Comparison of two typical regeneration methods to the spent biological activated carbon in drinking water

The spent biological activated carbon (BAC) should be disposed properly; regeneration was a better choice. Performances of thermal and ultrasonic regeneration to the BAC with various service time (3 years, 5 years, and 10 years) were compared comprehensively; the recovery of the BAC's pore structure, variation of mechanical hardness, influence of bioactivity, and removal efficiency of typical pollutants in the reuse were examined. The results showed that thermal regeneration was an utterly regeneration, and almost all the pore structure was restored, whose recovery rate was above 90% for BAC used 3 years and disfavored by the longer BAC's service time (83% for the BAC used 5 years). Ultrasonic regeneration could recover part of the BAC's pores (including micropores) and the restoration mainly focused on the BAC's surface, so the recovery rate was not influenced by the BAC's service time, and the recovery values of specific surface areas and iodine value were kept at 120 m²/g and 200 mg/g, respectively. In addition, the ultrasonic treatment enhanced the BAC's biological activity even with a significant decrease of the biomass on the BAC. The mechanical hardness of BAC decreased from 95 to 89% for the first regeneration and further to 79% for the second regeneration, whereas relatively lower decrease happened for the ultrasonic regeneration (less than 10% after 5 regeneration cycles). The mass losses in the thermal and ultrasonic regeneration were about 13%, 0.5%, and 25%, 3% for the first and second regeneration, respectively. The thermal-regenerated activated carbon (AC) exhibited excellent adsorption ability, good adherence of biofilm, and maintain higher removal rate for more than 2 years, which were similar with that of the fresh AC, but relatively lower removal performance was found. However, the ultrasonic regenerated BAC retained the biodegradation ability, restored the fast-adsorption ability, and the higher removal process lasted about 6 months. Taking the regeneration cost, operation, variation of the AC's characteristics, and the removal performance in reuse, ultrasonic regeneration was more suitable for the BAC filter and better used as a regular measure to maintain the higher removal performance, whereas thermal regeneration was more applicable to the activated carbon adsorption tank.