What should be the relationship between the height of troughs and sand loss during backwash?

The optimal relationship between the height of troughs and sand loss during backwash is that the troughs should prevent sand loss. This means that the design and positioning of troughs are crucial in managing sand retention effectively. The primary function of the troughs in this context is to contain the sand and prevent it from being washed away during backwash, which is the process of water reversing flow to clear out contaminants or settleings.

When troughs are correctly designed and placed, they act as barriers that capture and hold the sand particles, minimizing the risk of losing them to the water flow. If troughs are functioning properly, they will ensure that sand stays within the intended area rather than being lost to the environment, contributing to the effectiveness of sand filtration systems or other aquatic management technologies.

While other options highlight different relationships between trough height and sand loss, such as the idea that troughs must be lower or at ground level, these do not address the fundamental concern of preventing sand loss, which is crucial for maintaining an efficient system. Thus, preventing sand loss is a critical aspect of managing backwash effectively in water treatment and filtration processes.