In the context of Environmental Engineering, the process of disinfection of water primarily aims at eliminating harmful microorganisms to ensure that the water is safe for human consumption and use. Let us analyze each option to determine the correct one:

1. Complete grits: This term generally refers to the removal of solid particles, such as sand and gravel, from water during the treatment process. It is not directly related to disinfection.
2. Killing of bacteria: Disinfection primarily targets the destruction of pathogenic microorganisms, including bacteria. Methods like chlorination, UV radiation, or ozonation are commonly used for this purpose. This option is the correct one.
3. Reducing turbidity up to zero: Turbidity refers to the cloudiness or haziness of water caused by large numbers of individual particles. While reducing turbidity is part of water treatment, it is achieved through filtration and sedimentation, not specifically through disinfection.
4. Improve pH values: The pH value of water is adjusted during the treatment process to make it safe and pleasant for consumption. However, improving pH is part of chemical treatment processes and not directly related to disinfection.

Therefore, by focusing on eliminating harmful bacteria and making water safe for drinking, the primary result of disinfection is the killing of bacteria.