In a biological wastewater treatment system, what does the term 'bulking' refer to?

Bulking in a biological wastewater treatment system primarily refers to the excessive growth of microorganisms within the treatment process, particularly in activated sludge systems. When microorganisms proliferate significantly, they tend to form clumps or flocs, which can hinder the settling process during sedimentation. This excessive growth can result in fluffy, less dense biomass that does not settle well, leading to issues like poor effluent quality and increased total suspended solids.

This phenomenon is typically caused by overfeeding the microbial populations or certain operational factors that promote the rapid growth of specific microorganisms, such as filamentous bacteria. These factors can include nutrient imbalances, excessive organic loading, or suboptimal aeration conditions. As a result, bulking can become a significant operational challenge, requiring management adjustments to restore efficient settling characteristics.

While other options are related to challenges in wastewater treatment—such as sedimentation issues, low biochemical oxygen demand in the influent, or aeration rates—they do not specifically address the concept of bulking as connected to the excessive growth of microorganisms. Understanding bulking is essential in optimizing the performance of biological treatment systems and ensuring compliance with environmental standards.