Which of the following classifications indicates a higher solid concentration?

The classification that indicates a higher solid concentration is total suspended solids. Total suspended solids (TSS) refer to the solid particles that are suspended in wastewater and that may settle out under gravity. TSS is a vital measurement in wastewater treatment as it encompasses all particles present in the water, including both organic and inorganic materials, which means it captures a broader scope of solids compared to other classifications.

In contrast, primary solids specifically refer to the solids that are removed during the primary treatment phase of wastewater, which may not represent the entire concentration throughout the wastewater process. Volatile solids pertain to organic materials that can be lost upon combustion and focus on the organic fraction, while fixed solids represent the portion of solids that remain after ignition, effectively excluding any volatile component. Consequently, TSS provides a more comprehensive picture of the solid concentration in wastewater, making it the correct answer.