| Municipal wastewater treatments produce a large amount of residuals, which are characterized by high moisture content, high organic content and instability of chemical nature etc. The cost for sludge treatment and disposal accounted 30%~50% of total operating cost of WWTP. The sludge reduction is largely due to the sludge thickening during the sludge treatment process. Therefore the development of concentration technology with high efficiency has great significance in reducing sludge moisture content, improving the efficiency of dewatering devices, decreasing energy consumption in dewatering step. In this study, a long-term monitoring of sludge properties in membrane bioreactor(MBR) was performed, and the feasibility of using dissolved air floating technology for MBR surplus sludge thickening was investigated.According to the results of long-term monitoring of sludge characteristics, the sludge produced from MBR had very special features in comparison to traditional activated sludge, including high sludge concentration, smaller particle size, higher EPS content and poor settleability. In addition, the MBR sludge was very difficult to settle due to the high sludge concentration. Gravity and mechanical concentration had rather limited effect on decrease in moisture concentration of waste sludge. Therefore, it is very imperative and practical to develop the novel sludge concentration technologies.The experimental results of air-floating indicated that the chemical conditioning methods played important roles in sludge concentration efficiency. The effects of direct air floating without chemical preconditioning and surfactant conditioning were ineffective to decrease sludge moisture. The air floating thickening efficiency of sludge could be effectively enhanced by using chemical conditioning with PAM. The surfactants showed rather limited effect on sludge concentration. Finally, the raw sludge with water content of 99% was concentrated water content of 97%, so the air-floating had much better performance in concentration of MBR sludge than mechanical device in Beixiaohe WWTP.The pilot test of air floating concentration showed that the optimal operating conditions were solid loading-15kg/(m2·h), hydraulic loading-1.5m3/(m2·h), recirculation ratio-1, dosage of PAM-2 ‰(w/w dry solid), dissolved-air pressure-0.4MPa, air/solid ratio-0.03. After thickening, the moisture content of sludge reduced to around 97%. Furthermore, the content of loose-bound EPS, tightly-bound EPS were also sharply reduced, and addition of PAM enhanced the decrease in water content of concentrated sludge. Furthermore, the particle size of sludge underwent an unobvious change after direct air-floating, while addition of PAM resulted in increase of size due its flocculation effect. In addition, the cake moisture was linearly correlated with sludge loading of belt filter press. The cake moisture could be reduced to below 80% when the sludge loading of dewatering device was less than 80 kg/(m·h).Finally, the feasibility of gas-liquid multiphase pump(GLMP) for sludge concentration was also investigated. The results showed that this device has the advantages of small area occupation, low price of equipments and energy consumption, low operating cost, and the GLMP could be performed under the conditions of low labor intensity and an unattended backup. Meanwhile, the air-dissolution efficiency of GLMP was much higher than traditional air-floating equipment. |
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