What is the most common reason for wastewater to contain large amounts of sand, gravel, and grit?

The presence of large amounts of sand, gravel, and grit in wastewater is most commonly associated with storm events. During rainfall, stormwater runoff can carry various materials, including sediment and debris, from roads, construction sites, and landscapes into the sewer systems. This influx of surface materials, combined with the natural erosion that occurs during heavy rain, significantly increases the concentration of these particulates in the wastewater.

Storm events can lead to what is known as combined sewer overflow, where both stormwater and wastewater are collected in the same system. When these systems are overwhelmed, or when there is direct runoff into drainage systems, the additional sediment from the surface is transported into treatment facilities. As a result, the wastewater may contain higher levels of sand, gravel, and grit following storms.

Options related to industrial discharges and residential runoff could contribute some particulates, but they typically do not account for the same volume of sediment as storm events. Aging infrastructure may cause issues like leaks and blockages, but it is unlikely to directly introduce large quantities of sand and gravel into the wastewater flow.