What could cause solids to pass through and over a clarifier weir?

Short circuiting in a clarifier refers to the situation where water flows through the system faster than intended, bypassing the areas where solid settling occurs. When this happens, the flow can disrupt the settling process, preventing solids from properly settling at the bottom where they can be removed effectively. As a result, solids might not have enough time to settle out of the water before it reaches the weir, leading to them passing over the weir along with the treated water.

In contrast, insufficient aeration generally impacts biological treatment processes rather than the physical settling process in a clarifier. Low chemical concentration could affect coagulation and flocculation, but it does not directly cause the physical phenomena of solids spilling over the weir. Extended detention time typically allows for better settling of solids, thus reducing the likelihood of them passing over the weir. Therefore, short circuiting is the most relevant factor in causing solids to bypass the settling area in a clarifier.