The filtration efficiency and the inactivation performance of a dialdehyde cellulose (DAC) filter against airborne and waterborne bacteria and viruses were investigated. The DAC filter was synthesized by periodate oxidation of a cellulose filter. To evaluate the performance, the inactivation efficiency (determined by comparing the concentration of viable microbes obtained from control and experimental samples) and the relative survival fraction (calculated by comparing the viable microbes from an untreated filter and a treated filter) were introduced. For water filtration tests, the viable removal efficiency and the relative survival fraction of untreated and treated filters of different treatment times were investigated by testing with E. coil and MS2 bacteriophage. The treated filter presented a slightly higher viable removal efficiency and a significantly lower relative survival fraction as treatment time increased. Increasing the residence time by lowering the filtration velocity resulted in a higher viable removal efficiency and lower survival fraction. In air filtration tests, the physical removal efficiency, viable removal efficiency, and relative survival fraction of untreated and 12-h treated filters were investigated. The removal efficiency and relative survival fraction of the treated filter increased and decreased, respectively, with increasing relative humidity. The pressure drop of the treated filter was significantly lower than that of the untreated filter, which resulted in a higher filter quality. The DAC filter with sufficient moisture content had a higher removal efficiency, lower pressure drop, and better disinfection capability, which are all important attributes for practical biocidal applications.