent below 60% is the key to solve the above problems. However, sludge conditioning is a crucial approach in the improvement of its dewaterability in subsequent mechanical dewatering operation.Numerous works for sludge conditioning have been done by researchers including hydrothermal conditioning, microwave conditioning, ultrasonic conditioning, chemical conditioning, and so on. These methods could improve the sludge dewaterability to some extent, but there exists some serious shortcomings. For example, hydrothermal conditioning not only needs a lot of energy, and the filtrate after sludge dewatering contains a large amount of COD and ammonia nitrogen. Ultrasonic conditioning and microwave conditioning could disrupt the sludge flocs, to some extent, improve settling properties of sludge. However, the excess energy input into sludge system would significantly deteriorate sludge dewaterability because the disruption of floc structure and release of small molecules materials. These small molecules will clog the pores in the filter cloth, and increase the moisture content of sludge cake. It is more difficult to determine the optimal action time of ultrasonic and microwave with the different physicochemical characteristics of sludge. Therefore, there are certain risks to promote such methods in practical engineering application. Chemical conditioning could improve sludge dewaterability significantly and the conditioned sludge is dewatered to moisture content of sludge cake below 60% by chamber filter press. However, organic matter, TN, TP, and TK in dewatered sludge cake from chemical conditioning treatment are decrease dramatically, because of the introduction of large amounts of inorganic substances into sludge matrix. Expectedly, it was unfavorable for utilization of sludge.In the last two decades, bioleaching technology has been developed as a potential microbial method to remove heavy metals from sewage sludge facilitated by A. thiooxidans and A. ferrooxidans. In addition, our early laboratory experiments had confirmed that the sludge dewaterability could be improved after bioleaching, as exhibiting that SRF of bioleached sludge is drastically reduced with comparison to fresh sludge.However, many issues are still uncertainties in related research. What are the advantages and disadvantages of bioleaching technology compared with conventional physical, chemical conditioning methods? The best dosage of microbial nutrient subatance for microorganism in sludge bioleaching system, the stability of bioleaching system and sludge conditioning effect in pilot-scale study, and the disposal of bioleached sludge cake are the key issues need to be investigated for actual engineering application.In view of the above, these key issues had been systematic studied in this paper. The main conclusions are as follows:The dewaterability of municipal sewage sludge could be markedly enhanced after bioleaching or chemical conditioning with comparison to other conditioning approaches including hydrothermal, ultrasound, and microwave. However, unlike chemical conditioning, sludge bioleaching did not result in the obvious reduction of organic matter, TN, and TP in resulting dewatered sludge cake. Taking sludge dewaterability and chemical properties of sludge filtrate and dewatered sludge cake into account, bioleaching was a potential approach in improving sludge dewatering and subsequent disposal or reutilization of pre-treated sludge cake. Considering the acidification rate of bioleaching system,4g/L is considered to be the best initial dosage of microbial nutrient subatance in bioleaching system. Taking dewatering of municipal sewage sludge and the removal of sludge-borne heavy metals into account, pH-2.4 is the optimum end point for bioleaching. The removal efficiency of sludge-borne Cu could reach 85% in this pH level.Sequence batch and continuous plug flow bioleaching experiments were conducted through a bioleaching bio-reactor with 700 L of working volume. The bioleached sludge was dewatered by chamber filter press with 0.3-0.4 MPa working pressure for 2 hours. It is found that the moisture of dewatered sludge cake can be reduced to 58.0%. The dewatered sludge cake is of khaki appearance and didn’t emit any offensive odor. In addition, it is also observed that sludge organic matter did not result in the obvious reduction but 58% of sludge-borne Cu and 88% of sludge-borne Zn can be removed from sludge by bioleaching process. The moisture content of bioleached sludge cake was reduced to about 45% and less than 10% if the cake was placed at 25 "C for 15 h and 96 h, respectively. The longer sludge retention time, the easier bioleaching system reached stable. Taking the bio-reactor use efficiency and bioleaching system stability into account,2 d could be used as the optimum sludge retention time (SRT) in actual engineering application.Dewatered sludge after bioleaching has the faster evaporation rate compared with conventional dewatered sludge at the same drying conditions. The sewage sludge (higher heating value (d.b.):9250KJ/kg; moisture content:=32%; particle diameter:=8 mm) could achieve self maintaining incineration on the basis of sludge particles has a better flow state in combustor. The corresponding temperature of combustor could be maintained at about 850 ℃ and the ignition loss content of burned residue was 2.36%. It is also found that the initial sludge organic matter content, moisture content, degree of particle size breakage and the air amount on the distribution plate are determine the effect of sewage sludge incineration.The results of this study have an important guiding siginigicance for the engineering application of sewage sludge bioleaching. |
PDF Full Text Request