Abstarct: Nowadays, water treatment with maximum efficiency and minimum cost becomes necessary according to water resources reduction and pollution. One of the principal resources to supply drinking water is the water accumulated in the dam's reservoir. Many different pollution parameters in the raw water, including microbial contamination and the existence of the micro-organisms such as algae, are affected by the water quality. Therefore, novel technologies such as Micro-Nano Bubbles (MNBs) have been considered to achieve an efficient water treatment procedure. The effect of MNB technology on eliminating and reducing the different spectrums of the micro-organisms and the variation of the bacterial population in the dam reservoir's water was investigated by performing some experiments. For this purpose, the raw water entered the water treatment plant without any pretreatment, leading to the MNBs generator. Simultaneously, the air, oxygen, and ozone gases and the mixture of air and ozone gases were injected into the raw water. Since the experiments were performed on different days, the quality of the raw water and its microbial load was not controllable and this was one of the study's limitations. Investigating the impact of the type, quantity, and duration of the gas (air, oxygen, and ozone) injection on decreasing the micro-organisms and microbial load were the most important goals of this study. Various injection duration of the gases was applied to the system to assess the water quality parameters by identification and counting the number of the micro-organisms, measurement of the microbial load by the heterotrophic bacteria counting method, scanning electron microscope, analysis of the size of the MNBs, measurement of the zeta potential, measurement of ozone concentration and residual time in the water. The current study showed that the ozone MNBs have the maximum efficiency in eliminating and reducing all identified species of micro-organisms so that in the injection duration of 180 minutes, 68 to 100 percent of them were removed. Furthermore, it is found that using air, oxygen, and ozone MNBs for 30 minutes and the mixture of air and ozone MNBs for 60 minutes caused the maximum reduction rate of the micro-organisms, which can have considerable economic efficiency. Using the mix of air and ozone MNBs for 60 minutes leads to getting rid of 0.74 log (equivalent to 81 percent) of the Heterotrophic bacteria with acceptable efficiency. Generally, it could be concluded that the injection of MNBs in raw waters removes a wide range of micro-organisms and microbial load. Moreover, MNBs may decrease the doses of chemicals used, such as coagulants, in the next stage of the water treatment and prevent any disorders in the filtration process.