Effect of different sludge retention time (SRT) on municipal sewage sludge bioleaching continuous plug flow reaction system

A plug-flow bio-reactor of 700 L working volume for sludge bioleaching was used in this study. The reactor was divided into six sections along the direction of the sludge movement. Fourteen days of continuous operation of sludge bioleaching with different sludge retention time (SRT) under the condition of 1.2 m³·h^-1 aeration amount and 4 g·L^-1 of microbial nutritional substance was conducted. During sludge bioleaching, the dynamic changes of pH, DO, dewaterability (specific resistance to filtration, SRF)of sewage sludge in different sections were investigated in the present study. The results showed that sludge pH were maintained at 5.00, 3.00, 2.90, 2.70, 2.60 and 2.40 from section 1 to section 6 and the SRF of sludge was drastically decreased from initial 0.64×10¹³ m·kg^-1 to the final 0.33×10¹³ m·kg^-1 when bioleaching system reached stable at hour 72 with SRT 2.5d. In addition, the sludge pH were maintained at 5.10, 4.10, 3.20, 2.90, 2.70 and 2.60, the DO value were 0.43, 1.47, 3.29, 4.76, 5.75 and 5.88 mg·L^-1 from section 1 to section 6, and the SRF of sludge was drastically decreased from initial 0.56×10¹³ to the final 0.20×10¹³ m·kg^-1 when bioleaching system reached stable at hour 120 with SRT 2 d. The pH value was increased to 3.00 at section 6 at hour 48 h with SRT 1.25 d. The bioleaching system imbalanced in this operation conditions because of the utilization efficiency of microbial nutritional substance by Acidibacillus spp. was decreased. The longer sludge retention time, the easier bioleaching system reached stable. 2 d could be used as the optimum sludge retention time in engineering application. The bioleached sludge was collected and dewatered by plate-and-frame filter press to the moisture content of dewatered sludge cake under 60%.This study would provide the necessary data to the engineering application on municipal sewage sludge bioleaching.