What is the typical sludge concentration before digestion?

• A. 0.5-1%
• B. 10-12%
• C. 20-25%
• D. 4-8%

Answer: (D)

Thickening sludge before digestion concentrates the solids so the digester can operate efficiently without needing enormous volumes. The feed to the digester is typically around four to eight percent total solids, which strikes a balance between being pumpable and giving good mass transfer and gas production inside the digester. If the sludge is too dilute, you’d need much larger digesters and more energy to mix and heat it. If it’s too thick, handling, pumping, and mixing become difficult and digestion can suffer.