To achieve higher ammonia removal in treatment plants, what change in Mean Cell Residence Time (MCRT) is generally needed?

Increasing the Mean Cell Residence Time (MCRT) to 8-10 days is a common method to enhance ammonia removal in wastewater treatment processes, especially when utilizing biological treatment methods. A higher MCRT means that the microorganisms responsible for breaking down ammonia have more time to work, leading to greater digestion of nitrogenous compounds.

In biological treatment systems, particularly in activated sludge processes, the MCRT influences the growth rate and abundance of microorganisms. By extending the time that these microorganisms remain in the system, you allow the slow-growing nitrifying bacteria, which are critical for ammonia oxidation, to thrive. This longer retention facilitates the conversion of ammonia to nitrate, thus improving overall nitrogen removal efficiency.

While options suggesting a decrease in MCRT would generally lead to a more rapid turnover of biomass, this can limit the effectiveness of the biological processes that depend on slower-growing species. Conversely, maintaining MCRT at a mid-range level may not fully capitalize on the benefits of longer retention times for enhanced ammonia removal. Increasing MCRT to between 12-14 days might be effective in certain situations, but commonly, an increase to 8-10 days is seen as a practical target for optimal performance.