1. Biofilm Growth and Activity: The amount and activity of the biofilm on the media are critical indicators of performance. Techniques like microscopic examination, ATP (adenosine triphosphate) measurements, and oxygen uptake rates can be used to assess the health and activity level of the biofilm.

2. Removal Efficiencies: The core performance metric for any wastewater treatment process, including MBBR systems, is the efficiency of pollutant removal. This includes measurements of BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), TSS (Total Suspended Solids), nitrogen compounds (ammonia, nitrite, and nitrate), phosphorus, and any specific pollutants of concern. Efficiencies are calculated based on the influent and effluent concentrations of these compounds.

3. Specific Surface Area Utilization: Since the effectiveness of MBBR media is largely dependent on the available surface area for biofilm growth, measuring how well the biofilm utilizes this area is important. This can be done indirectly by assessing removal efficiencies in relation to the media volume and surface area within the reactor.

4. Oxygen Transfer Efficiency: Oxygen transfer is crucial in aerobic MBBR systems for the biofilm to oxidize organic matter. The efficiency of oxygen transfer can be evaluated through the oxygen transfer rate (OTR) or the standard oxygen transfer efficiency (SOTE). These metrics help determine if the aeration system is adequately supplying oxygen to the biofilm on the media.

5. Hydraulic Characteristics: The movement and mixing of MBBR media within the reactor ensure that wastewater contacts the biofilm evenly. Assessing hydraulic behavior, such as dead zones, channeling, or inadequate mixing, can be part of performance evaluation, typically using tracer studies or computational fluid dynamics (CFD) modeling.

6. Durability and Integrity of Media: Over time, assessing the physical condition of the MBBR media is essential to ensure they maintain their structure and surface area. Wear and tear, breakage, or fouling of the media can impact treatment efficiency and require maintenance or replacement.

7. Sludge Production Rate: In systems aiming for sludge reduction, measuring the rate of sludge production can indicate the biofilm's efficiency in breaking down organic matter without generating excessive biosolids.

Regular monitoring and assessment of these parameters are essential for optimizing MBBR system performance, ensuring compliance with discharge regulations, and identifying any needs for operational adjustments or maintenance.