What type of sludge is commonly pumped using a piston or diaphragm pump?

Primary sludge is the correct answer because it is typically characterized by its higher solids content and greater viscosity compared to secondary and tertiary sludges. Primary sludge results from the physical separation processes in wastewater treatment, where solids settle out of the liquid waste during primary treatment stages. Due to its thicker consistency and density, it is well-suited for pumping with piston or diaphragm pumps, which are designed to effectively handle the more viscous materials.

In contrast, secondary sludge, which comes from biological processes during secondary treatment, contains a higher proportion of water and may have a lower solids concentration. Tertiary sludge is not as commonly referenced in the same context, as tertiary treatment generally focuses on further purification rather than sludge management. Activated sludge specifically refers to the biomass involved in the biological treatment process but is generally managed differently than primary sludge during pumping operations.