Reduction of excess sludge and simultaneous removal of organic carbon and nitrogen by the porous carrier and membrane hybrid system

In a fluidized-bed porous carrier system, organic carbon and nitrogen in wastewater can be removed simultaneously. However, the excess sludge generated in the system has not been focused on. On the other hand, a membrane system can separate suspended solid from treated water and retain slowly growing microbes such as nitrifiers in the aeration tank. Porous carrier and membrane hybrid system with these respective advantages has been studied to enhance the nitrogen removal efficiency. In this study, the hybrid system was investigated to compare with a conventional carrier system in terms of the nitrogen removal rate from complex artificial wastewater (CAW) containing polypeptone as a nitrogen source. Moreover, the amounts of excess sludge generated in both systems were evaluated. Longer sludge retention time of the hybrid system was expected to enhance the microbial autolysis and lead to the reduction of the excess sludge. Consequently, the amount of excess sludge generated in the hybrid system was reduced to about one-fourth of that in the carrier system for about two months. In addition, a lot of slowly growing protozoa preyed on the microorganisms, which might have contributed to the reduction of excess sludge. The removal rates of organic carbon and nitrogen in the hybrid system were higher than in the carrier system, due to the highly concentrated microorganisms in the hybrid system. A batch operation revealed that the nitrogenous compounds of polypeptone in CAW were rapidly removed by assimilation, while urea in the artificial wastewater was mainly removed by nitrification. Consequently, total nitrogen concentration in CAW containing polypeptone decreased faster than in artificial wastewater containing urea. These results show that nitrogenous compounds in CAW are easy to be removed. The hybrid system is an efficient system to enhance the nitrogen removal and has a capability to reduce the excess sludge.