1. Wastewater Characteristics: The nature of the wastewater, including its chemical composition, presence of particulate matter, and variability in flow and load, dictates the choice of media. High-strength industrial wastewater might require more robust media with higher surface areas to support sufficient biofilm growth for effective treatment.

2. Treatment Goals: The specific objectives of the treatment process, such as BOD reduction, nitrification, denitrification, or phosphorus removal, influence the media choice. Different media types and sizes offer varying amounts of surface area for biofilm attachment, which is crucial for achieving specific treatment outcomes.

3. Biofilm Growth Conditions: The desired characteristics of the biofilm, including thickness and density, affect media selection. Media with larger surface areas can support denser and thicker biofilms, which might be necessary for treating certain types of wastewater or meeting specific effluent quality standards.

4. Hydraulic Conditions: The flow rate and hydraulic retention time of the wastewater within the MBBR system impact the media type and size. Media must be chosen to ensure proper mixing and contact with wastewater while preventing clogging or channeling.

5. Aeration and Mixing: The aeration system used to provide oxygen to the biofilm and keep the media in motion influences media selection. The size and density of the media need to be compatible with the aeration system to ensure efficient oxygen transfer and prevent settling or excessive wear.

6. Physical Constraints of the Reactor: The size of the reactor and the available volume for media can dictate the amount and size of media that can be used. Larger reactors might accommodate larger or more media, offering more flexibility in media selection.

7. Maintenance and Durability: Media that are easier to maintain, clean, and replace can be preferred in systems where regular maintenance is challenging. Durability under the specific chemical and physical conditions of the wastewater treatment process is also a critical consideration.

8. Cost: Budget constraints can influence the choice of MBBR media. More advanced or specialized media with higher surface areas or specific features for enhanced biofilm growth might be more expensive but offer better performance or lower operating costs over time.

9. Regulatory Requirements: Compliance with local and national regulations regarding effluent quality might necessitate the use of specific types of media to achieve the required treatment levels.

Selecting the appropriate MBBR media type and size involves balancing these factors to achieve the desired treatment efficiency, operational reliability, and cost-effectiveness of the wastewater treatment system.