What percent of sewage is solid?

• A. 10% solids
• B. 1% solids
• C. 0.1% solids
• D. 0.01% solids

Answer: (C)

The main idea is that sewage is mostly water, with only a small fraction being solids. In typical domestic wastewater, solids make up about 0.1% by weight. That means about 1 gram of solids per liter of sewage, with the rest being water and dissolved substances. This rough figure is a practical way to think about the system when sizing equipment or understanding treatment stages, even though actual solids can vary with flow and source.

Choosing a much higher value, like 10% or 1%, would imply a slurry far richer in solids than ordinary sewage, which isn’t representative. A much lower value, such as 0.01%, would understate the amount of solids seen in practice. So 0.1% is the commonly used approximation.